HAMSTRING FATIGUE AND MUSCLE ACTIVATION CHANGES DURING SIX SETS OF NORDIC HAMSTRING EXERCISE IN AMATEUR SOCCER PLAYERS PAUL W.M. MARSHALL, RIC LOVELL, MICHAEL F. KNOX, SCOTT L. BRENNAN, AND JASON C. SIEGLER Human Performance Laboratory, School of Science and Health, University of Western Sydney, Sydney, Australia ABSTRACT
INTRODUCTION
Marshall, PWM, Lovell, R, Knox, MF, Brennan, SL, and Siegler, JC. Hamstring fatigue and muscle activation changes during six sets of Nordic hamstring exercise in amateur soccer players. J Strength Cond Res 29(11): 3124–3133, 2015—The Nordic hamstring exercise (NHE) is a bodyweight movement commonly prescribed to increase eccentric hamstring strength and reduce the incidence of strain injury in sport. This study examined hamstring fatigue and muscle activation responses throughout 6 sets of 5 repetitions of the NHE. Ten amateur-level soccer players performed a single session of 6 sets of 5 repetitions of NHE. Maximal eccentric and concentric torque output (in newton meters) was measured after every set. Hamstrings electromyograms (EMG) were measured during all maximal contractions and exercise repetitions. Hamstring maximal eccentric torque was reduced throughout the range of motion after only a single set of NHE between 7.9 and 17.1% (p # 0.05), with further reductions in subsequent sets. Similarly, maximal concentric torque reductions between 7.8 and 17.2% were observed throughout the range of motion after 1 set of NHE (p # 0.05). During the descent phase of the NHE repetitions, hamstring muscle activity progressively increased as the number of sets performed increased. These increases were observed in the first half of the range of motion. During the ascent phase, biceps femoris muscle activity but not medial hamstrings was reduced from the start of exercise during latter sets of repetitions. These data provide unique insight into the extent of fatigue induced from a bodyweight only exercise after a single set of 5 repetitions. Strength and conditioning coaches need to be aware of the speed and extent of fatigue induced from NHE, particularly in practical settings in which this exercise is now prescribed before sportspecific training sessions (i.e., the FIFA-11 before soccer training).
KEY WORDS electromyography, FIFA-11, eccentric torque, concentric torque, biceps femoris, medial hamstrings
Address correspondence to Paul W.M. Marshall, [email protected]. 29(11)/3124–3133 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association
3124
the
T
he Nordic hamstring exercise (NHE; aka Nordic hamstring curl) has been used in many recreational (e.g., powerlifting and bodybuilding) and sports training environments (e.g., soccer, rugby union, Australian rules football) as a method of increasing eccentric hamstring muscle strength (16,28) and thus reducing the incidence of hamstring strain injuries (4,7,24). Briefly, the NHE is a bodyweight only exercise, which requires the individual to lower their upper body toward the ground from a kneeling position, with a partner or device holding the ankles down (the descent phase). The individual then uses the upper body to provide propulsion for returning to the start position (the ascent phase). The prescription of NHE is becoming more common as it only uses bodyweight as resistance and does not require specialized equipment. For example, the NHE is prescribed within the Fe´de´ration Internationale de Football Association (FIFA)-11 intervention program as part of a standard field warm-up for soccer players at the amateur and recreational levels (6,17). It is important to provide information about the use of the NHE to better inform the ability of strength and conditioning coaches to accurately prescribe this exercise. Currently, there is no research examining hamstring muscle fatigue, or changes in muscle activation during the descent and ascent phases of the exercise, when performing multiple sets of NHE. The extent of hamstring fatigue induced by NHE is of interest considering recommendations for the use of NHE before training sessions in the FIFA-11 intervention program (6,17). Examining hamstring muscle activation changes during multiple sets of NHE may provide insight into the optimal dose of exercise to prescribe to maximize recruitment of the hamstring muscles. To our knowledge, there are 2 studies that have described acute responses of the hamstring muscles to the NHE (9,16). Neither study examined hamstring fatigue using measures of concentric or eccentric torque, muscle activation changes that may have occurred within and between sets of repetitions, or the ascent phase of the exercise (9,16). Iga et al. reported increased hamstring muscle activity, measured by surface electromyography (EMG), in the last 608 of the descent phase (controlled movement cadence 308$s21)
TM
Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
the
TM
Journal of Strength and Conditioning Research compared with the first 308 during 1 set of 5 NHE repetitions (16). Ditroilo et al. (9) examined EMG and movement kinematics during 2 sets of 5 NHE repetitions and reported a wide range of movement velocities, peak accelerations, and activation patterns. The high intersubject variability was probably explained by the lack of cadence control during the exercise, which makes comparison of the results between the 2 identified studies difficult (9,16). The higher hamstring EMG activity through the latter twothirds of the NHE descent (16) suggests increased central motor output to the hamstrings. It is reasonable to speculate that eccentric muscle fatigue is more likely to occur in the latter two-thirds of the range of motion where activity is the highest. However, it is unclear how muscle activation during the NHE will change in the presence of eccentric fatigue. In conventional resistance exercise models, muscle activation seems to increase during multiple sets concomitant to fatigue in untrained (32), but not resistance trained individuals (13). Soccer players, particularly at the amateur level where the FIFA-11 is targeted, could probably be best described as not being resistance trained. Thus, it is reasonable to speculate that muscle activation of the hamstrings, particularly in the latter two-thirds of the descent phase, will increase concomitant to eccentric fatigue. The absence of information describing the ascent phase of the NHE makes it difficult to speculate about the extent of concentric fatigue and hamstring muscle activation during this part of the NHE. Therefore, we designed this study to address critical gaps in the understanding of the NHE to better inform prescription of this exercise in clinical practice. The objectives of this study were (a) to examine changes in eccentric and concentric hamstring muscle torque output during 6 sets of 5 repetitions of the NHE and (b) to measure EMG from the biceps femoris (BF) and medial hamstring (MH) muscles throughout all exercise repetitions and maximal torque trials. We hypothesized that the greater EMG amplitudes previously reported during the latter two-thirds of the descent phase (16) would contribute to a loss of eccentric torque measured in the same part of the range of motion but increased muscle activation during the repetitions of NHE. Although a clear hypothesis regarding the ascent phase and concentric fatigue could not be reached owing to an absence of available information, we also hypothesized that any observed reductions in concentric torque output would be associated with increased hamstring muscle activation during the ascent phase of the NHE.
METHODS Experimental Approach to the Problem
This was a cross-sectional study of the acute fatigue and muscle activation responses to a session of NHE. The test protocol consisted of 6 sets of 5 repetitions of the NHE using a controlled average cadence of 308$s21 for the descent phase and a self-paced ascent (participants asked to push off the floor and return to starting position as soon as possible).
| www.nsca.com
Two-minute interset rest intervals were provided. Primary dependent variables measured throughout the exercise session were maximal eccentric and concentric torque output and the activation of the BF and MH from continuous EMG recordings. Postprocessing, all torque and EMG variables (measured during maximal contractions and the exercise repetitions) were expressed and compared within each 158 movement epoch based on knee joint range of motion. Within the context of this study, the independent variables were the number of sets performed, the eccentric and concentric maximal contractions, and the descent and ascent phases of the exercise repetitions. Participants were familiarized with the NHE and all testing procedures at a session performed 5–7 days before the testing session. Maximal eccentric and concentric hamstring torque output was measured immediately before exercise, after each set, and immediately after completion of the sixth set. We chose to examine responses during 6 sets of 5 NHE repetitions as this encompasses most of the prescription volumes used in published training studies (16,24). The control of the average cadence during the descent phase of the NHE was chosen to match previous research (16) and to reduce intersubject variability in how the exercise is performed thus improving the ability to evaluate changes in results over time. This descent phase control was important to stop participants dropping to the ground from the start of the movement and ensured a comparable degree of active eccentric contraction that was performed in the early part of the descent. Thus, the average cadence is for the entire movement and does not reflect a consistent cadence for the entire descent. We used an uncontrolled ascent phase using the upper body to generate propulsion to replicate how the exercise is most commonly performed in clinical practice. Subjects
Ten healthy, injury-free amateur male soccer players (mean 6 SD; age = 22.7 6 3.9 years; height = 175.5 6 9.2 cm; mass = 69.5 6 7.9 kg) participated in this study. Although participants were familiar with the NHE and reported some history of performing the movement, no participant reported any regular (at least once per week) performance of the exercise as part of their usual training program. All participants were in the soccer off-season and reported performing between 3 and 7 training sessions per week consisting of cardiorespiratory exercise, stretching, and resistance exercise. The University’s Human Research Ethics Committee approved all procedures. The study was performed according to the principles of the Declaration of Helsinki. Written informed consent was received from all participants before enrollment in this study. Experimental Protocol
Each participant attended the laboratory in a 1-hour postprandial state on 2 separate occasions between 10 AM and 12 PM in the morning. Sessions were separated between 5 and 7 days. Participants were required to refrain from any strenuous or unaccustomed physical activity in the 24 hours VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
3125
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Nordic Hamstring Exercise
TABLE 1. Average angular velocities (in degrees per second) of the knee joint for the descent and ascent phases of the NHE during each set of 5 repetitions.*† Set 1 Descent phase Average 0–15 15–30 30–45 45–60 60–75 75–90 Ascent Phase Average 0–15 15–30 30–45 45–60 60–75 75–90
Set 2
Set 3
Set 4
Set 5
Set 6
29.3 13.4 38.5 75.1 118.7 141.2 43.2
6 6 6 6 6 6 6
4.6 4.2 22.9 54.9 71.9 62.3 25.3
27.5 14.7 38.4 67.9 109.7 131.3 47.3
6 6 6 6 6 6 6
4.9 7.2 32.8 56.1 77.6 71.1 26.9
29.7 14.1 36.7 73.4 119.9 143.6 46.1
6 6 6 6 6 6 6
2.4 4.6 18.9 40.2 62.2 69.5 26.7
29.4 14.1 40.1 74.9 119.9 142.6 46.4
6 6 6 6 6 6 6
3.0 6.0 33.2 53.8 63.9 60.1 21.9
29.3 14.6 37.8 71.4 118.6 139.9 47.7
6 6 6 6 6 6 6
3.7 5.4 25.9 45.2 65.8 64.7 26.1
33.2 14.5 36.8 67.8 113.1 130.4 46.9
6 6 6 6 6 6 6
3.8 5.6 31.4 46.5 61.1 60.6 23.6
62.9 21.5 105.1 153.2 178.2 154.7 48.6
6 6 6 6 6 6 6
5.6 6.2 46.3 44.5 37.5 35.5 18.2
60.9 19.9 104.5 145.1 166.4 157.7 51.4
6 6 6 6 6 6 6
8.2 7.7 55.6 54.6 47.8 43.9 25.4
67.8 25.7 109.3 158.6 180.4 158.9 45.0
6 6 6 6 6 6 6
8.5 11.7 46.2 43.2 32.8 47.1 17.0
60.6 19.9 105.4 149.5 172.8 156.0 48.9
6 6 6 6 6 6 6
10.4 12.2 53.3 46.3 36.1 41.2 21.3
60.4 19.5 105.2 147.9 172.0 157.0 50.7
6 6 6 6 6 6 6
9.0 1.04 54.0 51.1 34.9 34.9 17.9
58.8 18.91 101.8 144.5 167.1 154.4 50.1
6 6 6 6 6 6 6
11.8 11.8 66.6 56.5 38.6 31.8 16.9
*NHE, Nordic hamstring exercise. †Data presented are for the overall average velocity during each phase (within each set) and for the average velocity during each
158 movement epoch in the range of motion (where 08 represents the participant kneeling at the start of the NHE, 908 represents the bottom position of the NHE). There were no changes across sets of NHE for the average movement velocities.
before each session. The first visit was a familiarization session where participants were introduced to the NHE, electromyography preparation procedures, and performance of the maximal eccentric and concentric testing protocol. During this session, participants were familiarized with the specific cadence of the NHE. The descent phase of the exercise was performed to an average cadence of 308$s21 (16). Participants were required to return to the starting position as soon as possible using their upper body to initiate propulsion from
the ground. Descent cadence was controlled during all sessions using an audible metronome set to 1 Hz, so that contact with the ground was achieved on the third audible sound after the start of the movement. Familiarization continued until the participant could perform 1 set of 5 repetitions of NHE at the appropriate cadence. At the experimental session, participants performed the primary exercise protocol (6 sets of 5 repetitions of the NHE, 2-minute interset rest interval commenced after maximal torque measures with torque measures collected within a 30-second period). Before and immediately after each set of NHE maximal knee flexor, eccentric and concentric torque values were recorded using an isokinetic dynamometer (KinCom 125; Version 5.32, Chattanooga, TN, USA). Surface electromyograms (EMG) from the BF and MH were continuously recorded during all maximal torque measures and during all exercise repetitions. All torque and EMG recordings were made from the right Figure 1. Maximal eccentric and concentric torque output (in newton meters) measured pre-exercise and leg, as previous research indiafter every set of Nordic hamstring exercise performed. **p , 0.01 from pre-exercise. Data are expressed in mean and SD. cated no between-limb difference for recordings during the
3126
the
TM
Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
the
TM
Journal of Strength and Conditioning Research
| www.nsca.com
push their hands forward to break their descent as late as possible, ensuring minimal impact between the chest and floor occurred. Repetitions where a visible rebound from the chest occurred were identified and excluded from analyses. There was no delay between the descent and ascent phases. For the ascent phase of the exercise, participants were instructed to return to the initial position as soon as possible. Successive repetitions were commenced every 9 seconds (approximately 3-second interrepetition interval). Strong verbal encouragement was provided during all sets, and trunk position was continuously monitored by research assistants to ensure that a posture parallel to the thigh was maintained throughout all repetitions. Isokinetic Dynamometry
Testing was performed with the participant lying prone on the test bed, with the knee flexed to Figure 2. Average concentric and eccentric torque (in newton meters) across 158 epochs in the range of motion during the maximal voluntary contractions performed pre-exercise and after every set of Nordic hamstring exercise. 908. This position replicated the Significant reductions from pre-exercise (p # 0.05) and from set 1 for all grouped time points (p # 0.05) are relative trunk and lower limb anidentified within the figure. gles during the NHE, thus improving the validity of results. Straps were applied across the hips and torso to restrict movement. The axis of rotation of NHE (16). Knee joint kinematics were continuously recorded the dynamometer lever arm was aligned with the knee joint at 1,000 Hz (10 Hz low-pass filter) from a single axis electrocenter, and the lever arm was firmly strapped to the lower goniometer (ADI Instruments, Sydney, Australia) secured to leg approximately 2 cm superior to the lateral malleolus. the lateral aspect of the left knee joint (Table 1). The left knee Maximal eccentric and concentric knee flexor torque valwas used for kinematic tracking during the NHE to ensure ues were assessed at a movement velocity of 308$s21. that attachments to the goniometer did not interfere with the Range of motion was from the initial starting position to EMG site placements or the attachments of the dynamometer full extension of the knee. Pre-exercise, 3 maximal eccentric lever arm to the lower limb. and concentric contractions were performed, with Nordic Hamstring Exercise 3-second rest between each contraction. After each set, 1 Participants were required to kneel on the floor with the repetition of each contraction was performed with upper body vertical and straight. Pressure was applied to the 3-second rest between contractions. To commence moveheels by a research assistant to ensure that the feet remained ment and reduce acceleration during recorded testing, in contact with the floor throughout all exercise repetitions. movement of the dynamometer lever arm was designed The elbows were fully flexed and palms open. Participants to commence only after 20 N$m of knee flexor torque were required to lower themselves to the floor so that was generated. All torque values throughout the range of ground contact was made on the third audible sound from motion were corrected for the relative limb weight of the the metronome, thus a 3-second descent duration and participant (measured at rest with full leg extension) using average cadence of 308$s21. Participants were instructed to basic trigonometry. Dynamometer output was directly VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
3127
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Nordic Hamstring Exercise nals were filtered with a fourthorder Bessel filter between 20 and 500 Hz. Subsequently, the root-mean-square (RMS) was calculated for BF and MH using a 50-millisecond moving window (11). Data Processing
Data processing in this trial based on analysis within 158 movement epochs was based on established methods for analyzing dynamic EMG signals to improve the validity of analysis and interpretation of results (8,25). During all exercise repetitions and maximal torque trials, the average RMS (in millivolts) and torque (in newton meters) was calculated within 158 epochs throughout the range of motion (0–908 total range of motion) based on knee joint angle recordings, where 08 was defined as the starting position of both the NHE and maximal torque trials with the knee flexed to an absolute angle of 908 (thus the Figure 3. Average biceps femoris electromyogram amplitudes (in millivolts) across 158 epochs in the range of motion during the concentric and eccentric maximal voluntary contractions. *p # 0.05 from pre-exercise. epochs of relative movement from start position, 0–158, 15.01–308. 75.01–908). The sampled at 1,000 Hz, and a 10-Hz digital low-pass filter was start of each movement was visually confirmed from the first applied (PowerLab; ADI Instruments). derivative of the goniometer recording, indicating an increase in velocity from the starting position, which was Hamstrings Electromyograms maintained for 3 seconds before all exercise and torque trials. After careful skin preparation including removal of excess hair, The separation of the descent and ascent phases of the NHE abrasion with fine sandpaper, and cleaning the area with for analysis was based on the first derivative of the knee joint isopropyl alcohol swabs, pairs of Ag/AgCl electrodes (10 mm angle recording, with separation defined from the point contact diameter, 10 mm interelectrode distance, Maxsensor; where the first derivative crossed 08$s21 (indicating a change Medimax Global, Sydney, Australia) were applied parallel to in direction; positive angular velocity indicated descent, negthe direction of the muscle fibers on BF and MH. Placement ative ascent). Average velocity for each 158 epoch during the over BF and MH was in accordance with previous recomascent and descent phases of all NHE repetitions was remendations (26). The superior electrode was placed longitudicorded. For statistical analysis, all RMS and torque values nally 35% along a line from the ischial tuberosity to the lateral within each movement epoch during all NHE repetitions aspect of the popliteal cavity and 36% along a line from the were normalized to the respective values measured during ischial tuberosity to the medial side of the popliteal cavity for pre-exercise maximal torque trials (i.e., descent phase RMS BF and MH, respectively. A ground electrode (20 mm contact values were normalized to RMS values recorded during prediameter) was fixed to the right olecranon. To minimize moveexercise maximal eccentric torque, and ascent phase was ment artefacts, cables were held by a research assistant during normalized to concentric torque). all testing. Electromyographic signals were recorded using the Reliability ML138 BioAmp (common mode rejection ratio .85 dB at 50 Mean within-day within-subject coefficients of variation for Hz, input impedance 200 MV) with 16-bit analog-to-digital maximal concentric and eccentric torque were 7.2 6 2.6% conversion, sampled at 2,000 Hz (ADI instruments). Raw sig-
3128
the
TM
Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
the
TM
Journal of Strength and Conditioning Research
| www.nsca.com
normalized activity of the average value of the first 2 repetitions of the first set. In the event of a significant F ratio, post hoc comparisons were made using a Bonferroni correction. Unless otherwise stated, data are expressed as mean 6 SD. Two-tailed statistical significance was accepted at p # 0.05.
RESULTS Maximal and Average Torque Output Changes
No reductions were observed for maximal eccentric torque (Figure 1). A significant time effect was observed for reductions in maximal concentric torque (p , 0.001). Values were reduced from pre-exercise after the second set of NHE by 7.4% (Figure 1; p = 0.01), and no further reductions after the second set were observed. Average eccentric torque in each 158 epoch from 0 to 608 was reduced from pre-exercise Figure 4. Descent phase hamstring EMG activity recorded throughout the range of motion during the sets of values after the first set of NHE NHE. Average EMG values recorded during each part of the range of motion were normalized (in percent) to between 7.9 and 17.1% (Figure 2; values recorded in the same range of motion during the pre-exercise maximal eccentric contraction. *p # 0.05 for p , 0.001). Further reducall time points encompassed in the figure from values recorded in the first repetition of the first set of NHE. Data are expressed in mean and SD. EMG, electromyogram; NHE, Nordic hamstring exercise. tions from the first set were observed after the third set in the 0–158 and 15–308 epochs of 16.1 and 11.0%, respectively (p # 0.05) and after the fourth and 6.1 6 4.1%, respectively. Mean within-subject betweenset in the 30–458 epoch by 6.3% (p = 0.038). No changes day coefficients of variation for maximal concentric and were observed in the 60–758 epoch. In the 75–908 epoch, eccentric torque were 8.5 6 3.3% and 9.1 6 5.5%, respeca main effect of time was observed (p = 0.012). Torque was tively. Mean within-day, within-subject coefficients of variareduced from pre-exercise after the first set of NHE by 17.1% tion of maximal BF and MH values ranged from 10.1 to (p = 0.003), and no further reductions were observed. 16.0% for the concentric and eccentric contractions. Average concentric torque in each 158 epoch from 0 to 758 Within-subject between-day coefficients of variation for was reduced from pre-exercise values between 7.8 and 17.2% maximal BF and MH values ranged from 18.4 to 25.5%. after the first set of NHE (Figure 2; p , 0.01). In the 158 Statistical Analyses epochs from 0 to 608, average concentric torque was further All statistical analyses were performed using SPSS statistical reduced from the first set after the third set of NHE between software (IBM SPSS Statistics; version 22, Chicago, IL, USA). 6.6 and 8.2% (p # 0.05). In the 60–758 epoch, concentric Data were normally distributed, as assessed by inspection of torque was reduced from the first set after the sixth set by skewness and kurtosis values, and performing Kolmogorov9.1% (p = 0.012). In the 75–908 epoch, concentric torque was Smirnov normality tests. Repeated-measures analysis of varreduced from pre-exercise after 4 sets by 8.4% (p # 0.05). iance procedures with 7 levels of time were used to inspect Hamstrings Electromyogram During Maximal Contractions changes in maximal torque, average torque, and BF and MH Maximal BF and MH activity during the pre-exercise variables. For analysis of the EMG activity during the exercise concentric torque contraction were 0.49 6 0.16 and 0.50 6 trials, the normalized RMS values for last 2 repetitions of each 0.24 mV, respectively and during the eccentric contraction set were averaged to examine changes over time from the VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
3129
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Nordic Hamstring Exercise Hamstring Electromyogram During Nordic Hamstring Exercise
Descent Phase. Significant time effects were observed for increased BF activity across the 0–158 and 15–308 epochs (Figure 4; p , 0.001 and p = 0.004, respectively). In the 0–158 epoch, increases from the start of the first set were observed at the end of the third set by 49.4% (p = 0.008). No further changes were observed. In the 15–308 epoch, increases from the start of the first set were observed at the end of the fifth set by 26% (p = 0.018). No further changes were observed. No time effects were observed for BF activity in the movement epochs from 45 to 908. For MH (Figure 4), significant time effects were observed for increased activity in the 158 epochs from 0 to 458 (p , 0.01). Medial hamstring activity was increased from the start to the end of the first set in movement epochs 0–158, 15–308, and Figure 5. Ascent phase biceps femoris and medial hamstring EMG activity recorded throughout the range of 30–458 by 33.7, 21.7, and 24.8%, motion during the sets of NHE. Average EMG values recorded during each part of the range of motion were normalized (in percent) to values recorded in the same range of motion during the pre-exercise maximal concentric respectively (p # 0.05). No furcontraction. *p # 0.05 from values recorded in the first repetition of the first set of NHE. Data are expressed in ther increases were observed in mean and SD. EMG, electromyogram; NHE, Nordic hamstring exercise. the 0–158 and 15–308 epochs. In the 30–458 epoch, activity recorded at the end of the sixth were 0.42 6 0.12 and 0.45 6 0.23 mV for BF and MH, set was not different from pre-exercise (p = 0.11). No time respectively. No time effects were observed during the trial. effects were observed for MH activity in the movement epochs For average MH activity in all movement epochs during from 45 to 908. the maximal contractions, no time effects were observed. A Ascent Phase. For BF activity (Figure 5), significant time effects time effect was observed for average BF activity (Figure 3) were observed in the 158 epochs from 30 to 908 (p # 0.05). during the eccentric contractions in the 75–908 epoch (p = Biceps femoris activity in the 30–458, 45–608, and 75–908 0.011). Biceps femoris activity was reduced from preepochs was reduced from the start of the first set at the end exercise by 36% after the second set (p = 0.041) and reof the fifth set by 28.4, 29.6, and 42.1%, respectively (p # 0.05) mained reduced. and remained reduced. In the 60–758 epoch, BF activity was During the concentric contractions, main effects of time reduced from the start of the first set by 39.2% at the end of the were observed in each 158 movement epoch from 45 to 908 sixth set (p = 0.02). No changes were observed for MH. (p # 0.05). In the 45–608 epoch, BF activity was reduced from pre-exercise by 26.5% after the first set (p = 0.029) and DISCUSSION remained reduced till the end of the sixth set (p # 0.05). In The results of this study have provided unique and the 60–758 epoch, BF activity was reduced from pre-exercise important insights into understanding the acute fatigue by 16.6% after the second set (p = 0.042) and remained effect of performing NHE. The main findings of this study reduced. In the 75–908 epoch, BF activity was reduced from were (a) reductions in eccentric torque occurred after only 1 pre-exercise after the second, fourth, and sixth set by 17.0, set of NHE throughout the range of motion apart from the 22.4, and 20%, respectively (p # 0.05).
3130
the
TM
Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
the
TM
Journal of Strength and Conditioning Research 60–758 range where maximal torque was produced; (b) reduced eccentric torque in the end 158 range of motion occurred concomitant to decreased BF muscle activity; (c) during maximal contractions, only the BF muscle exhibited reduced activity across the sets; and (d) hamstring EMG activity during the sets of NHE exhibited range of motion specific changes that were increased during the descent phase and decreased during the ascent phase. The observed reductions in concentric and eccentric torque occurred throughout the range of motion after only 1 set of 5 NHE repetitions. This was a remarkable finding considering the NHE is a bodyweight only resistance exercise and highlights the demand that this exercise places on the hamstring muscles throughout the range of motion. As mentioned, the NHE is prescribed as part of the FIFA-11 injury prevention program (6,17) as 1 of 11 different exercises to be completed before training. The volume of NHE is recommended to start at 1 set of 3, 7, or 12 repetitions for beginners through advanced trainees, respectively (www.f-marc. com/11plus). Considering we have observed reductions in torque output after only the NHE, it may be likely that when combined with the remainder of the FIFA-11 exercises, acute fatigue responses are further exacerbated. In other research, the timing of the NHE around training was not clear, but the prescribed volumes throughout a 10-week program increased from 2 sets of 5 repetitions upward of 3 sets in the 8- to 12-repetition range (24). The results of this study demonstrate that greater volumes of NHE exacerbate the reductions in concentric and eccentric fatigue and increase the likelihood of observing reductions in central motor output to the BF during maximal contractions (Figure 3). We believe the most concerning observation in this study regarding the prescription of NHE before training sessions were the concomitant declines in eccentric torque and BF EMG amplitudes in the 158 range of motion before full knee extension during the maximal eccentric contraction. These reductions were manifested after only 1 set of 5 repetitions and may indicate a change that predisposes the BF to strain injury during a training session. It has been reported that extended hamstring muscle lengths near full knee extension is where injury occurs during the swing phase of sprinting (15,27). Imaging studies of acute hamstring strain injuries in normal and athletic populations report that greater than 80% of injuries are from the BF (19,20,31). The BF, particularly the long head of the muscle, undergoes the greatest lengthening of all the hamstring muscles during sprinting (30). Moreover, based on fine-wire EMG findings, the long head of BF is more active than all the hamstring muscles at extended knee joint angles (22). Combining these information suggests that the BF is susceptible to injury in the extended joint position where we observed reductions in eccentric torque and central motor output. The effect of performing NHE on subsequent fatigue responses during a soccer training session must be investigated to determine whether the overall fatigue profile, particularly at extended
| www.nsca.com
muscle lengths, is worsened by this exercise scheduling choice. Moreover, it will be important to explore muscle activation patterns during the sprinting to examine whether fatigue effects observed during isokinetic dynamometry, particularly at the slow contraction speed we used in this study, translate into impaired performance during high velocity dynamic exercise. During the first 30 and 458 of the NHE descent for BF and MH, respectively, muscle activity was increased. These increases were observed after 3 sets and at the end of the first set for BF and MH, respectively. These findings coincide with reductions in eccentric torque observed in the first half of the range of motion and suggest increased central motor output to the hamstrings to maintain the fixed task requirements. Whether the increased hamstring activity during the first half of the range of motion is suggestive of where training adaptation will occur is unclear. Training studies using the NHE have not delineated where in the eccentric range of motion strength changes occur (16,24). This is an important consideration for applying the NHE because the exercise may only provide a strength stimulus in certain parts of the range of motion. Also, the relevance of the earlier increase in activity for MH compared with BF is unclear. Although it is reasonable to speculate that the NHE differentially affects BF and MH when used as a training intervention over time, no study thus far has provided muscle-specific changes in neural adaptation or hypertrophy. In contrast to the increases observed during the descent phase of the NHE, BF activity during the ascent was progressively reduced from the first set in the first twothirds of the range of motion from ground contact. Increased upper-body propulsion owing to fatigue probably does not explain this finding, as ascent phase velocities did not change throughout the sets of exercise (Table 1). Moreover, there was an absence of any change in MH activity during the ascent phase. If greater upper-body propulsion (i.e., a bigger push off the floor) was the primary cause of reducing BF activity, it is reasonable to believe the same reductions should also be observed for MH. Thus, we believe the BF specific activity reductions are a fatigue mediated change driven by the nervous system. The purpose of this reduction is unclear because the task cadence was maintained throughout the sets of exercise despite the lower BF activity. However, this finding does suggest that NHE has specific effects within the hamstring muscle group that should be considered regarding training adaptation. It may be likely that aspects of neural or hypertrophic adaptation (including consideration for muscle architecture) may be muscle specific, and therefore the NHE may not be the most appropriate exercise for balanced hamstring training adaptations. An important strength of the methods in this study was the examination of concentric and eccentric torque during the maximal contractions in 158 increments throughout the range of motion. Previous research examining training adaptation using the NHE has only presented maximal torque and angle of peak torque data (16,17). VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
3131
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Nordic Hamstring Exercise The results of this study show a differential effect of the NHE throughout the range of motion, particularly during the eccentric phase. If we had solely relied on maximal eccentric torque, we would have reached the invalid conclusion that NHE had no effect on eccentric hamstring fatigue. Examination for how an NHE training program changes torque and central motor output to the hamstrings throughout different parts of the range of motion is required to provide insight and depth to ongoing discussion about whether this exercise is useful for prevention and rehabilitation of hamstring strain injuries (23). A primary limitation to this study was the use of surface electromyograms as the sole measure of central motor output. The EMG signal is potentially confounded by a number of factors, including modulation of output to and from the a-motoneuron, changes in sarcolemma action potential propagation, in addition to methodology constraints such as concurrent summation of negative and positive phases of action potentials thereby leading to signal cancellation (10,12,18). However, when carefully analyzed and compared directly with activity recorded within the same part of the range of motion, thus controlling for differences in the skin-muscle interaction introduced from dynamic movement (8,25), the EMG signal may be considered a gross estimate of central motor output to the hamstring muscles during repeated sets of NHE. Indeed, recent articles describing the regulation of central motor output during exercise to offset ongoing declines in peripheral muscle fatigue analyze the EMG signal during the dynamic movement (1–3,5). Future research should look to use interpolated twitch techniques during the different contractions, at different points in the range of motion, to provide greater insight into changes in central motor output to the hamstrings and the extent of peripheral muscle fatigue (14,21). Finally, we have only looked at the effect of NHE in isolation. It may be likely that fatigue responses are further exacerbated when the NHE is prescribed in the context of the entire FIFA-11 training program. Also, we did not look at recovery times after the performance of the NHE. Thus, although central motor output can be reduced for prolonged periods of time after fatiguing exercise (29), we are unsure whether a recovery period would offset the potentially deleterious changes we observed in this study. The lack of information pertaining to recovery times necessary to offset fatigue induced by NHE illustrates again the importance of understanding what happens during acute exercise sessions that are being prescribed to athletes in the real world.
PRACTICAL APPLICATIONS Although it may seem obvious to not perform strenuous strengthening exercises before training, a recent trend in soccer, particularly at the amateur and recreational levels, is for the use of the FIFA-11 intervention program before training. The FIFA-11 not only includes standard running and change of direction exercises as part of the warm-up but also includes strengthening movements such as the NHE.
3132
the
The NHE is a bodyweight only exercise that emphasizes the eccentric contraction of the hamstrings as a trainee is asked to lower their body from a kneeling position to the ground. Numerous videos are available on the Internet demonstrating appropriate exercise technique. Thus, the FIFA-11 is a pragmatic approach for amateur and recreational soccer players to accumulate a dose of strength exercise within a pre-training warm-up. However, there is limited information about how demanding the NHE is, and therefore, whether it is an appropriate exercise to use as part of a warm-up or as a stimulus for the hamstring. This was the first study to document the fatiguing effect of multiple sets of NHE. Nordic hamstring exercise elicited reductions in hamstring concentric and eccentric fatigue after only 1 set of 5 repetitions, which illustrates the high physical demand of this bodyweight only exercise. Reduced eccentric torque and central motor output to the BF muscle in the end range of motion is a particularly concerning finding regarding strain injury risk during soccer training sessions. Strain injury, particularly at extended hamstring muscle lengths, is associated with the fatigue that occurs in the latter stages of match play and training sessions. Thus, performing the NHE before training and inducing hamstring fatigue before the primary session does not seem the best scheduling choice. Coaches and practitioners should be cautioned about the high demands of the NHE when prescribing this exercise before training, particularly if the volume will be in excess of a single set of 5 repetitions. Based on the findings in this study, it would seem that 3 to 5 sets of 5 repetitions maximizes hamstring muscle activity during exercise, with further sets providing no further increases in muscle activity during exercise or the extent of muscular fatigue. It may be the best that NHE is prescribed after training or in a homebased training environment to reduce potential risks for inducing strain injury. Coaches should also be aware that during multiple sets of NHE, trainees do not appear to increase the amount of upper-body propulsion to aid the movement. This was based on the findings in this study in which the average movement velocities during the ascent phase (return to starting position) did not change throughout the 6 sets of 5 repetitions. Whether this exercise is a suitable replacement for conventional hamstring strengthening exercises is unclear. But, as a pragmatic solution for improving eccentric strength, the prevalence of NHE prescription is likely to increase. Thus, it is necessary for strength and conditioning coaches to be aware that significant fatigue can be induced by this relatively basic exercise after only 1 set of 5 repetitions. It must be noted that the results of this study only apply if strength and conditioning coaches control the descent phase of the exercise so that the average time it takes from the kneeling (start) to ground position is approximately 3 seconds. Controlling the descent phase of the exercise using a verbal count or metronome minimizes set-to-set
TM
Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
the
TM
Journal of Strength and Conditioning Research
| www.nsca.com
variability exercise performance and should reduce the between-trainee variability in how the exercise is performed during the descent phase.
15. Heiderscheit, BC, Hoerth, DM, Chumanov, ES, Swanson, WC, Thelen, BJ, and Thelen, DG. Identifying the time of occurrence of a hamstring strain injury during treadmill running: A case study. Clin Biomech 20: 1072–1078, 2005.
ACKNOWLEDGMENTS
16. Iga, J, Fruer, CS, Deighan, M, Croix, MDS, and James, DVB. Nordic hamstrings exercise—Engagement characteristics and training responses. Int J Sports Med 33: 1000–1004, 2012.
The authors have no conflicts of interest that are directly relevant to the contents of this manuscript. The authors would like to acknowledge the New South Wales Sporting Injuries Committee for project funding. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript (www.sportinginjuries.nsw.gov.au).
REFERENCES 1. Amann, M and Dempsey, JA. Locomotor muscle fatigue modifies central motor drive in healthy humans and imposes a limitation to exercise performance. J Physiol 586: 161–173, 2008. 2. Amann, M, Proctor, LT, Sebranek, JJ, Pegelow, DF, and Dempsey, JA. Opioid-mediated muscle afferents inhibit central motor drive and limit peripheral muscle fatigue development in humans. J Physiol 587: 271–283, 2009. 3. Amann, M, Romer, LM, Subudhi, AW, Pegelow, DF, and Dempsey, JA. Severity of arterial hypoxaemia affects the relative contributions of peripheral muscle fatigue to exercise performance in healthy humans. J Physiol 581: 389–403, 2007. 4. Arnason, A, Anderson, TE, Holme, I, Engebretsen, L, and Bahr, R. Prevention of hamstring strains in elite soccer: An intervention study. Scand J Med Sci Sports 18: 40–48, 2008. 5. Billaut, F, Kerris, JP, Rodriguez, RF, Martin, DT, Gore, CJ, and Bishop, DJ. Interaction of central and peripheral factors during repeated sprints at different levels of arterial O2 saturation. PLoS One 8: e77297, 2013. 6. Bizzini, M, Impellizzeri, FM, Dvorak, J, Bortolan, Schena, F, Modena, R, and Junge, A. Physiological and performance responses to the “FIFA 11+” (part 1): Is it an appropriate warm-up?. J Sports Sci 31: 1481–1490, 2013. 7. Brooks, JHM, Fuller, CW, Kemp, SPT, and Reddin, DB. Incidence, risk, and prevention of hamstring muscle injuries in professional rugby union. Am J Sports Med 34: 1297–1306, 2006. 8. De Luca, CJ. The use of surface electromyography in biomechanics. J Appl Biomech 13: 135–163, 1997. 9. Ditroilo, M, De Vito, G, and Delahunt, E. Kinematic and electromyographic analysis of the nordic hamstring exercise. J Electromyogr Kinesiol 23: 1111–1118, 2013. 10. Farina, D, Cescon, C, Negro, F, and Enoka, R. Amplitude cancellation of motor-unit action potentials in the surface electromyogram can be estimated with spike-triggered averaging. J Neurophysiol 100: 431–440, 2008. 11. Farina, D and Merletti, R. Comparison of algorithms for estimation of EMG variables during voluntary isometric contractions. J Electromyogr Kinesiol 10: 337–349, 2000. 12. Farina, D, Merletti, R, and Enoka, R. The extraction of neural strategies from the surface EMG. J Appl Physiol 96: 1486–1495, 2004. 13. Finn, HT, Brennan, SL, Gonano, BM, Knox, MF, Ryan, RC, Siegler, JC, and Marshall, PWM. Muscle activation does not increase after a fatigue plateau is reached during 8 sets of resistance exercise in trained individuals. J Strength Cond Res 28: 1226–1234, 2014. 14. Gandevia, SC. Spinal and supraspinal factors in human muscle fatigue. Physiol Rev 81: 1725–1789, 2001.
17. Impellizzeri, FM, Bizzini, M, Dvorak, J, Pellegrini, B, Schena, F, and Junge, A. Physiological and performance responses to the FIFA11+ (part 2): A randomised controlled trial on the training effects. J Sports Sci 31: 1491–1502, 2013. 18. Keenan, K, Farina, D, Merletti, R, and Enoka, R. Amplitude cancellation reduces the size of motor unit action potentials averaged from the surface EMG. J Appl Physiol 100: 1928–1937, 2006. 19. Koulouris, G and Connell, D. Evaluation of the hamstring muscle complex following acute injury. Skeletal Radiol 32: 582–589, 2003. 20. Koulouris, G, Connell, DA, Brukner, P, and Schneider-Kolsky, M. Magnetic resonance imaging parameters for assessing risk of recurrent hamstring injuries in elite athletes. Am J Sports Med 35: 1500–1506, 2007. 21. Merton, P. Voluntary strength and fatigue. J Physiol 123: 553–564, 1954. 22. Onishi, H, Yagi, R, Oyama, M, Akasaka, K, Ihashi, K, and Handa, Y. EMG-angle relationship of the hamstring muscles during maximum knee flexion. J Electromyogr Kinesiol 12: 399–406, 2002. 23. Opar, DA, Williams, MD, and Shield, AJ. Hamstring strain injuries. Sports Med 42: 209–226, 2012. 24. Petersen, J, Thorborg, K, Nielsen, MB, Budtz-Jørgensen, E, and Ho¨lmich, P. Preventive effect of eccentric training on acute hamstring injuries in men’s soccer: A cluster-randomized controlled trial. Am J Sports Med 39: 2296–2303, 2011. 25. Potvin, JR. Effects of muscle kinematics on surface EMG amplitude and frequency during fatiguing dynamic contractions. J Appl Physiol 82: 144–151, 1997. 26. Rainoldi, A, Melchiorri, G, and Caruso, I. A method for positioning electrodes during surface EMG recordings in lower limb muscles. J Neurosci Methods 134: 37–43, 2004. 27. Schache, AG, Wrigley, TV, Baker, R, and Pandy, MG. Biomechanical response to hamstring muscle strain injury. Gait Posture 29: 332–338, 2009. 28. Small, K, McNaughton, L, Greig, M, and Lovell, R. Effect of timing of eccentric hamstring strengthening exercises during soccer training: Implications for muscle fatigability. J Strength Cond Res 23: 1077–1083, 2009. 29. Taylor, JL and Gandevia, SC. A comparison of central aspects of fatigue in submaximal and maximal voluntary contractions. J Appl Phys 104: 542–550, 2008. 30. Thelen, DG, Chumanov, ES, Hoerth, DM, Best, TM, Swanson, SC, Li, L, Young, M, and Heiderscheit, BC. Hamstring muscle kinematics during treadmill sprinting. Med Sci Sports Exerc 37: 108– 114, 2005. 31. Verrall, GM, Slavotinek, JP, Barnes, PG, and Fon, GT. Diagnostic and prognostic value of clinical findings in 83 athletes with posterior thigh injury: Comparison of clinical findings with magnetic resonance imaging documentation of hamstring muscle strain. Am J Sports Med 31: 969–973, 2003. 32. Walker, S, Davis, L, Avela, J, and Hakkinen, K. Neuromuscular fatigue during dynamic maximal strength and hypertrophic resistance loadings. J Electromyogr Kinesiol 22: 356–362, 2012.
VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
3133
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
INTRODUCTION
Marshall, PWM, Lovell, R, Knox, MF, Brennan, SL, and Siegler, JC. Hamstring fatigue and muscle activation changes during six sets of Nordic hamstring exercise in amateur soccer players. J Strength Cond Res 29(11): 3124–3133, 2015—The Nordic hamstring exercise (NHE) is a bodyweight movement commonly prescribed to increase eccentric hamstring strength and reduce the incidence of strain injury in sport. This study examined hamstring fatigue and muscle activation responses throughout 6 sets of 5 repetitions of the NHE. Ten amateur-level soccer players performed a single session of 6 sets of 5 repetitions of NHE. Maximal eccentric and concentric torque output (in newton meters) was measured after every set. Hamstrings electromyograms (EMG) were measured during all maximal contractions and exercise repetitions. Hamstring maximal eccentric torque was reduced throughout the range of motion after only a single set of NHE between 7.9 and 17.1% (p # 0.05), with further reductions in subsequent sets. Similarly, maximal concentric torque reductions between 7.8 and 17.2% were observed throughout the range of motion after 1 set of NHE (p # 0.05). During the descent phase of the NHE repetitions, hamstring muscle activity progressively increased as the number of sets performed increased. These increases were observed in the first half of the range of motion. During the ascent phase, biceps femoris muscle activity but not medial hamstrings was reduced from the start of exercise during latter sets of repetitions. These data provide unique insight into the extent of fatigue induced from a bodyweight only exercise after a single set of 5 repetitions. Strength and conditioning coaches need to be aware of the speed and extent of fatigue induced from NHE, particularly in practical settings in which this exercise is now prescribed before sportspecific training sessions (i.e., the FIFA-11 before soccer training).
KEY WORDS electromyography, FIFA-11, eccentric torque, concentric torque, biceps femoris, medial hamstrings
Address correspondence to Paul W.M. Marshall, [email protected]. 29(11)/3124–3133 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association
3124
the
T
he Nordic hamstring exercise (NHE; aka Nordic hamstring curl) has been used in many recreational (e.g., powerlifting and bodybuilding) and sports training environments (e.g., soccer, rugby union, Australian rules football) as a method of increasing eccentric hamstring muscle strength (16,28) and thus reducing the incidence of hamstring strain injuries (4,7,24). Briefly, the NHE is a bodyweight only exercise, which requires the individual to lower their upper body toward the ground from a kneeling position, with a partner or device holding the ankles down (the descent phase). The individual then uses the upper body to provide propulsion for returning to the start position (the ascent phase). The prescription of NHE is becoming more common as it only uses bodyweight as resistance and does not require specialized equipment. For example, the NHE is prescribed within the Fe´de´ration Internationale de Football Association (FIFA)-11 intervention program as part of a standard field warm-up for soccer players at the amateur and recreational levels (6,17). It is important to provide information about the use of the NHE to better inform the ability of strength and conditioning coaches to accurately prescribe this exercise. Currently, there is no research examining hamstring muscle fatigue, or changes in muscle activation during the descent and ascent phases of the exercise, when performing multiple sets of NHE. The extent of hamstring fatigue induced by NHE is of interest considering recommendations for the use of NHE before training sessions in the FIFA-11 intervention program (6,17). Examining hamstring muscle activation changes during multiple sets of NHE may provide insight into the optimal dose of exercise to prescribe to maximize recruitment of the hamstring muscles. To our knowledge, there are 2 studies that have described acute responses of the hamstring muscles to the NHE (9,16). Neither study examined hamstring fatigue using measures of concentric or eccentric torque, muscle activation changes that may have occurred within and between sets of repetitions, or the ascent phase of the exercise (9,16). Iga et al. reported increased hamstring muscle activity, measured by surface electromyography (EMG), in the last 608 of the descent phase (controlled movement cadence 308$s21)
TM
Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
the
TM
Journal of Strength and Conditioning Research compared with the first 308 during 1 set of 5 NHE repetitions (16). Ditroilo et al. (9) examined EMG and movement kinematics during 2 sets of 5 NHE repetitions and reported a wide range of movement velocities, peak accelerations, and activation patterns. The high intersubject variability was probably explained by the lack of cadence control during the exercise, which makes comparison of the results between the 2 identified studies difficult (9,16). The higher hamstring EMG activity through the latter twothirds of the NHE descent (16) suggests increased central motor output to the hamstrings. It is reasonable to speculate that eccentric muscle fatigue is more likely to occur in the latter two-thirds of the range of motion where activity is the highest. However, it is unclear how muscle activation during the NHE will change in the presence of eccentric fatigue. In conventional resistance exercise models, muscle activation seems to increase during multiple sets concomitant to fatigue in untrained (32), but not resistance trained individuals (13). Soccer players, particularly at the amateur level where the FIFA-11 is targeted, could probably be best described as not being resistance trained. Thus, it is reasonable to speculate that muscle activation of the hamstrings, particularly in the latter two-thirds of the descent phase, will increase concomitant to eccentric fatigue. The absence of information describing the ascent phase of the NHE makes it difficult to speculate about the extent of concentric fatigue and hamstring muscle activation during this part of the NHE. Therefore, we designed this study to address critical gaps in the understanding of the NHE to better inform prescription of this exercise in clinical practice. The objectives of this study were (a) to examine changes in eccentric and concentric hamstring muscle torque output during 6 sets of 5 repetitions of the NHE and (b) to measure EMG from the biceps femoris (BF) and medial hamstring (MH) muscles throughout all exercise repetitions and maximal torque trials. We hypothesized that the greater EMG amplitudes previously reported during the latter two-thirds of the descent phase (16) would contribute to a loss of eccentric torque measured in the same part of the range of motion but increased muscle activation during the repetitions of NHE. Although a clear hypothesis regarding the ascent phase and concentric fatigue could not be reached owing to an absence of available information, we also hypothesized that any observed reductions in concentric torque output would be associated with increased hamstring muscle activation during the ascent phase of the NHE.
METHODS Experimental Approach to the Problem
This was a cross-sectional study of the acute fatigue and muscle activation responses to a session of NHE. The test protocol consisted of 6 sets of 5 repetitions of the NHE using a controlled average cadence of 308$s21 for the descent phase and a self-paced ascent (participants asked to push off the floor and return to starting position as soon as possible).
| www.nsca.com
Two-minute interset rest intervals were provided. Primary dependent variables measured throughout the exercise session were maximal eccentric and concentric torque output and the activation of the BF and MH from continuous EMG recordings. Postprocessing, all torque and EMG variables (measured during maximal contractions and the exercise repetitions) were expressed and compared within each 158 movement epoch based on knee joint range of motion. Within the context of this study, the independent variables were the number of sets performed, the eccentric and concentric maximal contractions, and the descent and ascent phases of the exercise repetitions. Participants were familiarized with the NHE and all testing procedures at a session performed 5–7 days before the testing session. Maximal eccentric and concentric hamstring torque output was measured immediately before exercise, after each set, and immediately after completion of the sixth set. We chose to examine responses during 6 sets of 5 NHE repetitions as this encompasses most of the prescription volumes used in published training studies (16,24). The control of the average cadence during the descent phase of the NHE was chosen to match previous research (16) and to reduce intersubject variability in how the exercise is performed thus improving the ability to evaluate changes in results over time. This descent phase control was important to stop participants dropping to the ground from the start of the movement and ensured a comparable degree of active eccentric contraction that was performed in the early part of the descent. Thus, the average cadence is for the entire movement and does not reflect a consistent cadence for the entire descent. We used an uncontrolled ascent phase using the upper body to generate propulsion to replicate how the exercise is most commonly performed in clinical practice. Subjects
Ten healthy, injury-free amateur male soccer players (mean 6 SD; age = 22.7 6 3.9 years; height = 175.5 6 9.2 cm; mass = 69.5 6 7.9 kg) participated in this study. Although participants were familiar with the NHE and reported some history of performing the movement, no participant reported any regular (at least once per week) performance of the exercise as part of their usual training program. All participants were in the soccer off-season and reported performing between 3 and 7 training sessions per week consisting of cardiorespiratory exercise, stretching, and resistance exercise. The University’s Human Research Ethics Committee approved all procedures. The study was performed according to the principles of the Declaration of Helsinki. Written informed consent was received from all participants before enrollment in this study. Experimental Protocol
Each participant attended the laboratory in a 1-hour postprandial state on 2 separate occasions between 10 AM and 12 PM in the morning. Sessions were separated between 5 and 7 days. Participants were required to refrain from any strenuous or unaccustomed physical activity in the 24 hours VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
3125
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Nordic Hamstring Exercise
TABLE 1. Average angular velocities (in degrees per second) of the knee joint for the descent and ascent phases of the NHE during each set of 5 repetitions.*† Set 1 Descent phase Average 0–15 15–30 30–45 45–60 60–75 75–90 Ascent Phase Average 0–15 15–30 30–45 45–60 60–75 75–90
Set 2
Set 3
Set 4
Set 5
Set 6
29.3 13.4 38.5 75.1 118.7 141.2 43.2
6 6 6 6 6 6 6
4.6 4.2 22.9 54.9 71.9 62.3 25.3
27.5 14.7 38.4 67.9 109.7 131.3 47.3
6 6 6 6 6 6 6
4.9 7.2 32.8 56.1 77.6 71.1 26.9
29.7 14.1 36.7 73.4 119.9 143.6 46.1
6 6 6 6 6 6 6
2.4 4.6 18.9 40.2 62.2 69.5 26.7
29.4 14.1 40.1 74.9 119.9 142.6 46.4
6 6 6 6 6 6 6
3.0 6.0 33.2 53.8 63.9 60.1 21.9
29.3 14.6 37.8 71.4 118.6 139.9 47.7
6 6 6 6 6 6 6
3.7 5.4 25.9 45.2 65.8 64.7 26.1
33.2 14.5 36.8 67.8 113.1 130.4 46.9
6 6 6 6 6 6 6
3.8 5.6 31.4 46.5 61.1 60.6 23.6
62.9 21.5 105.1 153.2 178.2 154.7 48.6
6 6 6 6 6 6 6
5.6 6.2 46.3 44.5 37.5 35.5 18.2
60.9 19.9 104.5 145.1 166.4 157.7 51.4
6 6 6 6 6 6 6
8.2 7.7 55.6 54.6 47.8 43.9 25.4
67.8 25.7 109.3 158.6 180.4 158.9 45.0
6 6 6 6 6 6 6
8.5 11.7 46.2 43.2 32.8 47.1 17.0
60.6 19.9 105.4 149.5 172.8 156.0 48.9
6 6 6 6 6 6 6
10.4 12.2 53.3 46.3 36.1 41.2 21.3
60.4 19.5 105.2 147.9 172.0 157.0 50.7
6 6 6 6 6 6 6
9.0 1.04 54.0 51.1 34.9 34.9 17.9
58.8 18.91 101.8 144.5 167.1 154.4 50.1
6 6 6 6 6 6 6
11.8 11.8 66.6 56.5 38.6 31.8 16.9
*NHE, Nordic hamstring exercise. †Data presented are for the overall average velocity during each phase (within each set) and for the average velocity during each
158 movement epoch in the range of motion (where 08 represents the participant kneeling at the start of the NHE, 908 represents the bottom position of the NHE). There were no changes across sets of NHE for the average movement velocities.
before each session. The first visit was a familiarization session where participants were introduced to the NHE, electromyography preparation procedures, and performance of the maximal eccentric and concentric testing protocol. During this session, participants were familiarized with the specific cadence of the NHE. The descent phase of the exercise was performed to an average cadence of 308$s21 (16). Participants were required to return to the starting position as soon as possible using their upper body to initiate propulsion from
the ground. Descent cadence was controlled during all sessions using an audible metronome set to 1 Hz, so that contact with the ground was achieved on the third audible sound after the start of the movement. Familiarization continued until the participant could perform 1 set of 5 repetitions of NHE at the appropriate cadence. At the experimental session, participants performed the primary exercise protocol (6 sets of 5 repetitions of the NHE, 2-minute interset rest interval commenced after maximal torque measures with torque measures collected within a 30-second period). Before and immediately after each set of NHE maximal knee flexor, eccentric and concentric torque values were recorded using an isokinetic dynamometer (KinCom 125; Version 5.32, Chattanooga, TN, USA). Surface electromyograms (EMG) from the BF and MH were continuously recorded during all maximal torque measures and during all exercise repetitions. All torque and EMG recordings were made from the right Figure 1. Maximal eccentric and concentric torque output (in newton meters) measured pre-exercise and leg, as previous research indiafter every set of Nordic hamstring exercise performed. **p , 0.01 from pre-exercise. Data are expressed in mean and SD. cated no between-limb difference for recordings during the
3126
the
TM
Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
the
TM
Journal of Strength and Conditioning Research
| www.nsca.com
push their hands forward to break their descent as late as possible, ensuring minimal impact between the chest and floor occurred. Repetitions where a visible rebound from the chest occurred were identified and excluded from analyses. There was no delay between the descent and ascent phases. For the ascent phase of the exercise, participants were instructed to return to the initial position as soon as possible. Successive repetitions were commenced every 9 seconds (approximately 3-second interrepetition interval). Strong verbal encouragement was provided during all sets, and trunk position was continuously monitored by research assistants to ensure that a posture parallel to the thigh was maintained throughout all repetitions. Isokinetic Dynamometry
Testing was performed with the participant lying prone on the test bed, with the knee flexed to Figure 2. Average concentric and eccentric torque (in newton meters) across 158 epochs in the range of motion during the maximal voluntary contractions performed pre-exercise and after every set of Nordic hamstring exercise. 908. This position replicated the Significant reductions from pre-exercise (p # 0.05) and from set 1 for all grouped time points (p # 0.05) are relative trunk and lower limb anidentified within the figure. gles during the NHE, thus improving the validity of results. Straps were applied across the hips and torso to restrict movement. The axis of rotation of NHE (16). Knee joint kinematics were continuously recorded the dynamometer lever arm was aligned with the knee joint at 1,000 Hz (10 Hz low-pass filter) from a single axis electrocenter, and the lever arm was firmly strapped to the lower goniometer (ADI Instruments, Sydney, Australia) secured to leg approximately 2 cm superior to the lateral malleolus. the lateral aspect of the left knee joint (Table 1). The left knee Maximal eccentric and concentric knee flexor torque valwas used for kinematic tracking during the NHE to ensure ues were assessed at a movement velocity of 308$s21. that attachments to the goniometer did not interfere with the Range of motion was from the initial starting position to EMG site placements or the attachments of the dynamometer full extension of the knee. Pre-exercise, 3 maximal eccentric lever arm to the lower limb. and concentric contractions were performed, with Nordic Hamstring Exercise 3-second rest between each contraction. After each set, 1 Participants were required to kneel on the floor with the repetition of each contraction was performed with upper body vertical and straight. Pressure was applied to the 3-second rest between contractions. To commence moveheels by a research assistant to ensure that the feet remained ment and reduce acceleration during recorded testing, in contact with the floor throughout all exercise repetitions. movement of the dynamometer lever arm was designed The elbows were fully flexed and palms open. Participants to commence only after 20 N$m of knee flexor torque were required to lower themselves to the floor so that was generated. All torque values throughout the range of ground contact was made on the third audible sound from motion were corrected for the relative limb weight of the the metronome, thus a 3-second descent duration and participant (measured at rest with full leg extension) using average cadence of 308$s21. Participants were instructed to basic trigonometry. Dynamometer output was directly VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
3127
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Nordic Hamstring Exercise nals were filtered with a fourthorder Bessel filter between 20 and 500 Hz. Subsequently, the root-mean-square (RMS) was calculated for BF and MH using a 50-millisecond moving window (11). Data Processing
Data processing in this trial based on analysis within 158 movement epochs was based on established methods for analyzing dynamic EMG signals to improve the validity of analysis and interpretation of results (8,25). During all exercise repetitions and maximal torque trials, the average RMS (in millivolts) and torque (in newton meters) was calculated within 158 epochs throughout the range of motion (0–908 total range of motion) based on knee joint angle recordings, where 08 was defined as the starting position of both the NHE and maximal torque trials with the knee flexed to an absolute angle of 908 (thus the Figure 3. Average biceps femoris electromyogram amplitudes (in millivolts) across 158 epochs in the range of motion during the concentric and eccentric maximal voluntary contractions. *p # 0.05 from pre-exercise. epochs of relative movement from start position, 0–158, 15.01–308. 75.01–908). The sampled at 1,000 Hz, and a 10-Hz digital low-pass filter was start of each movement was visually confirmed from the first applied (PowerLab; ADI Instruments). derivative of the goniometer recording, indicating an increase in velocity from the starting position, which was Hamstrings Electromyograms maintained for 3 seconds before all exercise and torque trials. After careful skin preparation including removal of excess hair, The separation of the descent and ascent phases of the NHE abrasion with fine sandpaper, and cleaning the area with for analysis was based on the first derivative of the knee joint isopropyl alcohol swabs, pairs of Ag/AgCl electrodes (10 mm angle recording, with separation defined from the point contact diameter, 10 mm interelectrode distance, Maxsensor; where the first derivative crossed 08$s21 (indicating a change Medimax Global, Sydney, Australia) were applied parallel to in direction; positive angular velocity indicated descent, negthe direction of the muscle fibers on BF and MH. Placement ative ascent). Average velocity for each 158 epoch during the over BF and MH was in accordance with previous recomascent and descent phases of all NHE repetitions was remendations (26). The superior electrode was placed longitudicorded. For statistical analysis, all RMS and torque values nally 35% along a line from the ischial tuberosity to the lateral within each movement epoch during all NHE repetitions aspect of the popliteal cavity and 36% along a line from the were normalized to the respective values measured during ischial tuberosity to the medial side of the popliteal cavity for pre-exercise maximal torque trials (i.e., descent phase RMS BF and MH, respectively. A ground electrode (20 mm contact values were normalized to RMS values recorded during prediameter) was fixed to the right olecranon. To minimize moveexercise maximal eccentric torque, and ascent phase was ment artefacts, cables were held by a research assistant during normalized to concentric torque). all testing. Electromyographic signals were recorded using the Reliability ML138 BioAmp (common mode rejection ratio .85 dB at 50 Mean within-day within-subject coefficients of variation for Hz, input impedance 200 MV) with 16-bit analog-to-digital maximal concentric and eccentric torque were 7.2 6 2.6% conversion, sampled at 2,000 Hz (ADI instruments). Raw sig-
3128
the
TM
Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
the
TM
Journal of Strength and Conditioning Research
| www.nsca.com
normalized activity of the average value of the first 2 repetitions of the first set. In the event of a significant F ratio, post hoc comparisons were made using a Bonferroni correction. Unless otherwise stated, data are expressed as mean 6 SD. Two-tailed statistical significance was accepted at p # 0.05.
RESULTS Maximal and Average Torque Output Changes
No reductions were observed for maximal eccentric torque (Figure 1). A significant time effect was observed for reductions in maximal concentric torque (p , 0.001). Values were reduced from pre-exercise after the second set of NHE by 7.4% (Figure 1; p = 0.01), and no further reductions after the second set were observed. Average eccentric torque in each 158 epoch from 0 to 608 was reduced from pre-exercise Figure 4. Descent phase hamstring EMG activity recorded throughout the range of motion during the sets of values after the first set of NHE NHE. Average EMG values recorded during each part of the range of motion were normalized (in percent) to between 7.9 and 17.1% (Figure 2; values recorded in the same range of motion during the pre-exercise maximal eccentric contraction. *p # 0.05 for p , 0.001). Further reducall time points encompassed in the figure from values recorded in the first repetition of the first set of NHE. Data are expressed in mean and SD. EMG, electromyogram; NHE, Nordic hamstring exercise. tions from the first set were observed after the third set in the 0–158 and 15–308 epochs of 16.1 and 11.0%, respectively (p # 0.05) and after the fourth and 6.1 6 4.1%, respectively. Mean within-subject betweenset in the 30–458 epoch by 6.3% (p = 0.038). No changes day coefficients of variation for maximal concentric and were observed in the 60–758 epoch. In the 75–908 epoch, eccentric torque were 8.5 6 3.3% and 9.1 6 5.5%, respeca main effect of time was observed (p = 0.012). Torque was tively. Mean within-day, within-subject coefficients of variareduced from pre-exercise after the first set of NHE by 17.1% tion of maximal BF and MH values ranged from 10.1 to (p = 0.003), and no further reductions were observed. 16.0% for the concentric and eccentric contractions. Average concentric torque in each 158 epoch from 0 to 758 Within-subject between-day coefficients of variation for was reduced from pre-exercise values between 7.8 and 17.2% maximal BF and MH values ranged from 18.4 to 25.5%. after the first set of NHE (Figure 2; p , 0.01). In the 158 Statistical Analyses epochs from 0 to 608, average concentric torque was further All statistical analyses were performed using SPSS statistical reduced from the first set after the third set of NHE between software (IBM SPSS Statistics; version 22, Chicago, IL, USA). 6.6 and 8.2% (p # 0.05). In the 60–758 epoch, concentric Data were normally distributed, as assessed by inspection of torque was reduced from the first set after the sixth set by skewness and kurtosis values, and performing Kolmogorov9.1% (p = 0.012). In the 75–908 epoch, concentric torque was Smirnov normality tests. Repeated-measures analysis of varreduced from pre-exercise after 4 sets by 8.4% (p # 0.05). iance procedures with 7 levels of time were used to inspect Hamstrings Electromyogram During Maximal Contractions changes in maximal torque, average torque, and BF and MH Maximal BF and MH activity during the pre-exercise variables. For analysis of the EMG activity during the exercise concentric torque contraction were 0.49 6 0.16 and 0.50 6 trials, the normalized RMS values for last 2 repetitions of each 0.24 mV, respectively and during the eccentric contraction set were averaged to examine changes over time from the VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
3129
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Nordic Hamstring Exercise Hamstring Electromyogram During Nordic Hamstring Exercise
Descent Phase. Significant time effects were observed for increased BF activity across the 0–158 and 15–308 epochs (Figure 4; p , 0.001 and p = 0.004, respectively). In the 0–158 epoch, increases from the start of the first set were observed at the end of the third set by 49.4% (p = 0.008). No further changes were observed. In the 15–308 epoch, increases from the start of the first set were observed at the end of the fifth set by 26% (p = 0.018). No further changes were observed. No time effects were observed for BF activity in the movement epochs from 45 to 908. For MH (Figure 4), significant time effects were observed for increased activity in the 158 epochs from 0 to 458 (p , 0.01). Medial hamstring activity was increased from the start to the end of the first set in movement epochs 0–158, 15–308, and Figure 5. Ascent phase biceps femoris and medial hamstring EMG activity recorded throughout the range of 30–458 by 33.7, 21.7, and 24.8%, motion during the sets of NHE. Average EMG values recorded during each part of the range of motion were normalized (in percent) to values recorded in the same range of motion during the pre-exercise maximal concentric respectively (p # 0.05). No furcontraction. *p # 0.05 from values recorded in the first repetition of the first set of NHE. Data are expressed in ther increases were observed in mean and SD. EMG, electromyogram; NHE, Nordic hamstring exercise. the 0–158 and 15–308 epochs. In the 30–458 epoch, activity recorded at the end of the sixth were 0.42 6 0.12 and 0.45 6 0.23 mV for BF and MH, set was not different from pre-exercise (p = 0.11). No time respectively. No time effects were observed during the trial. effects were observed for MH activity in the movement epochs For average MH activity in all movement epochs during from 45 to 908. the maximal contractions, no time effects were observed. A Ascent Phase. For BF activity (Figure 5), significant time effects time effect was observed for average BF activity (Figure 3) were observed in the 158 epochs from 30 to 908 (p # 0.05). during the eccentric contractions in the 75–908 epoch (p = Biceps femoris activity in the 30–458, 45–608, and 75–908 0.011). Biceps femoris activity was reduced from preepochs was reduced from the start of the first set at the end exercise by 36% after the second set (p = 0.041) and reof the fifth set by 28.4, 29.6, and 42.1%, respectively (p # 0.05) mained reduced. and remained reduced. In the 60–758 epoch, BF activity was During the concentric contractions, main effects of time reduced from the start of the first set by 39.2% at the end of the were observed in each 158 movement epoch from 45 to 908 sixth set (p = 0.02). No changes were observed for MH. (p # 0.05). In the 45–608 epoch, BF activity was reduced from pre-exercise by 26.5% after the first set (p = 0.029) and DISCUSSION remained reduced till the end of the sixth set (p # 0.05). In The results of this study have provided unique and the 60–758 epoch, BF activity was reduced from pre-exercise important insights into understanding the acute fatigue by 16.6% after the second set (p = 0.042) and remained effect of performing NHE. The main findings of this study reduced. In the 75–908 epoch, BF activity was reduced from were (a) reductions in eccentric torque occurred after only 1 pre-exercise after the second, fourth, and sixth set by 17.0, set of NHE throughout the range of motion apart from the 22.4, and 20%, respectively (p # 0.05).
3130
the
TM
Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
the
TM
Journal of Strength and Conditioning Research 60–758 range where maximal torque was produced; (b) reduced eccentric torque in the end 158 range of motion occurred concomitant to decreased BF muscle activity; (c) during maximal contractions, only the BF muscle exhibited reduced activity across the sets; and (d) hamstring EMG activity during the sets of NHE exhibited range of motion specific changes that were increased during the descent phase and decreased during the ascent phase. The observed reductions in concentric and eccentric torque occurred throughout the range of motion after only 1 set of 5 NHE repetitions. This was a remarkable finding considering the NHE is a bodyweight only resistance exercise and highlights the demand that this exercise places on the hamstring muscles throughout the range of motion. As mentioned, the NHE is prescribed as part of the FIFA-11 injury prevention program (6,17) as 1 of 11 different exercises to be completed before training. The volume of NHE is recommended to start at 1 set of 3, 7, or 12 repetitions for beginners through advanced trainees, respectively (www.f-marc. com/11plus). Considering we have observed reductions in torque output after only the NHE, it may be likely that when combined with the remainder of the FIFA-11 exercises, acute fatigue responses are further exacerbated. In other research, the timing of the NHE around training was not clear, but the prescribed volumes throughout a 10-week program increased from 2 sets of 5 repetitions upward of 3 sets in the 8- to 12-repetition range (24). The results of this study demonstrate that greater volumes of NHE exacerbate the reductions in concentric and eccentric fatigue and increase the likelihood of observing reductions in central motor output to the BF during maximal contractions (Figure 3). We believe the most concerning observation in this study regarding the prescription of NHE before training sessions were the concomitant declines in eccentric torque and BF EMG amplitudes in the 158 range of motion before full knee extension during the maximal eccentric contraction. These reductions were manifested after only 1 set of 5 repetitions and may indicate a change that predisposes the BF to strain injury during a training session. It has been reported that extended hamstring muscle lengths near full knee extension is where injury occurs during the swing phase of sprinting (15,27). Imaging studies of acute hamstring strain injuries in normal and athletic populations report that greater than 80% of injuries are from the BF (19,20,31). The BF, particularly the long head of the muscle, undergoes the greatest lengthening of all the hamstring muscles during sprinting (30). Moreover, based on fine-wire EMG findings, the long head of BF is more active than all the hamstring muscles at extended knee joint angles (22). Combining these information suggests that the BF is susceptible to injury in the extended joint position where we observed reductions in eccentric torque and central motor output. The effect of performing NHE on subsequent fatigue responses during a soccer training session must be investigated to determine whether the overall fatigue profile, particularly at extended
| www.nsca.com
muscle lengths, is worsened by this exercise scheduling choice. Moreover, it will be important to explore muscle activation patterns during the sprinting to examine whether fatigue effects observed during isokinetic dynamometry, particularly at the slow contraction speed we used in this study, translate into impaired performance during high velocity dynamic exercise. During the first 30 and 458 of the NHE descent for BF and MH, respectively, muscle activity was increased. These increases were observed after 3 sets and at the end of the first set for BF and MH, respectively. These findings coincide with reductions in eccentric torque observed in the first half of the range of motion and suggest increased central motor output to the hamstrings to maintain the fixed task requirements. Whether the increased hamstring activity during the first half of the range of motion is suggestive of where training adaptation will occur is unclear. Training studies using the NHE have not delineated where in the eccentric range of motion strength changes occur (16,24). This is an important consideration for applying the NHE because the exercise may only provide a strength stimulus in certain parts of the range of motion. Also, the relevance of the earlier increase in activity for MH compared with BF is unclear. Although it is reasonable to speculate that the NHE differentially affects BF and MH when used as a training intervention over time, no study thus far has provided muscle-specific changes in neural adaptation or hypertrophy. In contrast to the increases observed during the descent phase of the NHE, BF activity during the ascent was progressively reduced from the first set in the first twothirds of the range of motion from ground contact. Increased upper-body propulsion owing to fatigue probably does not explain this finding, as ascent phase velocities did not change throughout the sets of exercise (Table 1). Moreover, there was an absence of any change in MH activity during the ascent phase. If greater upper-body propulsion (i.e., a bigger push off the floor) was the primary cause of reducing BF activity, it is reasonable to believe the same reductions should also be observed for MH. Thus, we believe the BF specific activity reductions are a fatigue mediated change driven by the nervous system. The purpose of this reduction is unclear because the task cadence was maintained throughout the sets of exercise despite the lower BF activity. However, this finding does suggest that NHE has specific effects within the hamstring muscle group that should be considered regarding training adaptation. It may be likely that aspects of neural or hypertrophic adaptation (including consideration for muscle architecture) may be muscle specific, and therefore the NHE may not be the most appropriate exercise for balanced hamstring training adaptations. An important strength of the methods in this study was the examination of concentric and eccentric torque during the maximal contractions in 158 increments throughout the range of motion. Previous research examining training adaptation using the NHE has only presented maximal torque and angle of peak torque data (16,17). VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
3131
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Nordic Hamstring Exercise The results of this study show a differential effect of the NHE throughout the range of motion, particularly during the eccentric phase. If we had solely relied on maximal eccentric torque, we would have reached the invalid conclusion that NHE had no effect on eccentric hamstring fatigue. Examination for how an NHE training program changes torque and central motor output to the hamstrings throughout different parts of the range of motion is required to provide insight and depth to ongoing discussion about whether this exercise is useful for prevention and rehabilitation of hamstring strain injuries (23). A primary limitation to this study was the use of surface electromyograms as the sole measure of central motor output. The EMG signal is potentially confounded by a number of factors, including modulation of output to and from the a-motoneuron, changes in sarcolemma action potential propagation, in addition to methodology constraints such as concurrent summation of negative and positive phases of action potentials thereby leading to signal cancellation (10,12,18). However, when carefully analyzed and compared directly with activity recorded within the same part of the range of motion, thus controlling for differences in the skin-muscle interaction introduced from dynamic movement (8,25), the EMG signal may be considered a gross estimate of central motor output to the hamstring muscles during repeated sets of NHE. Indeed, recent articles describing the regulation of central motor output during exercise to offset ongoing declines in peripheral muscle fatigue analyze the EMG signal during the dynamic movement (1–3,5). Future research should look to use interpolated twitch techniques during the different contractions, at different points in the range of motion, to provide greater insight into changes in central motor output to the hamstrings and the extent of peripheral muscle fatigue (14,21). Finally, we have only looked at the effect of NHE in isolation. It may be likely that fatigue responses are further exacerbated when the NHE is prescribed in the context of the entire FIFA-11 training program. Also, we did not look at recovery times after the performance of the NHE. Thus, although central motor output can be reduced for prolonged periods of time after fatiguing exercise (29), we are unsure whether a recovery period would offset the potentially deleterious changes we observed in this study. The lack of information pertaining to recovery times necessary to offset fatigue induced by NHE illustrates again the importance of understanding what happens during acute exercise sessions that are being prescribed to athletes in the real world.
PRACTICAL APPLICATIONS Although it may seem obvious to not perform strenuous strengthening exercises before training, a recent trend in soccer, particularly at the amateur and recreational levels, is for the use of the FIFA-11 intervention program before training. The FIFA-11 not only includes standard running and change of direction exercises as part of the warm-up but also includes strengthening movements such as the NHE.
3132
the
The NHE is a bodyweight only exercise that emphasizes the eccentric contraction of the hamstrings as a trainee is asked to lower their body from a kneeling position to the ground. Numerous videos are available on the Internet demonstrating appropriate exercise technique. Thus, the FIFA-11 is a pragmatic approach for amateur and recreational soccer players to accumulate a dose of strength exercise within a pre-training warm-up. However, there is limited information about how demanding the NHE is, and therefore, whether it is an appropriate exercise to use as part of a warm-up or as a stimulus for the hamstring. This was the first study to document the fatiguing effect of multiple sets of NHE. Nordic hamstring exercise elicited reductions in hamstring concentric and eccentric fatigue after only 1 set of 5 repetitions, which illustrates the high physical demand of this bodyweight only exercise. Reduced eccentric torque and central motor output to the BF muscle in the end range of motion is a particularly concerning finding regarding strain injury risk during soccer training sessions. Strain injury, particularly at extended hamstring muscle lengths, is associated with the fatigue that occurs in the latter stages of match play and training sessions. Thus, performing the NHE before training and inducing hamstring fatigue before the primary session does not seem the best scheduling choice. Coaches and practitioners should be cautioned about the high demands of the NHE when prescribing this exercise before training, particularly if the volume will be in excess of a single set of 5 repetitions. Based on the findings in this study, it would seem that 3 to 5 sets of 5 repetitions maximizes hamstring muscle activity during exercise, with further sets providing no further increases in muscle activity during exercise or the extent of muscular fatigue. It may be the best that NHE is prescribed after training or in a homebased training environment to reduce potential risks for inducing strain injury. Coaches should also be aware that during multiple sets of NHE, trainees do not appear to increase the amount of upper-body propulsion to aid the movement. This was based on the findings in this study in which the average movement velocities during the ascent phase (return to starting position) did not change throughout the 6 sets of 5 repetitions. Whether this exercise is a suitable replacement for conventional hamstring strengthening exercises is unclear. But, as a pragmatic solution for improving eccentric strength, the prevalence of NHE prescription is likely to increase. Thus, it is necessary for strength and conditioning coaches to be aware that significant fatigue can be induced by this relatively basic exercise after only 1 set of 5 repetitions. It must be noted that the results of this study only apply if strength and conditioning coaches control the descent phase of the exercise so that the average time it takes from the kneeling (start) to ground position is approximately 3 seconds. Controlling the descent phase of the exercise using a verbal count or metronome minimizes set-to-set
TM
Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
the
TM
Journal of Strength and Conditioning Research
| www.nsca.com
variability exercise performance and should reduce the between-trainee variability in how the exercise is performed during the descent phase.
15. Heiderscheit, BC, Hoerth, DM, Chumanov, ES, Swanson, WC, Thelen, BJ, and Thelen, DG. Identifying the time of occurrence of a hamstring strain injury during treadmill running: A case study. Clin Biomech 20: 1072–1078, 2005.
ACKNOWLEDGMENTS
16. Iga, J, Fruer, CS, Deighan, M, Croix, MDS, and James, DVB. Nordic hamstrings exercise—Engagement characteristics and training responses. Int J Sports Med 33: 1000–1004, 2012.
The authors have no conflicts of interest that are directly relevant to the contents of this manuscript. The authors would like to acknowledge the New South Wales Sporting Injuries Committee for project funding. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript (www.sportinginjuries.nsw.gov.au).
REFERENCES 1. Amann, M and Dempsey, JA. Locomotor muscle fatigue modifies central motor drive in healthy humans and imposes a limitation to exercise performance. J Physiol 586: 161–173, 2008. 2. Amann, M, Proctor, LT, Sebranek, JJ, Pegelow, DF, and Dempsey, JA. Opioid-mediated muscle afferents inhibit central motor drive and limit peripheral muscle fatigue development in humans. J Physiol 587: 271–283, 2009. 3. Amann, M, Romer, LM, Subudhi, AW, Pegelow, DF, and Dempsey, JA. Severity of arterial hypoxaemia affects the relative contributions of peripheral muscle fatigue to exercise performance in healthy humans. J Physiol 581: 389–403, 2007. 4. Arnason, A, Anderson, TE, Holme, I, Engebretsen, L, and Bahr, R. Prevention of hamstring strains in elite soccer: An intervention study. Scand J Med Sci Sports 18: 40–48, 2008. 5. Billaut, F, Kerris, JP, Rodriguez, RF, Martin, DT, Gore, CJ, and Bishop, DJ. Interaction of central and peripheral factors during repeated sprints at different levels of arterial O2 saturation. PLoS One 8: e77297, 2013. 6. Bizzini, M, Impellizzeri, FM, Dvorak, J, Bortolan, Schena, F, Modena, R, and Junge, A. Physiological and performance responses to the “FIFA 11+” (part 1): Is it an appropriate warm-up?. J Sports Sci 31: 1481–1490, 2013. 7. Brooks, JHM, Fuller, CW, Kemp, SPT, and Reddin, DB. Incidence, risk, and prevention of hamstring muscle injuries in professional rugby union. Am J Sports Med 34: 1297–1306, 2006. 8. De Luca, CJ. The use of surface electromyography in biomechanics. J Appl Biomech 13: 135–163, 1997. 9. Ditroilo, M, De Vito, G, and Delahunt, E. Kinematic and electromyographic analysis of the nordic hamstring exercise. J Electromyogr Kinesiol 23: 1111–1118, 2013. 10. Farina, D, Cescon, C, Negro, F, and Enoka, R. Amplitude cancellation of motor-unit action potentials in the surface electromyogram can be estimated with spike-triggered averaging. J Neurophysiol 100: 431–440, 2008. 11. Farina, D and Merletti, R. Comparison of algorithms for estimation of EMG variables during voluntary isometric contractions. J Electromyogr Kinesiol 10: 337–349, 2000. 12. Farina, D, Merletti, R, and Enoka, R. The extraction of neural strategies from the surface EMG. J Appl Physiol 96: 1486–1495, 2004. 13. Finn, HT, Brennan, SL, Gonano, BM, Knox, MF, Ryan, RC, Siegler, JC, and Marshall, PWM. Muscle activation does not increase after a fatigue plateau is reached during 8 sets of resistance exercise in trained individuals. J Strength Cond Res 28: 1226–1234, 2014. 14. Gandevia, SC. Spinal and supraspinal factors in human muscle fatigue. Physiol Rev 81: 1725–1789, 2001.
17. Impellizzeri, FM, Bizzini, M, Dvorak, J, Pellegrini, B, Schena, F, and Junge, A. Physiological and performance responses to the FIFA11+ (part 2): A randomised controlled trial on the training effects. J Sports Sci 31: 1491–1502, 2013. 18. Keenan, K, Farina, D, Merletti, R, and Enoka, R. Amplitude cancellation reduces the size of motor unit action potentials averaged from the surface EMG. J Appl Physiol 100: 1928–1937, 2006. 19. Koulouris, G and Connell, D. Evaluation of the hamstring muscle complex following acute injury. Skeletal Radiol 32: 582–589, 2003. 20. Koulouris, G, Connell, DA, Brukner, P, and Schneider-Kolsky, M. Magnetic resonance imaging parameters for assessing risk of recurrent hamstring injuries in elite athletes. Am J Sports Med 35: 1500–1506, 2007. 21. Merton, P. Voluntary strength and fatigue. J Physiol 123: 553–564, 1954. 22. Onishi, H, Yagi, R, Oyama, M, Akasaka, K, Ihashi, K, and Handa, Y. EMG-angle relationship of the hamstring muscles during maximum knee flexion. J Electromyogr Kinesiol 12: 399–406, 2002. 23. Opar, DA, Williams, MD, and Shield, AJ. Hamstring strain injuries. Sports Med 42: 209–226, 2012. 24. Petersen, J, Thorborg, K, Nielsen, MB, Budtz-Jørgensen, E, and Ho¨lmich, P. Preventive effect of eccentric training on acute hamstring injuries in men’s soccer: A cluster-randomized controlled trial. Am J Sports Med 39: 2296–2303, 2011. 25. Potvin, JR. Effects of muscle kinematics on surface EMG amplitude and frequency during fatiguing dynamic contractions. J Appl Physiol 82: 144–151, 1997. 26. Rainoldi, A, Melchiorri, G, and Caruso, I. A method for positioning electrodes during surface EMG recordings in lower limb muscles. J Neurosci Methods 134: 37–43, 2004. 27. Schache, AG, Wrigley, TV, Baker, R, and Pandy, MG. Biomechanical response to hamstring muscle strain injury. Gait Posture 29: 332–338, 2009. 28. Small, K, McNaughton, L, Greig, M, and Lovell, R. Effect of timing of eccentric hamstring strengthening exercises during soccer training: Implications for muscle fatigability. J Strength Cond Res 23: 1077–1083, 2009. 29. Taylor, JL and Gandevia, SC. A comparison of central aspects of fatigue in submaximal and maximal voluntary contractions. J Appl Phys 104: 542–550, 2008. 30. Thelen, DG, Chumanov, ES, Hoerth, DM, Best, TM, Swanson, SC, Li, L, Young, M, and Heiderscheit, BC. Hamstring muscle kinematics during treadmill sprinting. Med Sci Sports Exerc 37: 108– 114, 2005. 31. Verrall, GM, Slavotinek, JP, Barnes, PG, and Fon, GT. Diagnostic and prognostic value of clinical findings in 83 athletes with posterior thigh injury: Comparison of clinical findings with magnetic resonance imaging documentation of hamstring muscle strain. Am J Sports Med 31: 969–973, 2003. 32. Walker, S, Davis, L, Avela, J, and Hakkinen, K. Neuromuscular fatigue during dynamic maximal strength and hypertrophic resistance loadings. J Electromyogr Kinesiol 22: 356–362, 2012.
VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
3133
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
