Journal of Sport Rehabilitation, 2015, 24, 13-20 http://dx.doi.org/10.1123/JSR.2013-0097 © 2015 Human Kinetics, Inc.
www.JSR-Journal.com ORIGINAL RESEARCH REPORT
The Validity of the Nordic Hamstring Lower for a Field-Based Assessment of Eccentric Hamstring Strength Emma Sconce, Paul Jones, Ellena Turner, Paul Comfort, and Philip Graham-Smith Context: Hamstring injury-risk assessment has primarily been investigated using isokinetic dynamometry. However, practical issues such as cost and availability limit the widespread application of isokinetics for injury-risk assessment; thus, field-based alternatives for assessing eccentric hamstring strength are needed. Objective: The aim of this study was to investigate the validity of the angle achieved during Nordic hamstring lowers (break-point angle) as a field-based test for eccentric hamstring strength. Design: Exploratory study. Setting: Laboratory. Participants: Sixteen male (n = 7) and female (n = 9) soccer players (mean ± SD age 24 ± 6 y, height 1.77 ± 0.12 m, and body mass 68.5 ± 16.5 kg) acted as subjects for the study. Main Outcome Measures: The authors explored relationships between the Nordic break-point angle (the point at which the subject can no longer resist the increasing gravitational moment during a Nordic hamstring lower) measured from video and isokinetic peak torque and angle of peak torque of right- and left-knee flexors. Results: The results revealed a meaningful relationship between eccentric knee-flexor peak torque (average of right and left limbs) and the Nordic break-point angle (r = –.808, r2 = 65%, P < .00001). However, there was a weak relationship observed (r = .480, r2 = 23%, P = .06) between break-point angle and the angle of peak torque (average of right and left limbs). Conclusions: The results suggest that the break-point angle achieved during Nordic hamstring lowers could be used as a field-based assessment of eccentric hamstring strength. Keywords: break-point angle, peak torque, angle of peak torque, angle of crossover, hamstring injury risk Hamstring injuries are common in a number of sprint-related sports, accounting for 29% of injuries among sprinters,1 whereas 12% of all injuries reported over 2 seasons in professional soccer players were hamstring strains.2 In an analysis of English professional rugby union match injuries over 2 seasons,3 hamstring injuries were the second-most-common injury, while in training,4 hamstring injuries were most frequent. Furthermore, players involved in more high-speed-running actions (eg, backs) accumulated more hamstring strains.3,4 The literature suggests that there are 2 noncontact mechanisms responsible for hamstring strain—one resulting from high-speed running, as in soccer5 and track and field events,6 and the other during stretching movements carried out to extreme range of motion,7 both resulting in high-velocity eccentric loading.8–10 Strain injury usually occurs late in the swing phase or early in the stance phase of the gait cycle.11,12 During late swing the hamstrings work eccentrically to decelerate both the thigh and lower
The authors are with the Research Centre for Health, Sport and Rehabilitation Sciences, University of Salford, Greater Manchester, United Kingdom. Address author correspondence to Paul Jones at [email protected]
leg in preparation for ground contact. Eccentric hamstring weakness has been associated with a history of hamstring strains,13,14 and many studies have shown that eccentric hamstring training can reduce the incidence of hamstring strains.7,15–18 Furthermore, the angle of peak torque has been considered as a risk factor for hamstring injuries,8,15,19,20 with angle of peak torque achieved at shorter muscle lengths associated with hamstring injury, as the muscle is vulnerable for a longer period.15,19 Many intrinsic risk factors exist, such as poor lumbar posture,21 previous injury,5,22 lack of flexibility,13 and insufficient hamstring strength in relation to the opposing quadriceps or contralateral hamstrings.23 Previous attempts at screening for potential hamstring injury have centered on the use of isokinetics to examine the ratio of flexor (hamstring) to extensor (quadriceps) muscle strength, through either a traditional concentric-flexor/ concentric-extensor peak-torque ratio9,22,23 or dynamic control ratio (eccentric knee flexor/concentric knee extensor).22,24 One criticism of these ratios is that they only take into account the peak-torque values, which can occur at different angular positions in the overall range of motion and are not joint-angle specific. Thus, an angle of crossover has recently been recommended that considers reciprocal knee-joint muscle balance throughout the full range of motion.25
13
14 Sconce et al
Although isokinetics have also been used to evaluate hamstring injuries retrospectively by assessing eccentric hamstring strength13 and angle of peak torque,8 a major drawback of using isokinetics for assessing hamstring strength and reciprocal knee-joint muscle balance for injury screening is the accessibility of isokinetic equipment for practitioners. Furthermore, isokinetic testing is expensive and time-consuming and may be regarded as nonfunctional with regard to most sporting actions. Moreover, often assessments are performed in a seated position,8,9,13,14,22–24 which may further limit the investigation of relationships to functional activities with the hip in an extended position. Therefore, there is a need to develop alternative tests of eccentric hamstring strength, particularly those that can be used in the field by practitioners. The inclusion of hamstring exercises such as Nordic curls (Nordic hamstring lowers) in a training program has been shown to improve hamstring strength by 11% and dynamic-control ratios in soccer players over 10 weeks,26 reduce the angle of peak torque (closer to full knee extension) for the hamstrings in athletic males,20 and reduce the incidence of hamstring injuries in various team-sport athletes during the subsequent season.16–18 Theoretically, the greater range achieved by an individual during a Nordic hamstring lower reflects the individual’s eccentric hamstring strength, as the gravitational moment progressively increases throughout the range of the exercise (Figure 1). Therefore, the “break point” (the angle at which the individual can no longer resist the increasing gravitational moment and falls to the floor) could be used as an assessment of eccentric hamstring strength. One previous investigation reported that the break-point angle was significantly correlated with peak concentric hamstring torque at both 60°/s and 240°/s.27 The aim of this study was to assess the validity of the Nordic hamstring-lower exercise to assess eccentric hamstring strength. The relationship between the break point achieved by individuals during Nordic hamstring lowers (Figure 1) and peak eccentric knee-flexor torque as measured during isokinetic dynamometry was explored. We hypothesized that the break point would be related to isokinetic peak eccentric knee-flexor torque. If such as relationship does exist, it provides support for the use of the Nordic break-point test as a field-based assessment for eccentric hamstring strength and would overcome many of the practical limitations of isokinetic testing.
Methods Design To investigate the study hypothesis and validate the use of the Nordic break-point angle as a field-based assessment of eccentric hamstring strength, a cross-sectional design was used to explore relationships between the Nordic break-point angle (the point at which the subject can no longer resist the increasing gravitational moment during a Nordic hamstring lower [Figure 1]) measured from video and traditional measures of eccentric hamstring
Figure 1 — Nordic hamstring-lower exercise: (a) start, (b) midpoint indicating the Nordic break-point angle, and (c) end.
strength such as isokinetic eccentric knee-flexor peak torque (average of right and left limbs). Furthermore, the relationship between Nordic break-point angle and angle of peak torque (average of right and left limbs) and a new method of reciprocal knee-joint muscle balance (average of right and left) and the angle of crossover,25 was also explored.
Participants Sixteen male (n = 7) and female (n = 9) soccer players of various playing positions, experience, and ability acted as subjects for the study (mean ± SD age 24 ± 6 y, height 1.77 ± 0.12 m, and body mass 68.5 ± 16.5 kg). The study was approved by the university’s ethical committee, and all subjects provided written informed consent to participate in the spirit of the Helsinki declaration.
Nordic Hamstring Lowers as a Measure of Eccentric Strength 15
Procedures After familiarization with the test procedures, each subject attended the laboratory for isokinetic assessment and video recording of 3 Nordic hamstring lowers. Each subject was asked to refrain from strenuous exercise or training 48 hours before assessments and to avoid eating 2 hours before testing. All equipment used was calibrated according to manufacturers’ standardized procedures. Gravity-corrected isokinetic peak torque and angle of peak torque of the knee flexors and extensors of the right and left legs was assessed at 60°/s in both concentric and eccentric modes using a Kin Com isokinetic dynamometer (Chattanooga Group, Tennessee). The subjects were seated with the hip joint at 90° (supine position = 0°). The seated position was chosen due to the consistent approach used in several previous studies.7–9,13,14,22–24 The axis of rotation of the dynamometer shaft was aligned with the best approximation of the knee axis of rotation, midway between the lateral condyles of the femur and tibia. The cuff of the dynamometer lever arm was attached to the ankle 5 cm proximal to the malleoli, and the moment arm recorded for gravity correction. Extraneous movement was prevented by straps positioned at the hip, shoulders, and tested thigh. Subjects were also instructed to hold onto the handles located underneath the seat. A range of motion was set as close to 90° as possible, zero being full knee extension. Eight submaximal concentric knee-extension and -flexion movements were performed as part of a warm-up. This followed 3 minutes of stationary cycling (60 rpm) on a cycle ergometer (Wattbike Ltd, Nottingham, UK). The
“overlay” method was selected for data collection, as this generates individual angle–torque graphs per repetition, enabling the graph with the greatest torque to be saved for subsequent analysis. Concentric and eccentric strength of the extensors were measured before concentric and eccentric strength of the knee flexors. Four maximaleffort movements were performed in each mode, with 10 seconds recovery between efforts.28 The resistance provided by the weight of the lower leg was recorded at 30° of flexion, to avoid excessive pull of the hamstrings to allow for gravity correction.28 Data were exported in ASCII format into Microsoft Excel, where a number of variables were calculated. The ends of range of movement that had corresponding phases of acceleration and deceleration were deleted from the analysis. A tolerance of ±1°/s was permitted. In addition, the dynamic-control profile25 was determined (Figure 2), which involves representing the net joint torque (eccentric flexor minus concentric extensor) over the entire range of motion and identifies the range of motion in which the quadriceps concentrically are stronger than the hamstrings eccentrically and hence the potential for injury risk. The point where the net joint torque crosses zero is known as the angle of crossover. The angle of crossover has been shown to be a reliable technique to assess knee flexor–extensor muscle balance and to discriminate retrospectively between hamstringinjured and -noninjured athletes.25 Each subject performed 3 trials of Nordic hamstring lowers (Figure 1). Each trial was recorded using a Casio Exlim-F1 camera (60 Hz) placed in the sagittal plane positioned 5 m away. Quintic Biomechanics v 17
Figure 2 — The dynamic control profile represents the net joint torque (eccentric hamstring – concentric quadriceps) throughout the full range of motion (0° [full knee extension] to 90° of flexion). The point at which the net joint torque crosses zero on the x-axis is the angle of crossover, whereby the torque generated by the quadriceps concentrically supersedes the hamstrings eccentrically.
16 Sconce et al
(Coventry, UK) was subsequently used to determine the angle from the greater trochanter (hip) to lateral epicondyle (knee) relative to the horizontal from the recorded video. Each video image was calibrated using a filmed 1-m × 1-m calibration frame before analysis. The exercise protocol included every individual being informed to remain with the hips fixed in line with the knee and shoulder joints throughout the range of movement.26 The subject assumed a kneeling position with a partner attempting to apply pressure onto the subject’s heels (Figure 1). From this position, the subject performed a forward-falling action. The subject’s head, trunk, and thighs were lowered forward via knee extension until he or she could no longer withstand the force of the “fall,” with the hamstrings providing the main resistance against gravity.29 Arms and hands were used to buffer the fall, letting the chest touch the surface.26 Every subject had 3 submaximal warm-up repetitions to ensure that correct technique was understood and followed. The best Nordic break-point-angle result (lowest, closer to floor) for every subject was determined and used in subsequent statistical analysis. Pilot data collected on 11 female soccer players revealed high relative (the degree to which individuals maintain their position in a sample of repeated measures30) (intraclass correlation coefficient [ICC] = .969, 95% confidence interval = .910–.992) and absolute (the degree to which repeated measurements vary for individuals30) within-session reliability (coefficient of variation [CV] = 4.85%, standard error of measurement [SEM] = 1.47°) for the Nordic break-point angle.
between 2 variables falls below 50% (Pearson correlation £ .71), it indicates that the variables are specific or somewhat independent in nature.31 Post hoc observed power calculations for correlation analysis were performed in G*Power v 3.1.5, Germany.32
Results Descriptive statistics for each variable are shown in Table 1. Women’s eccentric and concentric hamstring peak torques were significantly lower (P < .001) than men’s (Table 1). Women’s Nordic break-point angles were significantly higher (P < .05) than men’s (Table 1). Women’s angles of peak torque were significantly higher (P < .05) than men’s (Table 1). Women’s angles of crossover were significantly lower (P < .05) than men’s (Table 1). Pearson correlation and coefficient of determination revealed a meaningful relationship (r = –.808, r2 = 65%, n = 16, P < .00001, power = 0.96) between Nordic break-point angle and eccentric knee-flexor peak torque (average peak torque for right and left limbs; Figure 3). However, a weak relationship (r = .480, r2 = 23%, n = 16, P = .06, power = 0.95) was found between Nordic break-point angle and angle of peak torque (average of right and left limbs; Figure 4). The data also revealed a meaningful relationship (r = –.717, r2 = 51%, n = 16, P < .002, power = .96) between Nordic break-point angle and the angle of crossover (average of left and right limbs; Figure 5).
Discussion
Statistical Analyses All data were analyzed using statistical-analysis software, SPSS v 16.0 for Windows (Chicago, IL). Normality of data was confirmed using a Shapiro Wilks test. Significance was set at P < .05. An independent-samples T-test was used to determine any significant differences between genders (male vs female) for each variable. Pearson product–moment correlation (r) was used to explore relationships between eccentric flexor peak torque, angle of peak torque, angle of crossover (average of right and left limbs), and break-point angle during the Nordic hamstring lowers. Coefficients of determination (r2 × 100) were used to interpret the meaningfulness of the relationships. Using criteria from Thomas et al,31 when common variance (coefficient of determination)
The results of the study reveal that the Nordic break-point angle is related to eccentric knee-flexor peak torque, providing support for the use of this field-based test and substantiating the study hypothesis. The Nordic breakpoint angle was also considered to be highly reliable, with an ICC of .969, and possess low absolute error (CV = 4.85%, SEM = 1.47°), which is comparable to the findings of Lee et al,27 providing further support for its use in the field. Given that eccentric weakness is associated with hamstring injuries,13,14 future studies need to evaluate whether field-based tests such as the Nordic break-pointangle test can prospectively predict hamstring-injury occurrence and, possibly more important, determine whether, if this angle is decreased (ie, a lower angle
Table 1 Mean ± SD for Each Variable Considered in the Study Variable
Whole group (N = 16)
Men (n = 7)
Women (n = 9)
Eccentric knee-flexor peak torque (Nm)
131.9 ± 50.7
177.3 ± 38.2*
96.7 ± 23.0
Concentric knee-flexor peak torque (Nm)
107.0 ± 38.7
143.4 ± 27.7*
78.7 ± 13.3
41 ± 8.1
36 ± 5.9†
46 ± 7.0
Angle of peak torque (°)
27.2 ± 8.1
22.9 ± 5.4†
30.6 ± 8.5
Angle of crossover (°)
37.1 ± 11.9
43.6 ± 10.5‡
32 ± 10.9
Nordic break-point angle (°)
*Men significantly greater than women (P < .001). †Men significantly lower than women (P < .05). ‡Men significantly higher than women (P < .05).
Nordic Hamstring Lowers as a Measure of Eccentric Strength 17
Figure 3 — Scatter graph illustrating the relationship between Nordic break-point angle and eccentric knee-flexor peak torque (average of right and left limbs).
Figure 4 — Scatter graph illustrating a potential relationship between Nordic break-point angle and eccentric knee-flexor angle of peak torque (average of right and left limbs).
would indicate closer to floor, greater eccentric strength), it further decreases the risk of hamstring injury. The results revealed a greater than 50% shared common variance (r = –.808, r2 = 65%) between Nordic break-point angle and eccentric knee-flexor peak torque, suggesting that both measures are not independent and are related.31 This is in contrast to the findings of Lee et al,27 who found no relationship between break-point angle and eccentric peak torque. Lee et al,27 however, did find
a significant correlation with peak concentric hamstring torque at both 60°/s and 240°/s and break-point angle. Eccentric hamstring weakness has been associated with a history of hamstring strains,13,14 and many studies have shown that eccentric hamstring training can reduce the incidence of hamstring strains.7,16–18 Isokinetic assessment has been widely used in hamstring injury-risk assessment9,23 and is perhaps the only reliable way to measure eccentric hamstring strength. However, due to a
18 Sconce et al
Figure 5 — Scatter graph illustrating the relationship between Nordic break-point angle and angle of crossover (average of right and left limbs).
range of practical difficulties, the widespread application of isokinetics for injury risk assessment is limited. Thus, the need to develop field-based assessments of eccentric hamstring strength is of prime importance to aid in injury screening of athletes. A weak relationship was observed between Nordic break-point angle and knee-flexor angle of peak torque (r = .480, n = 16, r2 = 23%). With regard to hamstringinjury risk assessment, such a relationship is important, as Brockett et al,8 found that previously injured muscles reach peak torque at significantly shorter lengths (40.9° ± 2.7°) than the uninjured muscles of the opposite leg (29.8° ± 1.5°) and muscles of both legs in an uninjured group (right = 30.1° ± 1.5°, left = 17.3° ± 1.2°), the hypothesis being that the microscopic damage from eccentric exercise depends on the muscle’s optimum length for active tension, proposing that optimal length is a measure of susceptibility for muscle strains.8 It is reported that the greater the knee-extension angle (lower angle of flexion) at which peak torque is produced, the lower the risk of hamstring injury.8 Even though muscle balance may be satisfactory after rehabilitation, a major difference between the injured limb and the noninjured limb is the position of peak torque.8 Previous studies have also shown that acute19 and chronic (4 wk)20 adaptations to Nordic hamstring lowers involve alteration in the length–tension relationship of hamstrings, with a shift in the angle of peak torque to longer lengths (closer to full knee extension). The lack of a strong relationship between Nordic break-point angle and knee-flexor angle of peak torque is perhaps due to the different hip positions used in the 2 tests (flexed during isokinetics and straight during Nordic hamstring lowers). This is perhaps a limitation of the current study in that isokinetic assessments were
not made with the subjects in a supine rather than seated position and thus allow for length–tension status of the hamstrings similar to that in the Nordic hamstring lower. Given that both measures were significantly (P < .05) different between men and women in this study and that Figure 4 reveals a potential linear relationship, greater similarity between hip positions in both assessments may lead to a stronger relationship and provide further support for the use of the Nordic break-point angle for hamstring-injury risk assessment. Future studies should explore relationships between Nordic break-point angle and isokinetic knee-flexor peak torque and angle of peak torque in a supine position. A limitation of the Nordic break-point-angle assessment is that in regard to hamstring-injury risk screening it does not consider the role of the quadriceps and cocontraction synergy between the 2 muscle groups. In activities such as high-speed running or stretching movements carried out to the extreme range of motion, hamstring strain may result from an eccentric hamstrings muscle action failing to counteract excessive torque generated by the quadriceps due to deficits in intermuscular coordination between the 2 muscle groups. Assessments such as the Nordic break-point angle do not evaluate this potential cause of hamstring strain. The results of the current study revealed a greater than 50% shared common variance (r = –.717, r2 = 51%) between angle of crossover and Nordic break-point angle. This suggests that the variables are not independent and can be considered related measures.31 The angle of crossover is a new method of assessing reciprocal knee-joint muscle balance25 that identifies the range of motion in which the knee extensors are stronger than the knee flexors, with a greater angle desirable for reduced risk of hamstring
Nordic Hamstring Lowers as a Measure of Eccentric Strength 19
injury.25 Thus, the results of the current study illustrate that achieving a greater break-point angle is associated with a greater angle of crossover and, thus, an improved range in which the eccentric flexors are stronger than the concentric extensors and potentially reduced risk of hamstring injury. However, it should be cautioned that limited research has fully evaluated the angle of crossover with regard to hamstring-injury risk assessment. Potential limitations of using the Nordic breakpoint-angle test for field-based assessment of eccentric hamstring strength are that the Nordic hamstring lower is a bilateral leg-strength exercise and thus does not isolate bilateral differences in eccentric hamstring strength, unlike the use of an isokinetic dynamometer. Therefore, future studies need to evaluate whether such assessments can be performed unilaterally. Furthermore, no athletes in the current study could complete the Nordic hamstring lower throughout the whole range of motion. Typically, when athletes can achieve this, weight is added to the trunk through holding weight disks or wearing a weighted vest. Thus, further research needs to consider how adding weight can be integrated into the assessment for athletes who complete a Nordic hamstring lower throughout the full range of motion. A further limitation is whether the Nordic test can be applied across different sports; for example, rugby players, who have a greater upper-body mass, may achieve a larger (ie, farther from the floor) Nordic break-point angle. Furthermore, the gender differences observed in the current study may be partially explained by differences in upper-body mass between men and women. Therefore, future research needs to consider some form of correction factor based on trunk and upper-limb mass to allow comparisons between sports and gender. In conclusion, the results of the current study suggest that the Nordic break-point angle test is a reliable and valid field-based assessment of eccentric hamstring strength. Future research needs to evaluate whether such an assessment can prospectively identify athletes at risk for hamstring injury.
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www.JSR-Journal.com ORIGINAL RESEARCH REPORT
The Validity of the Nordic Hamstring Lower for a Field-Based Assessment of Eccentric Hamstring Strength Emma Sconce, Paul Jones, Ellena Turner, Paul Comfort, and Philip Graham-Smith Context: Hamstring injury-risk assessment has primarily been investigated using isokinetic dynamometry. However, practical issues such as cost and availability limit the widespread application of isokinetics for injury-risk assessment; thus, field-based alternatives for assessing eccentric hamstring strength are needed. Objective: The aim of this study was to investigate the validity of the angle achieved during Nordic hamstring lowers (break-point angle) as a field-based test for eccentric hamstring strength. Design: Exploratory study. Setting: Laboratory. Participants: Sixteen male (n = 7) and female (n = 9) soccer players (mean ± SD age 24 ± 6 y, height 1.77 ± 0.12 m, and body mass 68.5 ± 16.5 kg) acted as subjects for the study. Main Outcome Measures: The authors explored relationships between the Nordic break-point angle (the point at which the subject can no longer resist the increasing gravitational moment during a Nordic hamstring lower) measured from video and isokinetic peak torque and angle of peak torque of right- and left-knee flexors. Results: The results revealed a meaningful relationship between eccentric knee-flexor peak torque (average of right and left limbs) and the Nordic break-point angle (r = –.808, r2 = 65%, P < .00001). However, there was a weak relationship observed (r = .480, r2 = 23%, P = .06) between break-point angle and the angle of peak torque (average of right and left limbs). Conclusions: The results suggest that the break-point angle achieved during Nordic hamstring lowers could be used as a field-based assessment of eccentric hamstring strength. Keywords: break-point angle, peak torque, angle of peak torque, angle of crossover, hamstring injury risk Hamstring injuries are common in a number of sprint-related sports, accounting for 29% of injuries among sprinters,1 whereas 12% of all injuries reported over 2 seasons in professional soccer players were hamstring strains.2 In an analysis of English professional rugby union match injuries over 2 seasons,3 hamstring injuries were the second-most-common injury, while in training,4 hamstring injuries were most frequent. Furthermore, players involved in more high-speed-running actions (eg, backs) accumulated more hamstring strains.3,4 The literature suggests that there are 2 noncontact mechanisms responsible for hamstring strain—one resulting from high-speed running, as in soccer5 and track and field events,6 and the other during stretching movements carried out to extreme range of motion,7 both resulting in high-velocity eccentric loading.8–10 Strain injury usually occurs late in the swing phase or early in the stance phase of the gait cycle.11,12 During late swing the hamstrings work eccentrically to decelerate both the thigh and lower
The authors are with the Research Centre for Health, Sport and Rehabilitation Sciences, University of Salford, Greater Manchester, United Kingdom. Address author correspondence to Paul Jones at [email protected]
leg in preparation for ground contact. Eccentric hamstring weakness has been associated with a history of hamstring strains,13,14 and many studies have shown that eccentric hamstring training can reduce the incidence of hamstring strains.7,15–18 Furthermore, the angle of peak torque has been considered as a risk factor for hamstring injuries,8,15,19,20 with angle of peak torque achieved at shorter muscle lengths associated with hamstring injury, as the muscle is vulnerable for a longer period.15,19 Many intrinsic risk factors exist, such as poor lumbar posture,21 previous injury,5,22 lack of flexibility,13 and insufficient hamstring strength in relation to the opposing quadriceps or contralateral hamstrings.23 Previous attempts at screening for potential hamstring injury have centered on the use of isokinetics to examine the ratio of flexor (hamstring) to extensor (quadriceps) muscle strength, through either a traditional concentric-flexor/ concentric-extensor peak-torque ratio9,22,23 or dynamic control ratio (eccentric knee flexor/concentric knee extensor).22,24 One criticism of these ratios is that they only take into account the peak-torque values, which can occur at different angular positions in the overall range of motion and are not joint-angle specific. Thus, an angle of crossover has recently been recommended that considers reciprocal knee-joint muscle balance throughout the full range of motion.25
13
14 Sconce et al
Although isokinetics have also been used to evaluate hamstring injuries retrospectively by assessing eccentric hamstring strength13 and angle of peak torque,8 a major drawback of using isokinetics for assessing hamstring strength and reciprocal knee-joint muscle balance for injury screening is the accessibility of isokinetic equipment for practitioners. Furthermore, isokinetic testing is expensive and time-consuming and may be regarded as nonfunctional with regard to most sporting actions. Moreover, often assessments are performed in a seated position,8,9,13,14,22–24 which may further limit the investigation of relationships to functional activities with the hip in an extended position. Therefore, there is a need to develop alternative tests of eccentric hamstring strength, particularly those that can be used in the field by practitioners. The inclusion of hamstring exercises such as Nordic curls (Nordic hamstring lowers) in a training program has been shown to improve hamstring strength by 11% and dynamic-control ratios in soccer players over 10 weeks,26 reduce the angle of peak torque (closer to full knee extension) for the hamstrings in athletic males,20 and reduce the incidence of hamstring injuries in various team-sport athletes during the subsequent season.16–18 Theoretically, the greater range achieved by an individual during a Nordic hamstring lower reflects the individual’s eccentric hamstring strength, as the gravitational moment progressively increases throughout the range of the exercise (Figure 1). Therefore, the “break point” (the angle at which the individual can no longer resist the increasing gravitational moment and falls to the floor) could be used as an assessment of eccentric hamstring strength. One previous investigation reported that the break-point angle was significantly correlated with peak concentric hamstring torque at both 60°/s and 240°/s.27 The aim of this study was to assess the validity of the Nordic hamstring-lower exercise to assess eccentric hamstring strength. The relationship between the break point achieved by individuals during Nordic hamstring lowers (Figure 1) and peak eccentric knee-flexor torque as measured during isokinetic dynamometry was explored. We hypothesized that the break point would be related to isokinetic peak eccentric knee-flexor torque. If such as relationship does exist, it provides support for the use of the Nordic break-point test as a field-based assessment for eccentric hamstring strength and would overcome many of the practical limitations of isokinetic testing.
Methods Design To investigate the study hypothesis and validate the use of the Nordic break-point angle as a field-based assessment of eccentric hamstring strength, a cross-sectional design was used to explore relationships between the Nordic break-point angle (the point at which the subject can no longer resist the increasing gravitational moment during a Nordic hamstring lower [Figure 1]) measured from video and traditional measures of eccentric hamstring
Figure 1 — Nordic hamstring-lower exercise: (a) start, (b) midpoint indicating the Nordic break-point angle, and (c) end.
strength such as isokinetic eccentric knee-flexor peak torque (average of right and left limbs). Furthermore, the relationship between Nordic break-point angle and angle of peak torque (average of right and left limbs) and a new method of reciprocal knee-joint muscle balance (average of right and left) and the angle of crossover,25 was also explored.
Participants Sixteen male (n = 7) and female (n = 9) soccer players of various playing positions, experience, and ability acted as subjects for the study (mean ± SD age 24 ± 6 y, height 1.77 ± 0.12 m, and body mass 68.5 ± 16.5 kg). The study was approved by the university’s ethical committee, and all subjects provided written informed consent to participate in the spirit of the Helsinki declaration.
Nordic Hamstring Lowers as a Measure of Eccentric Strength 15
Procedures After familiarization with the test procedures, each subject attended the laboratory for isokinetic assessment and video recording of 3 Nordic hamstring lowers. Each subject was asked to refrain from strenuous exercise or training 48 hours before assessments and to avoid eating 2 hours before testing. All equipment used was calibrated according to manufacturers’ standardized procedures. Gravity-corrected isokinetic peak torque and angle of peak torque of the knee flexors and extensors of the right and left legs was assessed at 60°/s in both concentric and eccentric modes using a Kin Com isokinetic dynamometer (Chattanooga Group, Tennessee). The subjects were seated with the hip joint at 90° (supine position = 0°). The seated position was chosen due to the consistent approach used in several previous studies.7–9,13,14,22–24 The axis of rotation of the dynamometer shaft was aligned with the best approximation of the knee axis of rotation, midway between the lateral condyles of the femur and tibia. The cuff of the dynamometer lever arm was attached to the ankle 5 cm proximal to the malleoli, and the moment arm recorded for gravity correction. Extraneous movement was prevented by straps positioned at the hip, shoulders, and tested thigh. Subjects were also instructed to hold onto the handles located underneath the seat. A range of motion was set as close to 90° as possible, zero being full knee extension. Eight submaximal concentric knee-extension and -flexion movements were performed as part of a warm-up. This followed 3 minutes of stationary cycling (60 rpm) on a cycle ergometer (Wattbike Ltd, Nottingham, UK). The
“overlay” method was selected for data collection, as this generates individual angle–torque graphs per repetition, enabling the graph with the greatest torque to be saved for subsequent analysis. Concentric and eccentric strength of the extensors were measured before concentric and eccentric strength of the knee flexors. Four maximaleffort movements were performed in each mode, with 10 seconds recovery between efforts.28 The resistance provided by the weight of the lower leg was recorded at 30° of flexion, to avoid excessive pull of the hamstrings to allow for gravity correction.28 Data were exported in ASCII format into Microsoft Excel, where a number of variables were calculated. The ends of range of movement that had corresponding phases of acceleration and deceleration were deleted from the analysis. A tolerance of ±1°/s was permitted. In addition, the dynamic-control profile25 was determined (Figure 2), which involves representing the net joint torque (eccentric flexor minus concentric extensor) over the entire range of motion and identifies the range of motion in which the quadriceps concentrically are stronger than the hamstrings eccentrically and hence the potential for injury risk. The point where the net joint torque crosses zero is known as the angle of crossover. The angle of crossover has been shown to be a reliable technique to assess knee flexor–extensor muscle balance and to discriminate retrospectively between hamstringinjured and -noninjured athletes.25 Each subject performed 3 trials of Nordic hamstring lowers (Figure 1). Each trial was recorded using a Casio Exlim-F1 camera (60 Hz) placed in the sagittal plane positioned 5 m away. Quintic Biomechanics v 17
Figure 2 — The dynamic control profile represents the net joint torque (eccentric hamstring – concentric quadriceps) throughout the full range of motion (0° [full knee extension] to 90° of flexion). The point at which the net joint torque crosses zero on the x-axis is the angle of crossover, whereby the torque generated by the quadriceps concentrically supersedes the hamstrings eccentrically.
16 Sconce et al
(Coventry, UK) was subsequently used to determine the angle from the greater trochanter (hip) to lateral epicondyle (knee) relative to the horizontal from the recorded video. Each video image was calibrated using a filmed 1-m × 1-m calibration frame before analysis. The exercise protocol included every individual being informed to remain with the hips fixed in line with the knee and shoulder joints throughout the range of movement.26 The subject assumed a kneeling position with a partner attempting to apply pressure onto the subject’s heels (Figure 1). From this position, the subject performed a forward-falling action. The subject’s head, trunk, and thighs were lowered forward via knee extension until he or she could no longer withstand the force of the “fall,” with the hamstrings providing the main resistance against gravity.29 Arms and hands were used to buffer the fall, letting the chest touch the surface.26 Every subject had 3 submaximal warm-up repetitions to ensure that correct technique was understood and followed. The best Nordic break-point-angle result (lowest, closer to floor) for every subject was determined and used in subsequent statistical analysis. Pilot data collected on 11 female soccer players revealed high relative (the degree to which individuals maintain their position in a sample of repeated measures30) (intraclass correlation coefficient [ICC] = .969, 95% confidence interval = .910–.992) and absolute (the degree to which repeated measurements vary for individuals30) within-session reliability (coefficient of variation [CV] = 4.85%, standard error of measurement [SEM] = 1.47°) for the Nordic break-point angle.
between 2 variables falls below 50% (Pearson correlation £ .71), it indicates that the variables are specific or somewhat independent in nature.31 Post hoc observed power calculations for correlation analysis were performed in G*Power v 3.1.5, Germany.32
Results Descriptive statistics for each variable are shown in Table 1. Women’s eccentric and concentric hamstring peak torques were significantly lower (P < .001) than men’s (Table 1). Women’s Nordic break-point angles were significantly higher (P < .05) than men’s (Table 1). Women’s angles of peak torque were significantly higher (P < .05) than men’s (Table 1). Women’s angles of crossover were significantly lower (P < .05) than men’s (Table 1). Pearson correlation and coefficient of determination revealed a meaningful relationship (r = –.808, r2 = 65%, n = 16, P < .00001, power = 0.96) between Nordic break-point angle and eccentric knee-flexor peak torque (average peak torque for right and left limbs; Figure 3). However, a weak relationship (r = .480, r2 = 23%, n = 16, P = .06, power = 0.95) was found between Nordic break-point angle and angle of peak torque (average of right and left limbs; Figure 4). The data also revealed a meaningful relationship (r = –.717, r2 = 51%, n = 16, P < .002, power = .96) between Nordic break-point angle and the angle of crossover (average of left and right limbs; Figure 5).
Discussion
Statistical Analyses All data were analyzed using statistical-analysis software, SPSS v 16.0 for Windows (Chicago, IL). Normality of data was confirmed using a Shapiro Wilks test. Significance was set at P < .05. An independent-samples T-test was used to determine any significant differences between genders (male vs female) for each variable. Pearson product–moment correlation (r) was used to explore relationships between eccentric flexor peak torque, angle of peak torque, angle of crossover (average of right and left limbs), and break-point angle during the Nordic hamstring lowers. Coefficients of determination (r2 × 100) were used to interpret the meaningfulness of the relationships. Using criteria from Thomas et al,31 when common variance (coefficient of determination)
The results of the study reveal that the Nordic break-point angle is related to eccentric knee-flexor peak torque, providing support for the use of this field-based test and substantiating the study hypothesis. The Nordic breakpoint angle was also considered to be highly reliable, with an ICC of .969, and possess low absolute error (CV = 4.85%, SEM = 1.47°), which is comparable to the findings of Lee et al,27 providing further support for its use in the field. Given that eccentric weakness is associated with hamstring injuries,13,14 future studies need to evaluate whether field-based tests such as the Nordic break-pointangle test can prospectively predict hamstring-injury occurrence and, possibly more important, determine whether, if this angle is decreased (ie, a lower angle
Table 1 Mean ± SD for Each Variable Considered in the Study Variable
Whole group (N = 16)
Men (n = 7)
Women (n = 9)
Eccentric knee-flexor peak torque (Nm)
131.9 ± 50.7
177.3 ± 38.2*
96.7 ± 23.0
Concentric knee-flexor peak torque (Nm)
107.0 ± 38.7
143.4 ± 27.7*
78.7 ± 13.3
41 ± 8.1
36 ± 5.9†
46 ± 7.0
Angle of peak torque (°)
27.2 ± 8.1
22.9 ± 5.4†
30.6 ± 8.5
Angle of crossover (°)
37.1 ± 11.9
43.6 ± 10.5‡
32 ± 10.9
Nordic break-point angle (°)
*Men significantly greater than women (P < .001). †Men significantly lower than women (P < .05). ‡Men significantly higher than women (P < .05).
Nordic Hamstring Lowers as a Measure of Eccentric Strength 17
Figure 3 — Scatter graph illustrating the relationship between Nordic break-point angle and eccentric knee-flexor peak torque (average of right and left limbs).
Figure 4 — Scatter graph illustrating a potential relationship between Nordic break-point angle and eccentric knee-flexor angle of peak torque (average of right and left limbs).
would indicate closer to floor, greater eccentric strength), it further decreases the risk of hamstring injury. The results revealed a greater than 50% shared common variance (r = –.808, r2 = 65%) between Nordic break-point angle and eccentric knee-flexor peak torque, suggesting that both measures are not independent and are related.31 This is in contrast to the findings of Lee et al,27 who found no relationship between break-point angle and eccentric peak torque. Lee et al,27 however, did find
a significant correlation with peak concentric hamstring torque at both 60°/s and 240°/s and break-point angle. Eccentric hamstring weakness has been associated with a history of hamstring strains,13,14 and many studies have shown that eccentric hamstring training can reduce the incidence of hamstring strains.7,16–18 Isokinetic assessment has been widely used in hamstring injury-risk assessment9,23 and is perhaps the only reliable way to measure eccentric hamstring strength. However, due to a
18 Sconce et al
Figure 5 — Scatter graph illustrating the relationship between Nordic break-point angle and angle of crossover (average of right and left limbs).
range of practical difficulties, the widespread application of isokinetics for injury risk assessment is limited. Thus, the need to develop field-based assessments of eccentric hamstring strength is of prime importance to aid in injury screening of athletes. A weak relationship was observed between Nordic break-point angle and knee-flexor angle of peak torque (r = .480, n = 16, r2 = 23%). With regard to hamstringinjury risk assessment, such a relationship is important, as Brockett et al,8 found that previously injured muscles reach peak torque at significantly shorter lengths (40.9° ± 2.7°) than the uninjured muscles of the opposite leg (29.8° ± 1.5°) and muscles of both legs in an uninjured group (right = 30.1° ± 1.5°, left = 17.3° ± 1.2°), the hypothesis being that the microscopic damage from eccentric exercise depends on the muscle’s optimum length for active tension, proposing that optimal length is a measure of susceptibility for muscle strains.8 It is reported that the greater the knee-extension angle (lower angle of flexion) at which peak torque is produced, the lower the risk of hamstring injury.8 Even though muscle balance may be satisfactory after rehabilitation, a major difference between the injured limb and the noninjured limb is the position of peak torque.8 Previous studies have also shown that acute19 and chronic (4 wk)20 adaptations to Nordic hamstring lowers involve alteration in the length–tension relationship of hamstrings, with a shift in the angle of peak torque to longer lengths (closer to full knee extension). The lack of a strong relationship between Nordic break-point angle and knee-flexor angle of peak torque is perhaps due to the different hip positions used in the 2 tests (flexed during isokinetics and straight during Nordic hamstring lowers). This is perhaps a limitation of the current study in that isokinetic assessments were
not made with the subjects in a supine rather than seated position and thus allow for length–tension status of the hamstrings similar to that in the Nordic hamstring lower. Given that both measures were significantly (P < .05) different between men and women in this study and that Figure 4 reveals a potential linear relationship, greater similarity between hip positions in both assessments may lead to a stronger relationship and provide further support for the use of the Nordic break-point angle for hamstring-injury risk assessment. Future studies should explore relationships between Nordic break-point angle and isokinetic knee-flexor peak torque and angle of peak torque in a supine position. A limitation of the Nordic break-point-angle assessment is that in regard to hamstring-injury risk screening it does not consider the role of the quadriceps and cocontraction synergy between the 2 muscle groups. In activities such as high-speed running or stretching movements carried out to the extreme range of motion, hamstring strain may result from an eccentric hamstrings muscle action failing to counteract excessive torque generated by the quadriceps due to deficits in intermuscular coordination between the 2 muscle groups. Assessments such as the Nordic break-point angle do not evaluate this potential cause of hamstring strain. The results of the current study revealed a greater than 50% shared common variance (r = –.717, r2 = 51%) between angle of crossover and Nordic break-point angle. This suggests that the variables are not independent and can be considered related measures.31 The angle of crossover is a new method of assessing reciprocal knee-joint muscle balance25 that identifies the range of motion in which the knee extensors are stronger than the knee flexors, with a greater angle desirable for reduced risk of hamstring
Nordic Hamstring Lowers as a Measure of Eccentric Strength 19
injury.25 Thus, the results of the current study illustrate that achieving a greater break-point angle is associated with a greater angle of crossover and, thus, an improved range in which the eccentric flexors are stronger than the concentric extensors and potentially reduced risk of hamstring injury. However, it should be cautioned that limited research has fully evaluated the angle of crossover with regard to hamstring-injury risk assessment. Potential limitations of using the Nordic breakpoint-angle test for field-based assessment of eccentric hamstring strength are that the Nordic hamstring lower is a bilateral leg-strength exercise and thus does not isolate bilateral differences in eccentric hamstring strength, unlike the use of an isokinetic dynamometer. Therefore, future studies need to evaluate whether such assessments can be performed unilaterally. Furthermore, no athletes in the current study could complete the Nordic hamstring lower throughout the whole range of motion. Typically, when athletes can achieve this, weight is added to the trunk through holding weight disks or wearing a weighted vest. Thus, further research needs to consider how adding weight can be integrated into the assessment for athletes who complete a Nordic hamstring lower throughout the full range of motion. A further limitation is whether the Nordic test can be applied across different sports; for example, rugby players, who have a greater upper-body mass, may achieve a larger (ie, farther from the floor) Nordic break-point angle. Furthermore, the gender differences observed in the current study may be partially explained by differences in upper-body mass between men and women. Therefore, future research needs to consider some form of correction factor based on trunk and upper-limb mass to allow comparisons between sports and gender. In conclusion, the results of the current study suggest that the Nordic break-point angle test is a reliable and valid field-based assessment of eccentric hamstring strength. Future research needs to evaluate whether such an assessment can prospectively identify athletes at risk for hamstring injury.
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