International Journal of Sports Physiology and Performance, 2015, 10, 418 -425 http://dx.doi.org/10.1123/ijspp.2014-0316 © 2015 Human Kinetics, Inc.
Original Investigation
Combined Strength and Endurance Session Order: Differences in Force Production and Oxygen Uptake Ritva S. Taipale, Jussi Mikkola, Ari T. Nummela, Juha Sorvisto, Kai Nyman, Heikki Kyröläinen, and Keijo Häkkinen Purpose: To examine acute responses of force production and oxygen uptake to combined strength (S) and endurance-running (E) loading sessions in which the order of exercises is reversed (ES vs SE). Methods: This crossover study design included recreationally endurance-trained men and women (age 21–45 y; n = 12 men, 10 women) who performed ES and SE loadings. Force production of the lower extremities including countermovement-jump height (CMJ) and maximal isometric strength (MVC) was measured pre-, mid-, and post-ES and -SE, and ground-reaction forces, ground-reaction times, and running economy were measured during E. Results: A significant decrease in CMJ was observed after combined ES and SE in men (4.5% ± 7.0% and 6.6% ± 7.7%, respectively) but not in women (0.2% ± 8.5% and 1.4% ± 7.3% in ES and SE). MVC decreased significantly in both men (20.7% ± 6.1% ES and 19.3% ± 9.4% SE) and women (12.4% ± 9.3% ES and 11.6% ± 12.0% SE). Stride length decreased significantly in ES and SE men, but not in women. No changes were observed in ground-reaction times during running in men or women. Performing S before E caused greater (P < .01) oxygen uptake during running in both men and women than if E was performed before S, although heart rate and blood lactate were similar between ES and SE. Conclusions: Performing S before E increased oxygen uptake during E, which is explained, in part, by a decrease in MVC in both men and women, decreased CMJ and stride length in men, and/or an increase in postexercise oxygen consumption. Keywords: postexercise oxygen consumption, combined strength and endurance training, biomechanics and kinematics of running The neuromuscular responses to endurance exercise sessions/ loadings are sport specific and depend on movement type, while the magnitude of fatigue depends on the intensity and/or duration of the endurance-exercise session/loading. One of the most popular forms of endurance exercise is running, which typically involves a lesser amount of force production than strength exercises due to distinct differences in force requirements.1 Even running uphill at maximal speed does not provoke maximal activation of the muscles,2 and only slight acute decreases in strength and neural activation are observed after a typical endurance-running session.3 Running, however, does demand repetitive force production and utilizes the stretch-shortening cycle,4 suggesting that races and long distances like the marathon can induce a significant amount of neuromuscular fatigue.5 This fatigue may be characterized by significant decreases in, for example, maximal force production and muscle activation of the lower extremities. For example, after a 5-km time trial, a 15% decrease in maximal force has been observed,6 while 30 km of high-intensity trail running and a simulated marathon caused 24% and 22% decreases in maximal force, respectively.7,8 This neuromuscular fatigue may have an impact on running-specific force-production abilities including decreased reaction times9 and a reduction in coordination.10 As such, fatigue may also alter the mechanics and kinematics of running.11,12 Significant fatigue may even lead to changes in muscle-activation patterns such as those Taipale, Sorvisto, Kyröläinen, and Häkkinen are with the Dept of Biology of Physical Activity, University of Jyväskylä, Jyväskylä, Finland. Mikkola and Nummela are with the KIHU Research Inst for Olympic Sport, Jyväskylä, Finland. Nyman is with the Central Hospital of Central Finland (KSSHP), Jyväskylä, Finland. Address author correspondence to Ritva Taipale at [email protected]. 418
observed in landing tasks while also reducing joint stabilization,13 which may result in overuse or stress, causing injuries. The fatigue observed in neuromuscular function during running may parallel or reflect changes that occur in cardiorespiratory factors like running economy and heart rate.14 Whether the fatigue is the result of central15 or peripheral16 factors may also play a role in understanding its duration and the time course of recovery, which may assist in planning more effective training programs. Both maximal and explosive strength training are recognized as a necessary component of endurance training17–20 and have been shown to effectively enhance running performance by improving speed and running economy.17,21 Strength training improves fastforce production, which is an important component of peak running speed and running economy,22 while explosive jumping ability has been shown to correlate significantly with both middle- and longdistance running performance.23 Acute decreases in maximal force production after a strength-training session may vary 10% to 25% in men and ~20% in women.24,25 In general, women appear to be less fatigable than men,26,27,28 and this difference may be related, in part, to muscle fiber-type distribution and may influence fatigue mechanisms.29,30 A typical strength-training session generally requires greater force production than an endurance-running session, although as already noted, high-intensity running (time trials/ races) and long distances may induce decreases in maximal force production that are similar to decreases in maximal force production observed immediately after a strength-training session. The divergent responses and adaptations to strength and endurance exercises and training31,32 suggest that training needs to be carefully planned for optimal results, even in recreational endurance runners. As part of an optimal training regimen, or to save time, recreational and elite athletes may perform strength and endurance exercise in the same combined training session, but it
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is unclear whether strength or endurance exercise should be performed first. The purpose of this study was to examine the acute effects of 2 different orders of combined strength and endurance training performed in a single training session on force production of the lower extremities. We measured maximal isometric strength and countermovement-jump height (CMJ) before, during, and after combined sessions, as well as ground-reaction forces (GRF), ground-reaction times, and economy during the endurance-running loading. The combined strength and endurance sessions used in the current study are representative of typical training sessions that a recreational athlete might undertake. Using a cross-sectional design, we hypothesized that the first loading of the combined exercise sessions would induce specific fatigue responses that might differ in magnitude, thus affecting the subsequent exercise session.33 More specifically, we expected that a strength-training session performed before an endurance-training session would cause impairments in force production during running, including GRFs and groundreaction times, as well as running economy.
Methods Subjects Recreationally endurance-trained subjects (age 21–45 y; n = 12 men n = 10 women) were recruited to participate in this study (Table 1). Subjects regularly participated in endurance activities (primarily running) at least 3 times per week but had no systematic background in strength training. Exclusion criteria included body-mass index >28 kg/m2, illness, disease, injury, or use of medications that would contraindicate participation in the study. Subjects received specific written and oral information about the study design and measurement procedures and were informed of the possible risks and benefits of participation in the study before they signed an informed-consent document. Each subject had a resting electrocardiogram and health-history questionnaire reviewed and approved by a qualified physician before participation in the study. Ethical approval was granted by the university ethical committee, and the study was conducted according to the provisions of the most recent Declaration of Helsinki.
Design Before the combined experimental training sessions, body composition, height, and body mass were measured (Table 1). Subjects then completed a set of tests to become familiarized with the strength-training equipment and to determine appropriate loads and intensities for the specific loadings. These tests included 1-repetition maximum using a leg-press device (David 210 dynamometer,
David Sports Ltd, Helsinki, Finland) and a squat rack (Kraftwerk, Hot Milling Oy, Tuusula, Finland), as well as CMJ and maximal bilateral isometric leg press. A maximal treadmill running test was used to determine maximal oxygen uptake (VO2max). After completion of these preliminary tests, subjects performed, in random order, 2 combined strength- and endurance-training sessions. One training session started with and endurance-exercise session (loading) that included running for 60 minutes on an indoor track at a moderate intensity, which was immediately followed by a strength-exercise session (loading) that included a mixture of maximal and explosive exercises for the lower extremities (ES). The other combined training session started with the strength loading, which was immediately followed by the endurance loading (SE). Possible time-of-day variations in neuromuscular and cardiorespiratory performance were controlled for by ensuring that each subject performed the experimental training sessions ±1 hour from the time of his or her preliminary testing (Figure 1). The current combined strength and endurance sessions are representative of typical training sessions that a recreational athlete might undertake and thus were planned to be fatiguing but not too exhaustive. In addition, the current study design tested runners at the same physiological intensities34 rather than at a fixed running speed for all subjects to help control for individual fitness level. CMJ and maximal bilateral isometric force production were measured before, during, and after each combined experimental training session, whereas GRFs and ground-reaction times, oxygen uptake, and blood lactate were measured before and after the endurance loading (pre-E and post-E). Oxygen consumption was measured breath by breath at the beginning and end of the endurance loading. Percent VO2max and oxygen demand/running economy were calculated from oxygen uptake, while heart rate was recorded continuously throughout the endurance session. The endurance loading included running on a 200-m indoor track for a total of 60 minutes. The intensity of running was at a steady-state speed between each subject’s previously determined individual lactate threshold and respiratory-compensation threshold. This specific running speed for each subject was guided by a so-called “light rabbit” calibrated lamp system (Protom, Naakka, Finland) installed in the edge of the indoor running track. The average intensity of running was ~80% of VO2max. The average intensity in terms of VO2 during running was determined from expired gases during the first and last 10 minutes of the endurance loading. The percentage of VO2max was calculated for each runner based on his
Table 1 Subject Characteristics, Mean ± SD Age (y) Height (cm) Body mass (kg) Body fat (%) VO2max (L/min) VO2max (mL ∙ kg–1 ∙ min–1)
Men (n = 12) 38.8 ± 7.1 177.4 ± 6.4 75.7 ± 3.6 12.9 ± 3.6 4.1 ± 0.3 54.5 ± 4.0
Abbreviation: VO2max, maximal oxygen uptake.
Women (n = 10) 33.5 ± 8.3 165.9 ± 7.6 59.8 ± 5.1 22.0 ± 3.8 2.9 ± 0.4 48.5 ± 4.6
Figure 1 — Study design. Arrows indicate measurement time points where countermovement jump and maximal isometric strength were measured. Stars indicate the measurement points for ground-reaction forces including running speed, stride length, stride rate, braking time, push-off, and total contact times and measurement of oxygen consumption. Heart rate and blood lactate measurements were recorded at 4 intervals throughout the 60-minute endurance-running loading.
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or her VO2max test completed on a treadmill. Running distance was calculated based on laps. The strength loading focused primarily on the lower extremities and included both maximal and explosive strength exercises (Table 2). Strength exercises were performed in a circuit in which the exercise changed after every set. The total duration of strengthexercise sessions was approximately 45 minutes. The 2 combined experimental training sessions were similar in terms of volume and intensity (Table 3). No significant differences were observed in running distance, speed, % of VO2max, or heart rate between the 2 combined strength- and endurance-training sessions in men or in women. Heart rate and blood lactate (Table 3) increased gradually over the course of the endurance-training session. A significant difference (P < .05) was observed between blood lactate concentrations during the endurance loadings of the combined strength- and endurancetraining sessions. Lactate concentrations were higher in ES than SE at the first 2 measurement points.
Methodology Body Composition. In addition to standing height, body mass and
body composition were measured using bioimpedance (InBody720
body-composition analyzer, Biospace Co Ltd, Seoul, South Korea). Measurements were always taken between 7:30 and 8:00 AM. Subjects were instructed to arrive for testing in a fasted state, to help keep the possible confounding variables of diet and hydration status to a minimum. Subjects were measured in their undergarments. Aerobic Capacity. Endurance performance characteristics were
measured using a treadmill running protocol.35 Running started at the speed of 8 km/h and was increased by 1 km/h every 3 minutes until volitional exhaustion. Treadmill incline remained a constant 0.5° throughout the test. Heart rate was recorded continuously using a heart-rate monitor (Suunto t6, Vantaa, Finland). Mean heart-rate values from the last minute of each stage were used for analysis. Oxygen consumption was measured breath by breath throughout the test using a portable gas analyzer (Oxycon Mobile, Jaeger, Hoechberg, Germany). VO2max was determined to the highest average of 60-second VO2 values. Other factors such as heart rate, VO2, and respiratory-exchange ratio were monitored for determination of maximal effort. Fingertip blood samples were taken every 3 minutes to measure blood lactate concentrations. For blood sampling, the treadmill was stopped for approximately 15 to 20 seconds. Blood lactates were analyzed using a Biosen S_line Laboratory+ lactate analyzer (EKF Diagnostic, Magdeburg, Germany). Lactate threshold
Table 2 Strength Loading Performed in a Circuit in Which the Exercise Changed After Every Set Exercise Maximal bilateral leg press Explosive bilateral leg press Squat Squat jump Calf raise
Sets 3 3 3 3 2
Loads (%1-repetition maximum) 70–85% 30–40% 70–85% 30–40% 70–85%
Repetitions 5–8 8–10 5–8 8–10 5–8
Rest (min) 2 2 2 2 2
Table 3 Endurance Loadings of Running Distance and Speed, as Well as Percentages of Maximal Speed, Oxygen Uptake, and Heart Rate for Endurance Loadings Performed Either Before or After Strength Loading, Mean ± SD Men
Running distance (km) Running speed (km/h) Speed (%max) Oxygen uptake (%max) Heart rate (%max) Heart rate (beats/min) start 15 min 45 min end Blood lactate (mmol/L) start 15 min 45 min end
Women
Endurance before strength 11.6 ± 0.9 12.2 ± 1.0 76.9 ± 1.8 81.1 ± 4.1 87.7 ± 4.1
Strength before endurance 11.7 ± 1.0 12.2 ± 1.0 76.9 ± 1.8 83.1 ± 5.2 86.7 ± 4.8
Endurance before strength 10.3 ± 0.9 10.7 ± 0.9 76.2 ± 2.7 79.5 ± 5.8 86.8 ± 5.2
Strength before endurance 10.3 ± 0.8 10.7 ± 0.9 76.2 ± 2.7 81.2 ± 5.8 84.7 ± 4.9
154 ± 11 153 ± 11 159 ± 10 162 ± 10
151 ± 8 150 ± 9 157 ± 10 160 ± 10
160 ± 17 159 ± 16 164 ± 16 167 ± 17
159 ± 13 155 ± 16 161 ± 15 166 ± 12
1.2 ± 0.5* 2.7 ± 1.0* 2.5 ± 0.9 2.5 ± 1.1
0.8 ± 0.2* 2.2 ± 0.5* 2.1 ± 0.9 1.9 ± 0.7
1.0 ± 0.5 2.1 ± 1.1 2.3 ± 1.1 2.2 ± 1.4
1.1 ± 0.5 1.7 ± 0.7 1.8 ± 0.6 1.6 ± 0.5
Note: No significant differences between heart rates were observed between the current orders of combined strength- and endurance-training sessions. *Significant difference (P < .05) between blood lactate concentrations during the loadings of the combined strength- and endurance-training sessions. IJSPP Vol. 10, No. 4, 2015
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and respiratory-compensation threshold were determined using blood lactate, ventilation, VO2, and VCO2 (production of carbon dioxide) according to Meyer et al.36 Ground-Reaction Forces. Force platforms (total length 9 m, TR-test, Jyväskylä, Finland, natural frequency ≥170 Hz, nonlinearity ≤1%, cross talk ≤2%) were used to record GRFs, which were analyzed using a Signal Software (v 4.1, Cambridge Electronic Design, UK). GRF data were first low-pass filtered using a fourthorder Butterworth filter with cutoff frequency of 50 Hz. Then custom-written script was used to analyze stride rate and length from ground-contact times based on 60-N vertical-force thresholds, as well as the duration of the braking and propulsion phases and the peak values of the vertical and horizontal forces. Loading rate was calculated as the total change in vertical force divided by the total change in time period between 60 N and individual subject body weight (in Newtons).
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Isometric Leg-Press Force. An electromechanical isometric
leg-press device (designed and manufactured by the Department of Biology of Physical Activity, University of Jyväskylä, Finland) was used to measure maximal bilateral isometric strength (maximum voluntary contractions [MVC]) and average force over the first 500 milliseconds of MVC in a horizontal leg-press position. The subject’s knee angle was set at 107° while the hip angle was 110°. Subjects were instructed to produce force “as fast and as hard as possible” for approximately 3 seconds. Subjects performed at least 3 MVCs.37 If the maximum force during the last trial differed more than 5% from the previous trial, an additional trial was performed. The best performance was used for statistical analysis. During the experimental sessions, maximal force was measured only 2 times with a 15-second interval of rest to record acute fatigue. Countermovement Jump. A force platform (Department of Biology of Physical Activity, Jyväskylä, Finland) was used to measure maximal dynamic explosive force by CMJ height. 38
Subjects were instructed to stand with their feet approximately hip width apart and hands on their hips. They were then instructed to perform a quick and explosive CMJ on verbal command so that knee angle for the jump was no less than 90°. Force data were collected and analyzed by computer software (Signal 2.14, CED, Cambridge, UK), which used the equation h = I2 ∙ 2gm2 to calculate jump height from impulse, where I = impulse, g = gravity, and m = mass of subject.
Statistical Analysis Standard statistical methods were used for calculation of means, standard deviations (SD), and Pearson product–moment correlation coefficients. Group differences and group-by-loading interaction were analyzed by a repeated analysis using mixed models and an unstructured covariance matrix. Groups were compared with leastsignificant-difference post hoc analysis in a mixed-models analysis when appropriate. The criterion for significance was set at *P = .05, **P < .01, and ***P < .001. Statistical analysis was completed with PASW Statistics 18 (SPSS Inc, Chicago, IL, USA).
Results Force Production CMJ height decreased significantly from baseline (from 33.4 ± 5.1 cm to 32.3 ± 4.8 cm, P < .01) at mid-SE but not ES (from 33.2 ± 4.9 cm to 32.8 ± 4.1 cm, P > .05; Figure 2) in men. The significant decrease in jumping height after combined ES and SE sessions was similar between ES (down to 31.8 ± 5.0 cm, P < .05) and SE (down to 31.5 ± 4.2 cm, P < .01). CMJ height did not change significantly in women over the combined strength and endurance sessions (from 23.0 ± 3.5 cm to 23.2 ± 4.5 cm and 22.7 ± 3.7 cm to 22.4 ± 4.5 cm in ES and SE, respectively).
Figure 2 — Countermovement-jump height during the combined strength- and endurance-training sessions (Δ%) performed before or after a strength loading (E). *, **Significant change (P < .05, P < .01) from pre-E. ++Significant change (P < .01) from mid-E. Bracketed asterisks, significant difference between men and women (P < .05). IJSPP Vol. 10, No. 4, 2015
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Maximal bilateral isometric force before the combined training sessions was 2757 ± 371 N and 2807 ± 485 N in ES and SE, respectively, in men. In women, maximal bilateral isometric force before ES and SE was 1808 ± 423 N and 1811 ± 429 N, respectively. Maximal bilateral isometric force decreased significantly more (P < .01) at mid in men after S of SE (18.9% ± 9.2%, P < .001) than after E of ES (8.1% ± 7.3%, P < .01). At post, the cumulative force decrease was 19.3% ± 9.4% (P < .001) in SE and 20.7% ± 6.1% (P < .001) in ES. Decreases in force production in women were generally smaller, and the difference in %Δ change between men and women was significant at post in ES (P < .05). In women, a significant decrease in maximal force production (14.4% ± 7.9%, P < .01) was observed at mid only after S of SE. At post, the cumulative decrease in force was 11.6% ± 12.0% (P < .05) in SE and 12.4% ± 9.3% (P < .01) in ES. In men, stride length shortened significantly from 1.26 ± 0.10 to 1.20 ± 0.13 m and from 1.27 ± 0.07 to 1.22 ± 0.09 m (4.1% ± 8.3% and 4.1% ± 4.3%, P < .05 for both; Figure 3) in ES and SE sessions, respectively, but stride rate remained statistically the same. No significant changes occurred in braking time, push-off time, or total contact time in either order of combined strengthand endurance-training sessions in men. In women, stride length remained unaltered for both orders of combined strength- and endurance-training sessions (from 1.05 ± 0.11 to 1.05 ± 0.11 m and 1.07 ± 0.11 to 1.05 ± 0.11 m, P > .05; Figure 3). In addition, running speed, stride rate, braking time, push-off, and total contact time remained statistically unaltered in both ES and SE sessions.
Running Economy When strength loading preceded endurance loading in men, oxygen demand did not increase significantly (0.6% ± 2.8%, P > .05) in SE; however, it increased significantly during the endurance loading in men performing the endurance loading before the strength
loading (2.4% ± 2.9%, P < .05 in ES; Figure 4). In women, oxygen demand did not increase significantly in either combined training session (SE 0.4% ± 1.7% and ES 0.5% ± 2.5%). In absolute terms, oxygen uptake was significantly greater in both men and women when strength preceded endurance in the current loadings (P < .01). Furthermore, oxygen uptake remained significantly increased in women when the current strength loading preceded the current endurance loading (P < .01).
Discussion The 2 combined strength- and endurance-training sessions (SE and ES) were similar in terms of duration, distance/volume, and intensity. Both combined sessions led to significant decreases in CMJ height and stride length in men but not in women. A significant decrease in maximal isometric force production was observed in both men and women, which was observed to be greater in men than in women. The combined strength- and endurance-training-session orders (SE or ES) had no effect on ground-reaction times including braking time, push-off time, and total contact time during the current endurance loading that included 60 minutes of running in either men or women. Heart rate and blood lactate increased slightly during the combined sessions, while performing the current strength loading before the current endurance loading increased oxygen uptake during the endurance loading in both men and women. CMJ height and stride length decreased significantly in men but not in women. While it appears that fast-force production decreased more in men than in women, maximal force production decreased in both men and women, but this decrease was significantly greater in men than it was in women. Women are known to be less fatigable than men,26,27,28 which may be linked, in part, to muscle-fiber distribution.29,30 Nevertheless, maximal and explosive strengthtraining sessions can induce significant acute fatigue, although the mechanisms (central or peripheral) behind fatigue differ to some
Figure 3 — Running stride length during the 60-minute endurance loading performed before or after a strength loading. *Significant change (P < .05) in Δ% in running stride length from preloading to postloading.
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Figure 4 — Average oxygen uptake during the 60-minute endurance loading (mL · kg–1 · min–1) performed before or after a strength loading (E). *, **Significant change (P < .05, P < .01) in running economy from pre-E. Bracketed asterisks, significant difference between men and women.
extent.26 Central factors including decreased recruitment of motor units and reduced firing frequency of active motor units15 may have contributed, at least in part, to fatigue in the current study. In addition, eccentric muscle actions such as those used in running may result in mechanical stress that results in muscle damage,39 thereby contributing to decreases in force production. The accumulated fatigue in the current study appeared to affect fast-force-production abilities similar to Lepers et al40 but did not cause any significant lengthening of contact times, as might have been expected.12,41 Running involves all major muscles and joints in the body34 and makes use of a repetitive stretch-shortening cycle4; however, endurance running typically requires less force production than strength exercises.1,2 At lower intensities of running, we did not expect large decreases in force production due to the current endurance-running loading.3 At the beginning of the stance phase in the running cycle, the quadriceps are the primary contributor to the backward and upward mass-center acceleration, while later in the stance phase, plantar flexors are responsible for acceleration.42 Running technique, of course, influences how the gluteal muscles and plantar flexors may be used by an individual. It has been reported that plantar flexors take care of 60% to 70% of running reaction forces,43 suggesting that the current strength loading, which focused on the quadriceps muscles, may have fatigued, to some extent, different muscles than those actually used in the current endurance loading. Previous research has shown that a 5-km time trial decreased strength and power of the lower extremities by 15%,6 while a 2-hour run decreased maximal isometric force of the quadriceps by 19%40 and longer/more-intense distances have caused still greater decreases in force production and muscle activation.7,8 These decreases in force-production capabilities are comparable to those after heavy strength loadings.24,25 The decreases in maximal force production in the current study are in line with the previously mentioned studies. Maximal force production decreased by 19% and 14% after the strength loading in men and women, respectively,
and by 8% and 7% after the endurance loading. Cumulative fatigue after both combined sessions, however, was ~20% in men and ~12% in women, regardless of order. While fatigue and a decrease in force-production capabilities are expected outcomes of physical exertion, fatigue negatively affects the protective neuromuscular mechanism of muscles and joints dispersing dynamic loads on the lower extremities.44 This fatigue allows for more injuries and possibly affects optimal training adaptations. While GRFs in the current study were not statistically altered, we did not measure and thus cannot be sure that coordination10 or other kinematic changes in terms of inversion/eversion did not occur with neuromuscular fatigue.11 It is, nevertheless, plausible that the running- and strength-loading-induced fatigue may have affected running biomechanics on level other than that measured, for example, the kinematics of the entire lower extremity.11 It is furthermore possible that muscle activation during running decreased, as well. Nummela et al,45 for example, reported that distance-running performance is affected by neural input, which may have been slightly altered in the current running loadings, especially when they were preceded by the current strength loading. Previous studies have shown that running at a slightly higher intensity (above anaerobic threshold) causes decreases in knee-extensor strength that appear to affect running mechanics.7,40 Decreased neural input and ability to produce force may affect running economy. In the current study, we observed that the increase in oxygen consumption in men coincided with decreased force production, but in women this was not the case. As an increase in oxygen demand or a decrease in running economy cannot be explained by running mechanics alone,41 excess postexercise oxygen consumption (EPOC) appears to be a logical explanation for the observed increase in oxygen consumption in both men and women. Resistance training has previously been shown to increase EPOC,46,47 which has been linked to mechanisms related to the biochemical processes resulting from an acute bout of strength or endurance training.46
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Conclusions In the current study, our hypothesis was that the first loading of the combined strength- and endurance-exercise sessions would induce a specific fatigue response that would differ in magnitude, thus affecting the subsequent exercise session. Both combined experimental strength- and endurance-training sessions led to significant decreases in force production including CMJ height and stride length in men, but not in women. No significant changes occurred in the ground-reaction times recorded during running, including braking time, push-off time, or total contact time, in either order of the combined sessions for men or women, although a significant decrease in accumulated maximal force production was observed at post in both men and women. No significant differences in heart rate and blood lactate were observed between training-session orders. With the current combined SE and ES sessions, there were no significant differences in the magnitude of accumulated fatigue in terms of force production, including GRFs, ground-reaction times, CMJ height, and maximal force production. Nevertheless, a significant increase in oxygen demand was observed when the current strength loading preceded the current endurance loading. The increase in oxygen demand observed during running may be interpreted as an expression of accumulated neuromuscular fatigue and thus a decrease in running economy, or it could be understood as a reflection of EPOC.46 Performing the current strength loading before the current endurance loading increased oxygen uptake during the endurance loading; however, the reverse order does not appear to affect accumulated neuromuscular-fatigue response to the current combined loadings.
Practical Applications The order of the current combined strength- and endurance-training sessions (ES or SE) appears to influence fatigue; however, these fatigue responses are somewhat larger in men than in women. Performing the current strength loading before the current endurance loading caused a greater need for oxygen during the endurance loading in both men and women, which may be related to decreases in force-production abilities, resulting in a decrease in stride length and kinematic changes, or it could simply be the result of strength-loading-induced EPOC. It is important to keep in mind that by changing the volume/intensity of a combined strength- and endurance-training session or by using different modes of endurance training (running vs cycling) or strength training (maximal vs hypertrophic vs explosive), the acute responses may be quite different. The current findings may be applied in planning training and are of greater interest when examined alongside the previously published studies from our group.48 Acknowledgments This study was completed at the Department of Biology of Physical Activity at the University of Jyväskylä (JYU) with assistance from the Research Institute for Olympic Sport (KIHU). Funding was provided by the Department of Biology of Physical Activity. The authors wish to thank Moritz Schumann (JYU) and Juha-Pekka Kulmala (JYU) for their assistance in data collection. In addition, the authors wish to thank the Department of Biology of Physical Activity’s technical staff (Pirkko Puttonen, Risto Puurtinen, Sirpa Roivas, and Markku Ruuskanen) and statistician (Elina Vaara), as well as technical staff from KIHU (Sirpa Vänttinen), for their
contributions to the completion of this study. The results of the current study do not constitute endorsement of the product by the authors or the journal. The first author had full access to all of the data in this study and takes full responsibility for their integrity and analysis.
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34. Tartaruga MP, Brisswalter J, Peyré-Tartaruga LA, et al. The relationship between running economy and biomechanical variables in distance runners. Res Q Exerc Sport. 2012;83(3):367–375. PubMed doi:10.1080/02701367.2012.10599870 35. Mikkola J, Rusko H, Nummela A, Pollari T, Häkkinen K. Concurrent endurance and explosive type strength training improves neuromuscular and anaerobic characteristics in young distance runners. Int J Sports Med. 2007;28(7):602–611. PubMed doi:10.1055/s-2007-964849 36. Meyer T, Lucia A, Earnest CP, Kindermann W. A conceptual framework for performance diagnosis and training prescription from submaximal gas exchange parameters—theory and application. Int J Sports Med. 2005;26(Supplement 1):S38–S48. PubMed doi:10.1055/s-2004-830514 37. Häkkinen K, Kallinen M, Izquierdo M, et al. Changes in agonist– antagonist EMG, muscle CSA, and force during strength training in middle-aged and older people. J Appl Physiol. 1998;84(4):1341–1349. PubMed 38. Komi PV, Bosco C. Utilization of stored elastic energy in leg extensor muscles by men and women. Med Sci Sports. 1978;10(4):261–265. PubMed 39. Bell GJ, Syrotuik D, Martin TP, Burnham R, Quinney HA. Effect of concurrent strength and endurance training on skeletal muscle properties and hormone concentrations in humans. Eur J Appl Physiol. 2000;81(5):418–427. PubMed doi:10.1007/s004210050063 40. Lepers R, Pousson M, Maffiuletti N, Martin A, Van Hoecke J. The effects of a prolonged running exercise on strength characteristics. Int J Sports Med. 2000;21(4):275–280. PubMed doi:10.1055/s-2000-308 41. Kyröläinen H, Pullinen T, Candau R, Avela J, Huttunen P, Komi P. Effects of marathon running on running economy and kinematics. Eur J Appl Physiol. 2000;82(4):297–304. PubMed doi:10.1007/ s004210000219 42. Hamner SR, Delp SL. Muscle contributions to fore–aft and vertical body mass center accelerations over a range of running speeds. J Biomech. 2013;46(4):780–787. PubMed 43. Dorn TW, Schache AG, Pandy MG. Muscular strategy shift in human running: dependence of running speed on hip and ankle muscle performance. J Exp Biol. 2012;215(Pt 11):1944–1956. PubMed doi:10.1242/ jeb.064527 44. Radin EL. Role of muscles in protecting athletes from injury. Acta Med Scand Suppl. 1986;711:143–147. PubMed doi:10.1111/j.0954-6820.1986.tb08943.x 45. Nummela AT, Paavolainen LM, Sharwood KA, Lambert MI, Noakes TD, Rusko HK. Neuromuscular factors determining 5 km running performance and running economy in well-trained athletes. Eur J Appl Physiol. 2006;97(1):1–8. PubMed doi:10.1007/s00421-006-0147-3 46. Børsheim E, Bahr R. Effect of exercise intensity, duration and mode on post-exercise oxygen consumption. Sports Med. 2003;33(14):1037– 1060. PubMed doi:10.2165/00007256-200333140-00002 47. Burleson MA, Jr, O’Bryant HS, Stone MH, Collins MA, TriplettMcBride T. Effect of weight training exercise and treadmill exercise on post-exercise oxygen consumption. Med Sci Sports Exerc. 1998;30(4):518–522. PubMed doi:10.1097/00005768-19980400000008 48. Taipale RS, Häkkinen K. Acute hormonal and force responses to combined strength and endurance loadings in men and women: the “order effect.” PLoS ONE. 2013;8(2):e55051. PubMed doi:10.1371/ journal.pone.0055051
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Original Investigation
Combined Strength and Endurance Session Order: Differences in Force Production and Oxygen Uptake Ritva S. Taipale, Jussi Mikkola, Ari T. Nummela, Juha Sorvisto, Kai Nyman, Heikki Kyröläinen, and Keijo Häkkinen Purpose: To examine acute responses of force production and oxygen uptake to combined strength (S) and endurance-running (E) loading sessions in which the order of exercises is reversed (ES vs SE). Methods: This crossover study design included recreationally endurance-trained men and women (age 21–45 y; n = 12 men, 10 women) who performed ES and SE loadings. Force production of the lower extremities including countermovement-jump height (CMJ) and maximal isometric strength (MVC) was measured pre-, mid-, and post-ES and -SE, and ground-reaction forces, ground-reaction times, and running economy were measured during E. Results: A significant decrease in CMJ was observed after combined ES and SE in men (4.5% ± 7.0% and 6.6% ± 7.7%, respectively) but not in women (0.2% ± 8.5% and 1.4% ± 7.3% in ES and SE). MVC decreased significantly in both men (20.7% ± 6.1% ES and 19.3% ± 9.4% SE) and women (12.4% ± 9.3% ES and 11.6% ± 12.0% SE). Stride length decreased significantly in ES and SE men, but not in women. No changes were observed in ground-reaction times during running in men or women. Performing S before E caused greater (P < .01) oxygen uptake during running in both men and women than if E was performed before S, although heart rate and blood lactate were similar between ES and SE. Conclusions: Performing S before E increased oxygen uptake during E, which is explained, in part, by a decrease in MVC in both men and women, decreased CMJ and stride length in men, and/or an increase in postexercise oxygen consumption. Keywords: postexercise oxygen consumption, combined strength and endurance training, biomechanics and kinematics of running The neuromuscular responses to endurance exercise sessions/ loadings are sport specific and depend on movement type, while the magnitude of fatigue depends on the intensity and/or duration of the endurance-exercise session/loading. One of the most popular forms of endurance exercise is running, which typically involves a lesser amount of force production than strength exercises due to distinct differences in force requirements.1 Even running uphill at maximal speed does not provoke maximal activation of the muscles,2 and only slight acute decreases in strength and neural activation are observed after a typical endurance-running session.3 Running, however, does demand repetitive force production and utilizes the stretch-shortening cycle,4 suggesting that races and long distances like the marathon can induce a significant amount of neuromuscular fatigue.5 This fatigue may be characterized by significant decreases in, for example, maximal force production and muscle activation of the lower extremities. For example, after a 5-km time trial, a 15% decrease in maximal force has been observed,6 while 30 km of high-intensity trail running and a simulated marathon caused 24% and 22% decreases in maximal force, respectively.7,8 This neuromuscular fatigue may have an impact on running-specific force-production abilities including decreased reaction times9 and a reduction in coordination.10 As such, fatigue may also alter the mechanics and kinematics of running.11,12 Significant fatigue may even lead to changes in muscle-activation patterns such as those Taipale, Sorvisto, Kyröläinen, and Häkkinen are with the Dept of Biology of Physical Activity, University of Jyväskylä, Jyväskylä, Finland. Mikkola and Nummela are with the KIHU Research Inst for Olympic Sport, Jyväskylä, Finland. Nyman is with the Central Hospital of Central Finland (KSSHP), Jyväskylä, Finland. Address author correspondence to Ritva Taipale at [email protected]. 418
observed in landing tasks while also reducing joint stabilization,13 which may result in overuse or stress, causing injuries. The fatigue observed in neuromuscular function during running may parallel or reflect changes that occur in cardiorespiratory factors like running economy and heart rate.14 Whether the fatigue is the result of central15 or peripheral16 factors may also play a role in understanding its duration and the time course of recovery, which may assist in planning more effective training programs. Both maximal and explosive strength training are recognized as a necessary component of endurance training17–20 and have been shown to effectively enhance running performance by improving speed and running economy.17,21 Strength training improves fastforce production, which is an important component of peak running speed and running economy,22 while explosive jumping ability has been shown to correlate significantly with both middle- and longdistance running performance.23 Acute decreases in maximal force production after a strength-training session may vary 10% to 25% in men and ~20% in women.24,25 In general, women appear to be less fatigable than men,26,27,28 and this difference may be related, in part, to muscle fiber-type distribution and may influence fatigue mechanisms.29,30 A typical strength-training session generally requires greater force production than an endurance-running session, although as already noted, high-intensity running (time trials/ races) and long distances may induce decreases in maximal force production that are similar to decreases in maximal force production observed immediately after a strength-training session. The divergent responses and adaptations to strength and endurance exercises and training31,32 suggest that training needs to be carefully planned for optimal results, even in recreational endurance runners. As part of an optimal training regimen, or to save time, recreational and elite athletes may perform strength and endurance exercise in the same combined training session, but it
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is unclear whether strength or endurance exercise should be performed first. The purpose of this study was to examine the acute effects of 2 different orders of combined strength and endurance training performed in a single training session on force production of the lower extremities. We measured maximal isometric strength and countermovement-jump height (CMJ) before, during, and after combined sessions, as well as ground-reaction forces (GRF), ground-reaction times, and economy during the endurance-running loading. The combined strength and endurance sessions used in the current study are representative of typical training sessions that a recreational athlete might undertake. Using a cross-sectional design, we hypothesized that the first loading of the combined exercise sessions would induce specific fatigue responses that might differ in magnitude, thus affecting the subsequent exercise session.33 More specifically, we expected that a strength-training session performed before an endurance-training session would cause impairments in force production during running, including GRFs and groundreaction times, as well as running economy.
Methods Subjects Recreationally endurance-trained subjects (age 21–45 y; n = 12 men n = 10 women) were recruited to participate in this study (Table 1). Subjects regularly participated in endurance activities (primarily running) at least 3 times per week but had no systematic background in strength training. Exclusion criteria included body-mass index >28 kg/m2, illness, disease, injury, or use of medications that would contraindicate participation in the study. Subjects received specific written and oral information about the study design and measurement procedures and were informed of the possible risks and benefits of participation in the study before they signed an informed-consent document. Each subject had a resting electrocardiogram and health-history questionnaire reviewed and approved by a qualified physician before participation in the study. Ethical approval was granted by the university ethical committee, and the study was conducted according to the provisions of the most recent Declaration of Helsinki.
Design Before the combined experimental training sessions, body composition, height, and body mass were measured (Table 1). Subjects then completed a set of tests to become familiarized with the strength-training equipment and to determine appropriate loads and intensities for the specific loadings. These tests included 1-repetition maximum using a leg-press device (David 210 dynamometer,
David Sports Ltd, Helsinki, Finland) and a squat rack (Kraftwerk, Hot Milling Oy, Tuusula, Finland), as well as CMJ and maximal bilateral isometric leg press. A maximal treadmill running test was used to determine maximal oxygen uptake (VO2max). After completion of these preliminary tests, subjects performed, in random order, 2 combined strength- and endurance-training sessions. One training session started with and endurance-exercise session (loading) that included running for 60 minutes on an indoor track at a moderate intensity, which was immediately followed by a strength-exercise session (loading) that included a mixture of maximal and explosive exercises for the lower extremities (ES). The other combined training session started with the strength loading, which was immediately followed by the endurance loading (SE). Possible time-of-day variations in neuromuscular and cardiorespiratory performance were controlled for by ensuring that each subject performed the experimental training sessions ±1 hour from the time of his or her preliminary testing (Figure 1). The current combined strength and endurance sessions are representative of typical training sessions that a recreational athlete might undertake and thus were planned to be fatiguing but not too exhaustive. In addition, the current study design tested runners at the same physiological intensities34 rather than at a fixed running speed for all subjects to help control for individual fitness level. CMJ and maximal bilateral isometric force production were measured before, during, and after each combined experimental training session, whereas GRFs and ground-reaction times, oxygen uptake, and blood lactate were measured before and after the endurance loading (pre-E and post-E). Oxygen consumption was measured breath by breath at the beginning and end of the endurance loading. Percent VO2max and oxygen demand/running economy were calculated from oxygen uptake, while heart rate was recorded continuously throughout the endurance session. The endurance loading included running on a 200-m indoor track for a total of 60 minutes. The intensity of running was at a steady-state speed between each subject’s previously determined individual lactate threshold and respiratory-compensation threshold. This specific running speed for each subject was guided by a so-called “light rabbit” calibrated lamp system (Protom, Naakka, Finland) installed in the edge of the indoor running track. The average intensity of running was ~80% of VO2max. The average intensity in terms of VO2 during running was determined from expired gases during the first and last 10 minutes of the endurance loading. The percentage of VO2max was calculated for each runner based on his
Table 1 Subject Characteristics, Mean ± SD Age (y) Height (cm) Body mass (kg) Body fat (%) VO2max (L/min) VO2max (mL ∙ kg–1 ∙ min–1)
Men (n = 12) 38.8 ± 7.1 177.4 ± 6.4 75.7 ± 3.6 12.9 ± 3.6 4.1 ± 0.3 54.5 ± 4.0
Abbreviation: VO2max, maximal oxygen uptake.
Women (n = 10) 33.5 ± 8.3 165.9 ± 7.6 59.8 ± 5.1 22.0 ± 3.8 2.9 ± 0.4 48.5 ± 4.6
Figure 1 — Study design. Arrows indicate measurement time points where countermovement jump and maximal isometric strength were measured. Stars indicate the measurement points for ground-reaction forces including running speed, stride length, stride rate, braking time, push-off, and total contact times and measurement of oxygen consumption. Heart rate and blood lactate measurements were recorded at 4 intervals throughout the 60-minute endurance-running loading.
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or her VO2max test completed on a treadmill. Running distance was calculated based on laps. The strength loading focused primarily on the lower extremities and included both maximal and explosive strength exercises (Table 2). Strength exercises were performed in a circuit in which the exercise changed after every set. The total duration of strengthexercise sessions was approximately 45 minutes. The 2 combined experimental training sessions were similar in terms of volume and intensity (Table 3). No significant differences were observed in running distance, speed, % of VO2max, or heart rate between the 2 combined strength- and endurance-training sessions in men or in women. Heart rate and blood lactate (Table 3) increased gradually over the course of the endurance-training session. A significant difference (P < .05) was observed between blood lactate concentrations during the endurance loadings of the combined strength- and endurancetraining sessions. Lactate concentrations were higher in ES than SE at the first 2 measurement points.
Methodology Body Composition. In addition to standing height, body mass and
body composition were measured using bioimpedance (InBody720
body-composition analyzer, Biospace Co Ltd, Seoul, South Korea). Measurements were always taken between 7:30 and 8:00 AM. Subjects were instructed to arrive for testing in a fasted state, to help keep the possible confounding variables of diet and hydration status to a minimum. Subjects were measured in their undergarments. Aerobic Capacity. Endurance performance characteristics were
measured using a treadmill running protocol.35 Running started at the speed of 8 km/h and was increased by 1 km/h every 3 minutes until volitional exhaustion. Treadmill incline remained a constant 0.5° throughout the test. Heart rate was recorded continuously using a heart-rate monitor (Suunto t6, Vantaa, Finland). Mean heart-rate values from the last minute of each stage were used for analysis. Oxygen consumption was measured breath by breath throughout the test using a portable gas analyzer (Oxycon Mobile, Jaeger, Hoechberg, Germany). VO2max was determined to the highest average of 60-second VO2 values. Other factors such as heart rate, VO2, and respiratory-exchange ratio were monitored for determination of maximal effort. Fingertip blood samples were taken every 3 minutes to measure blood lactate concentrations. For blood sampling, the treadmill was stopped for approximately 15 to 20 seconds. Blood lactates were analyzed using a Biosen S_line Laboratory+ lactate analyzer (EKF Diagnostic, Magdeburg, Germany). Lactate threshold
Table 2 Strength Loading Performed in a Circuit in Which the Exercise Changed After Every Set Exercise Maximal bilateral leg press Explosive bilateral leg press Squat Squat jump Calf raise
Sets 3 3 3 3 2
Loads (%1-repetition maximum) 70–85% 30–40% 70–85% 30–40% 70–85%
Repetitions 5–8 8–10 5–8 8–10 5–8
Rest (min) 2 2 2 2 2
Table 3 Endurance Loadings of Running Distance and Speed, as Well as Percentages of Maximal Speed, Oxygen Uptake, and Heart Rate for Endurance Loadings Performed Either Before or After Strength Loading, Mean ± SD Men
Running distance (km) Running speed (km/h) Speed (%max) Oxygen uptake (%max) Heart rate (%max) Heart rate (beats/min) start 15 min 45 min end Blood lactate (mmol/L) start 15 min 45 min end
Women
Endurance before strength 11.6 ± 0.9 12.2 ± 1.0 76.9 ± 1.8 81.1 ± 4.1 87.7 ± 4.1
Strength before endurance 11.7 ± 1.0 12.2 ± 1.0 76.9 ± 1.8 83.1 ± 5.2 86.7 ± 4.8
Endurance before strength 10.3 ± 0.9 10.7 ± 0.9 76.2 ± 2.7 79.5 ± 5.8 86.8 ± 5.2
Strength before endurance 10.3 ± 0.8 10.7 ± 0.9 76.2 ± 2.7 81.2 ± 5.8 84.7 ± 4.9
154 ± 11 153 ± 11 159 ± 10 162 ± 10
151 ± 8 150 ± 9 157 ± 10 160 ± 10
160 ± 17 159 ± 16 164 ± 16 167 ± 17
159 ± 13 155 ± 16 161 ± 15 166 ± 12
1.2 ± 0.5* 2.7 ± 1.0* 2.5 ± 0.9 2.5 ± 1.1
0.8 ± 0.2* 2.2 ± 0.5* 2.1 ± 0.9 1.9 ± 0.7
1.0 ± 0.5 2.1 ± 1.1 2.3 ± 1.1 2.2 ± 1.4
1.1 ± 0.5 1.7 ± 0.7 1.8 ± 0.6 1.6 ± 0.5
Note: No significant differences between heart rates were observed between the current orders of combined strength- and endurance-training sessions. *Significant difference (P < .05) between blood lactate concentrations during the loadings of the combined strength- and endurance-training sessions. IJSPP Vol. 10, No. 4, 2015
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and respiratory-compensation threshold were determined using blood lactate, ventilation, VO2, and VCO2 (production of carbon dioxide) according to Meyer et al.36 Ground-Reaction Forces. Force platforms (total length 9 m, TR-test, Jyväskylä, Finland, natural frequency ≥170 Hz, nonlinearity ≤1%, cross talk ≤2%) were used to record GRFs, which were analyzed using a Signal Software (v 4.1, Cambridge Electronic Design, UK). GRF data were first low-pass filtered using a fourthorder Butterworth filter with cutoff frequency of 50 Hz. Then custom-written script was used to analyze stride rate and length from ground-contact times based on 60-N vertical-force thresholds, as well as the duration of the braking and propulsion phases and the peak values of the vertical and horizontal forces. Loading rate was calculated as the total change in vertical force divided by the total change in time period between 60 N and individual subject body weight (in Newtons).
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Isometric Leg-Press Force. An electromechanical isometric
leg-press device (designed and manufactured by the Department of Biology of Physical Activity, University of Jyväskylä, Finland) was used to measure maximal bilateral isometric strength (maximum voluntary contractions [MVC]) and average force over the first 500 milliseconds of MVC in a horizontal leg-press position. The subject’s knee angle was set at 107° while the hip angle was 110°. Subjects were instructed to produce force “as fast and as hard as possible” for approximately 3 seconds. Subjects performed at least 3 MVCs.37 If the maximum force during the last trial differed more than 5% from the previous trial, an additional trial was performed. The best performance was used for statistical analysis. During the experimental sessions, maximal force was measured only 2 times with a 15-second interval of rest to record acute fatigue. Countermovement Jump. A force platform (Department of Biology of Physical Activity, Jyväskylä, Finland) was used to measure maximal dynamic explosive force by CMJ height. 38
Subjects were instructed to stand with their feet approximately hip width apart and hands on their hips. They were then instructed to perform a quick and explosive CMJ on verbal command so that knee angle for the jump was no less than 90°. Force data were collected and analyzed by computer software (Signal 2.14, CED, Cambridge, UK), which used the equation h = I2 ∙ 2gm2 to calculate jump height from impulse, where I = impulse, g = gravity, and m = mass of subject.
Statistical Analysis Standard statistical methods were used for calculation of means, standard deviations (SD), and Pearson product–moment correlation coefficients. Group differences and group-by-loading interaction were analyzed by a repeated analysis using mixed models and an unstructured covariance matrix. Groups were compared with leastsignificant-difference post hoc analysis in a mixed-models analysis when appropriate. The criterion for significance was set at *P = .05, **P < .01, and ***P < .001. Statistical analysis was completed with PASW Statistics 18 (SPSS Inc, Chicago, IL, USA).
Results Force Production CMJ height decreased significantly from baseline (from 33.4 ± 5.1 cm to 32.3 ± 4.8 cm, P < .01) at mid-SE but not ES (from 33.2 ± 4.9 cm to 32.8 ± 4.1 cm, P > .05; Figure 2) in men. The significant decrease in jumping height after combined ES and SE sessions was similar between ES (down to 31.8 ± 5.0 cm, P < .05) and SE (down to 31.5 ± 4.2 cm, P < .01). CMJ height did not change significantly in women over the combined strength and endurance sessions (from 23.0 ± 3.5 cm to 23.2 ± 4.5 cm and 22.7 ± 3.7 cm to 22.4 ± 4.5 cm in ES and SE, respectively).
Figure 2 — Countermovement-jump height during the combined strength- and endurance-training sessions (Δ%) performed before or after a strength loading (E). *, **Significant change (P < .05, P < .01) from pre-E. ++Significant change (P < .01) from mid-E. Bracketed asterisks, significant difference between men and women (P < .05). IJSPP Vol. 10, No. 4, 2015
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Maximal bilateral isometric force before the combined training sessions was 2757 ± 371 N and 2807 ± 485 N in ES and SE, respectively, in men. In women, maximal bilateral isometric force before ES and SE was 1808 ± 423 N and 1811 ± 429 N, respectively. Maximal bilateral isometric force decreased significantly more (P < .01) at mid in men after S of SE (18.9% ± 9.2%, P < .001) than after E of ES (8.1% ± 7.3%, P < .01). At post, the cumulative force decrease was 19.3% ± 9.4% (P < .001) in SE and 20.7% ± 6.1% (P < .001) in ES. Decreases in force production in women were generally smaller, and the difference in %Δ change between men and women was significant at post in ES (P < .05). In women, a significant decrease in maximal force production (14.4% ± 7.9%, P < .01) was observed at mid only after S of SE. At post, the cumulative decrease in force was 11.6% ± 12.0% (P < .05) in SE and 12.4% ± 9.3% (P < .01) in ES. In men, stride length shortened significantly from 1.26 ± 0.10 to 1.20 ± 0.13 m and from 1.27 ± 0.07 to 1.22 ± 0.09 m (4.1% ± 8.3% and 4.1% ± 4.3%, P < .05 for both; Figure 3) in ES and SE sessions, respectively, but stride rate remained statistically the same. No significant changes occurred in braking time, push-off time, or total contact time in either order of combined strengthand endurance-training sessions in men. In women, stride length remained unaltered for both orders of combined strength- and endurance-training sessions (from 1.05 ± 0.11 to 1.05 ± 0.11 m and 1.07 ± 0.11 to 1.05 ± 0.11 m, P > .05; Figure 3). In addition, running speed, stride rate, braking time, push-off, and total contact time remained statistically unaltered in both ES and SE sessions.
Running Economy When strength loading preceded endurance loading in men, oxygen demand did not increase significantly (0.6% ± 2.8%, P > .05) in SE; however, it increased significantly during the endurance loading in men performing the endurance loading before the strength
loading (2.4% ± 2.9%, P < .05 in ES; Figure 4). In women, oxygen demand did not increase significantly in either combined training session (SE 0.4% ± 1.7% and ES 0.5% ± 2.5%). In absolute terms, oxygen uptake was significantly greater in both men and women when strength preceded endurance in the current loadings (P < .01). Furthermore, oxygen uptake remained significantly increased in women when the current strength loading preceded the current endurance loading (P < .01).
Discussion The 2 combined strength- and endurance-training sessions (SE and ES) were similar in terms of duration, distance/volume, and intensity. Both combined sessions led to significant decreases in CMJ height and stride length in men but not in women. A significant decrease in maximal isometric force production was observed in both men and women, which was observed to be greater in men than in women. The combined strength- and endurance-training-session orders (SE or ES) had no effect on ground-reaction times including braking time, push-off time, and total contact time during the current endurance loading that included 60 minutes of running in either men or women. Heart rate and blood lactate increased slightly during the combined sessions, while performing the current strength loading before the current endurance loading increased oxygen uptake during the endurance loading in both men and women. CMJ height and stride length decreased significantly in men but not in women. While it appears that fast-force production decreased more in men than in women, maximal force production decreased in both men and women, but this decrease was significantly greater in men than it was in women. Women are known to be less fatigable than men,26,27,28 which may be linked, in part, to muscle-fiber distribution.29,30 Nevertheless, maximal and explosive strengthtraining sessions can induce significant acute fatigue, although the mechanisms (central or peripheral) behind fatigue differ to some
Figure 3 — Running stride length during the 60-minute endurance loading performed before or after a strength loading. *Significant change (P < .05) in Δ% in running stride length from preloading to postloading.
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Figure 4 — Average oxygen uptake during the 60-minute endurance loading (mL · kg–1 · min–1) performed before or after a strength loading (E). *, **Significant change (P < .05, P < .01) in running economy from pre-E. Bracketed asterisks, significant difference between men and women.
extent.26 Central factors including decreased recruitment of motor units and reduced firing frequency of active motor units15 may have contributed, at least in part, to fatigue in the current study. In addition, eccentric muscle actions such as those used in running may result in mechanical stress that results in muscle damage,39 thereby contributing to decreases in force production. The accumulated fatigue in the current study appeared to affect fast-force-production abilities similar to Lepers et al40 but did not cause any significant lengthening of contact times, as might have been expected.12,41 Running involves all major muscles and joints in the body34 and makes use of a repetitive stretch-shortening cycle4; however, endurance running typically requires less force production than strength exercises.1,2 At lower intensities of running, we did not expect large decreases in force production due to the current endurance-running loading.3 At the beginning of the stance phase in the running cycle, the quadriceps are the primary contributor to the backward and upward mass-center acceleration, while later in the stance phase, plantar flexors are responsible for acceleration.42 Running technique, of course, influences how the gluteal muscles and plantar flexors may be used by an individual. It has been reported that plantar flexors take care of 60% to 70% of running reaction forces,43 suggesting that the current strength loading, which focused on the quadriceps muscles, may have fatigued, to some extent, different muscles than those actually used in the current endurance loading. Previous research has shown that a 5-km time trial decreased strength and power of the lower extremities by 15%,6 while a 2-hour run decreased maximal isometric force of the quadriceps by 19%40 and longer/more-intense distances have caused still greater decreases in force production and muscle activation.7,8 These decreases in force-production capabilities are comparable to those after heavy strength loadings.24,25 The decreases in maximal force production in the current study are in line with the previously mentioned studies. Maximal force production decreased by 19% and 14% after the strength loading in men and women, respectively,
and by 8% and 7% after the endurance loading. Cumulative fatigue after both combined sessions, however, was ~20% in men and ~12% in women, regardless of order. While fatigue and a decrease in force-production capabilities are expected outcomes of physical exertion, fatigue negatively affects the protective neuromuscular mechanism of muscles and joints dispersing dynamic loads on the lower extremities.44 This fatigue allows for more injuries and possibly affects optimal training adaptations. While GRFs in the current study were not statistically altered, we did not measure and thus cannot be sure that coordination10 or other kinematic changes in terms of inversion/eversion did not occur with neuromuscular fatigue.11 It is, nevertheless, plausible that the running- and strength-loading-induced fatigue may have affected running biomechanics on level other than that measured, for example, the kinematics of the entire lower extremity.11 It is furthermore possible that muscle activation during running decreased, as well. Nummela et al,45 for example, reported that distance-running performance is affected by neural input, which may have been slightly altered in the current running loadings, especially when they were preceded by the current strength loading. Previous studies have shown that running at a slightly higher intensity (above anaerobic threshold) causes decreases in knee-extensor strength that appear to affect running mechanics.7,40 Decreased neural input and ability to produce force may affect running economy. In the current study, we observed that the increase in oxygen consumption in men coincided with decreased force production, but in women this was not the case. As an increase in oxygen demand or a decrease in running economy cannot be explained by running mechanics alone,41 excess postexercise oxygen consumption (EPOC) appears to be a logical explanation for the observed increase in oxygen consumption in both men and women. Resistance training has previously been shown to increase EPOC,46,47 which has been linked to mechanisms related to the biochemical processes resulting from an acute bout of strength or endurance training.46
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Conclusions In the current study, our hypothesis was that the first loading of the combined strength- and endurance-exercise sessions would induce a specific fatigue response that would differ in magnitude, thus affecting the subsequent exercise session. Both combined experimental strength- and endurance-training sessions led to significant decreases in force production including CMJ height and stride length in men, but not in women. No significant changes occurred in the ground-reaction times recorded during running, including braking time, push-off time, or total contact time, in either order of the combined sessions for men or women, although a significant decrease in accumulated maximal force production was observed at post in both men and women. No significant differences in heart rate and blood lactate were observed between training-session orders. With the current combined SE and ES sessions, there were no significant differences in the magnitude of accumulated fatigue in terms of force production, including GRFs, ground-reaction times, CMJ height, and maximal force production. Nevertheless, a significant increase in oxygen demand was observed when the current strength loading preceded the current endurance loading. The increase in oxygen demand observed during running may be interpreted as an expression of accumulated neuromuscular fatigue and thus a decrease in running economy, or it could be understood as a reflection of EPOC.46 Performing the current strength loading before the current endurance loading increased oxygen uptake during the endurance loading; however, the reverse order does not appear to affect accumulated neuromuscular-fatigue response to the current combined loadings.
Practical Applications The order of the current combined strength- and endurance-training sessions (ES or SE) appears to influence fatigue; however, these fatigue responses are somewhat larger in men than in women. Performing the current strength loading before the current endurance loading caused a greater need for oxygen during the endurance loading in both men and women, which may be related to decreases in force-production abilities, resulting in a decrease in stride length and kinematic changes, or it could simply be the result of strength-loading-induced EPOC. It is important to keep in mind that by changing the volume/intensity of a combined strength- and endurance-training session or by using different modes of endurance training (running vs cycling) or strength training (maximal vs hypertrophic vs explosive), the acute responses may be quite different. The current findings may be applied in planning training and are of greater interest when examined alongside the previously published studies from our group.48 Acknowledgments This study was completed at the Department of Biology of Physical Activity at the University of Jyväskylä (JYU) with assistance from the Research Institute for Olympic Sport (KIHU). Funding was provided by the Department of Biology of Physical Activity. The authors wish to thank Moritz Schumann (JYU) and Juha-Pekka Kulmala (JYU) for their assistance in data collection. In addition, the authors wish to thank the Department of Biology of Physical Activity’s technical staff (Pirkko Puttonen, Risto Puurtinen, Sirpa Roivas, and Markku Ruuskanen) and statistician (Elina Vaara), as well as technical staff from KIHU (Sirpa Vänttinen), for their
contributions to the completion of this study. The results of the current study do not constitute endorsement of the product by the authors or the journal. The first author had full access to all of the data in this study and takes full responsibility for their integrity and analysis.
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