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Influence of Length-Restricted Strength Training on Athlete's Power-Load Curves of Knee Extensors and Flexors

Ullrich, Boris1; Kleinöder, Heinz2; Brüggemann, Gert-Peter1

Journal of Strength and Conditioning Research: March 2010 - Volume 24 - Issue 3 - p 668-678
doi: 10.1519/JSC.0b013e3181c088ce
Original Research
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Ullrich, B, Kleinöder, H, and Brüggemann, G-P. Influence of length-restricted strength training on athlete's power-load curves of knee extensors and flexors. J Strength Cond Res 24(3): 668-678, 2010-This study investigated whether different length-restricted strength training regimens affect voluntary explosive concentric power-load curves of the quadriceps femoris (QF) and hamstring (HAM) muscles. Thirty-two athletes were divided into 3 different training groups (G1-G3): G1 performed isometric training at knee joint angles corresponding to long muscle-tendon unit (MTU) length for QF and HAM; G2 conducted concentric-eccentric contraction cycles that were restricted to a knee joint range of motion corresponding to predominantly long MTU length for QF and HAM; and G3 combined the protocols of G1 and G2. Knee joint angle-dependent power-load curves during maximal voluntary explosive concentric knee extensions and flexions were measured for loads corresponding to 40, 60, and 80% of individual 1 repetition maximum at 5 different occasions: 2 times before, after 5 and 8 weeks of training, and 4 weeks post training. Power values of each subject were normalized to the largest value produced at any knee joint position (percent maximum). Obtained by curve fitting, the optimal knee joint angle for power production of QF and HAM remained unaltered throughout the course of the study for all testing loads and training groups. Therefore, different strength training regimens with a common restriction to long MTU lengths failed to induce length-dependent alterations in athlete's voluntary concentric power-load curves of knee extensors and flexors. The approach to develop strength training programs that induce systematic shifts in length-dependent power production of QF and HAM is of direct practical relevance for athletic activities such as cycling, ice skating, and skiing. However, restricting the muscle excursion range during loading seems to be an inappropriate trigger to cause length-dependent alterations in athlete's voluntary concentric power-load curves.

1Institute of Biomechanics and Orthopaedics; and 2Institute of Training Science and Sport Informatics, German Sport University Cologne, Cologne, Germany

Address correspondence to Boris Ullrich, boris_U@gmx.de.

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Introduction

Findings from animal (8,9,21) and human in vivo studies (5,12,27) strongly suggest that length-dependent force generation capacities are not a constant property but can adapt to specific loading imposed through long-term athletic activities or strength training interventions. A key mechanism to explain these alterations seems to be the muscle's ability to adjust the number of serial sarcomeres (9,21). Therefore, muscle architecture is highly plastic and direct influences force-length properties (5). However, despite considerable research, the most important trigger mechanism for adaptations in force-length properties remains unclear. Although some studies suggested contraction mode being a major trigger for architectural changes that underpin force-length adaptations (9,21,32), others failed to prove this hypothesis (5,19). For example, recent findings of Blazevich et al. (5) proposed the predominant muscle excursion length during loading rather than contraction mode being a major trigger for alterations in human force-length properties. Most human in vivo studies used isometric joint angle measurements (12,17,27,30,31) or relatively slow isovelocity contractions (7,30,32) to investigate alterations in length-dependent force generation capacities. These approaches enabled force measurements under highly controlled conditions with respect to the muscle's activation, length, and speed of contraction (12,30,31).

Importantly, many athletic activities involve movements that require generation of force over short periods (11,14,22). In these situations, muscular power, that is, the product of voluntary rapid force production and velocity, is regarded as a key determinant of performance (11,16). Although it would be of direct practical relevance for athletic performance (30) and the prevention of injuries (7,13), no study investigated to date if length-restricted strength training exercise can induce shifts in athlete's power-load curves of knee extensors and flexors. For example, in physical activities such as cycling, ice skating, and skiing and also during change of direction movements in many team games, high amounts of knee extensor muscular power are needed at more flexed knee joint positions (12,27,30). Additionally, hamstring muscles are prone to injuries when lengthened during rapid knee extensions (4,13). Therefore, a higher contractive capacity of these muscle groups at longer muscle-tendon unit (MTU) length might enhance athletic performance and reduce the risk of injuries.

There is evidence in the literature that strength training exercise affects human force-length properties (5,7,17). For example, joint angle-specific isometric training produced significant strength increases restricted to MTU lengths around the training angle (18). Additionally, eccentric exercise shifted the optimal joint angle for moment production in the direction of longer MTU length and this was suggested to be attributable to an increased number of serial sarcomeres (7,23,30). However, a recent strength training study strongly indicates that the predominant muscle excursion length during loading has to be considered as a key trigger of human force-length adaptations (5). This hypothesis is in line with Herzog et al. (12) who presented evidence on that human muscles can adapt to the length they are most frequently confronted with.

This study examined if different strength training regimens that were restricted to knee joint angles corresponding to predominantly long MTU length for quadriceps femoris (QF) and hamstring (HAM) can shift voluntary explosive concentric power-load curves of knee extensors and flexors in the direction of the training length. Additionally, the time course of possible length-dependent adaptations was studied. Therefore, knee joint angle-dependent explosive concentric power-load curves of knee extensors and flexors were determined at different occasions in a sample of athletes who performed a supervised strength training period. Testing loads corresponding to 40, 60, and 80% of individual 1 repetition maximum (1 RPM) were used because optimal power production in athlete's lower extremity muscles was reported to occur within this range of loads (16). Quadriceps femoris and HAM were chosen because these muscle groups play an important role in many athletic activities and work as antagonists concerning knee joint movements (4,13,28).

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Methods

Experimental Approach to the Problem

The main purpose of this study was to examine if different strength training regimens that were restricted to knee joint angles corresponding to predominantly long MTU length for QF and HAM can shift athlete's voluntary explosive concentric power-load curves of knee extensors and flexors in the direction of the training length. This approach is of direct practical relevance for athletic activities that require high power production of the QF at more flexed knee angle positions (30) and furthermore might help to reduce the risk of hamstring injuries (4,13). For example, high amounts of knee extensor power production at more flexed knee joint positions are required during physical activities such as cycling, ice skating, or skiing and furthermore during change of direction movements in many team games (12,30). A longitudinal design was used to examine length-dependent alterations in athlete's power-load curves for QF and HAM throughout a controlled period of specific strength training exercises. As many athletic movements require rapid voluntary power production in the lower extremity (11,22), voluntary concentric power-load curves were measured and no isometric or isovelocity testing was performed. The total duration of the experimental period was 15 weeks. Before start of the study, 36 athletes were divided randomly into 3 different training groups (G1-G3). G1 (isometric training group) performed cyclic (hold and relax) isometric contractions at knee joint angles corresponding to long MTU length for QF and HAM; G2 (dynamic training group) conducted concentric-eccentric contraction cycles that were restricted to a knee joint range of motion (ROM) corresponding to predominantly long MTU length for QF and HAM; and G3 (combined training group) combined the training protocols of G1 and G2. For all training groups, the first 2 weeks were used as a control period during which time no strength training was carried out but subjects maintained their normal athletic activities. Thereafter, all subjects participated in a 9-week supervised strength training period for QF and HAM. Notably, subjects were allowed to maintain their sports-specific training sessions but performed strength training exercises for QF and HAM exclusively in the present study. Week 6 of the training period was exclusively used for testing (i.e., no training was carried out). Knee joint angle-dependent power-load curves during maximal voluntary explosive concentric knee extension and flexion contractions were measured for testing loads corresponding to 40, 60, and 80% of individual 1 RPM at 5 different occasions: 2 times during the control period (weeks −2, −1), during and directly after the end of the training period (weeks 6, 10), and 4 weeks post training (follow-up, week 14). During testing, randomization clarified if subjects started with either the knee extensions or knee flexions. Single-leg 1 RPM testing was conducted with the subject's stronger side using standardized methods (29). Afterward, individual testing loads corresponding to 40, 60, and 80% of 1 RPM were calculated. The stronger leg was identified in familiarization testing approximately 10 days before start of the study. The leg where the highest 1 RPM value for knee extensions occurred was defined as the stronger leg and used for testing. For each muscle group, body position of the subjects and knee joint angle ROM were the same during 1 RPM testing and power-load testing. During maximal voluntary explosive concentric knee extensions, subjects were seated upright with a constant hip joint angle of 105°. Knee joint angle ROM was set from 80° (starting position) to 140° (final position). During maximal voluntary explosive knee flexions, subjects lay prone with a constant hip joint angle of 170°. Knee joint angle ROM was set from 170° (starting position) to 100° (final position). Knee and hip joint angles of 180° correspond to full knee and hip extension, respectively. Subjects executed 2 trials of 3 consecutive single-leg concentric-eccentric power cycles for each testing load. The order of testing loads was clarified by randomization, and a 3-minute rest period was given between trials. HAM extensibility was evaluated with manual testing as described in a previous study (30). This was done to guarantee a high quality of measurements throughout the testing ROM and furthermore to prohibit risks of injury.

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Subjects

The investigation started with 36 young athletes (27 men and 9 women). To account for an equal number of women in each training group, 3 women were randomly assessed to each group. In the second step, 9 men were randomly divided into different training groups. Therefore, the present study started with 12 subjects in each training group, but 4 subjects suffered from injuries during the course of the study and in consequence were eliminated from the data pool. This explains why there is a different number of subjects among the training groups. Subjects were recruited from the university's elite sport teams in soccer, handball, basketball, field hockey, and tennis who regularly competed at the highest national university level. Additionally, most of the subjects were members of the clubs who competed at top regional or even second national division level. All subjects were older than 18 years (range 19-28 years) and performed in their physical activity in the previous 5 years. Anthropometric data during the course of the study are presented for all training groups in Table 1. Experimental training started in October and finished in December when most of the athletes were engaged in the first half of the winter (indoor) season of their respective sport. During the course of the investigation, all athletes maintained their sport-specific athletic training but performed QF and HAM strength training exclusively in the present study. During their sport-specific training sessions, subjects used approximately 3-4 hours per week for technical training issues and approximately 6-8 hours per week for competition-related training including training of team tactics. Additionally, subjects spent approximately 3-5 hours per week for physical training sessions such as aerobic endurance training, speed and agility exercises, and whole body strength training exercises that had nothing to do with the muscles that were examined in the present study. Overall training and competition volume (including the strength training sessions of the present study) ranged between 16 and 22 hours per week for all athletes during the course of the study. Notably, the overall training volume and training contents remained rather constant for all athletes throughout the time of the present investigation. All subjects had experience with strength training exercises for at least 4 years, but none of the athletes were engaged in systematic strength training for QF and HAM during the last 3 months before start of this study. All subjects gave their written informed consent to participate in the study, and furthermore, all subjects were informed of the experimental risks and signed an informed consent document before the investigation. The study was approved by the German Sport University of Cologne Ethics Committee for use of human subjects in research.

Table 1

Table 1

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Procedures

Knee Extensor Power-Load Curves

Subjects were seated upright, and their upper body was fixed to the chair of the testing apparatus with a rigid belt. The upper extremities were maintained in the crossed position during all contractions. To determine knee joint angle-dependent concentric power-load curves during explosive maximal voluntary knee extensions (maximum voluntary contraction), a traditional knee extensor strength training machine (Edition-Line; Gym80 International MbH, Gelsenkirchen, Germany) was armed with a 5 kN force sensor (Mechatronik MbH, Hamm, Germany) and a displacement transducer with an 1-mm accuracy (Model S501; Megatron Elektronik AG & Co., München, Germany). Force sensor and displacement transducer were mounted in line with the belt that carries the testing loads. Therefore, the measured force represents the force the belt carries the load with, and the displacement transducer recognizes the vertical load displacement. An electrical goniometer was synchronized to the signals of the force sensor and displacement transducer. The rotational center of the goniometer was placed at the lateral femoral condyle, the lower shank was directed out to the lateral malleolus, and the upper shank was brought in line with the greater trochanter. The upper and lower shanks of the goniometer were carefully fixed at the segments by straps. The knee joint angle was defined as the included angle between shank and thigh, and the hip joint angle was defined as the included angle between thigh and upper body. The shank segment was defined as a line connecting the lateral malleolus and the lateral femoral condyle; the thigh was defined as a line connecting the lateral femoral condyle and the greater trochanter; and the upper body was defined as a line connecting an estimated midpoint on the iliac crest and the greater tubercle of the humerus. The lateral femoral condyle that was defined as the rotational axis of the knee joint was carefully brought in line with the rotational center of the testing machine. To ensure identical placement of the foot role between sessions, the distance from the distal point of the foot role to the lateral malleolus was measured. During each testing session, the position of the foot role was marked with a pencil to ensure its identical placement between trials. The contralateral leg was allowed to hang freely during all contractions. Before starting a measurement, the initial knee angle and the knee joint position with respect to the axis of rotation of the testing machine were carefully controlled. Each trial with a given load (40, 60, and 80% of 1 RPM) consisted of 3 consecutive MVC knee extensions throughout the testing ROM. Therefore, subjects were asked to accelerate the load as “forcefully and explosive as possible” throughout the ROM (concentric phase). Afterward, they had to lower the weight to the knee joint angle's starting position within 1 second (eccentric phase) and to relax in the starting position for 1 second before starting the next contraction cycle. Pilot data demonstrated that sets of 3 consecutive contractions were optimal to elicit a peak power measurement. Trials were repeated if a subject judged the attempt to be less than maximal or when any of the methodical guidelines was violated. An individual written computer program (DasyLab; National Instruments Corp., Austin, TX, USA) presented direct online feedback about the ROM, required time of the lowering (eccentric) phase, and required time in the relaxing phase and initial knee joint angle at start of any contraction cycle.

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Knee Flexor Power-Load Curves

Subjects lay in the prone position, and their upper body was fixed at the testing apparatus with 2 rigid belts. The upper extremities were maintained in the crossed position during all contractions. To avoid rotational movements of the tibia, subjects were advised to hold the ankle joint in the neutral position during all knee flexions. Trials with visible tibia rotation were recognized by an investigator and immediately deleted. The contralateral leg was held approximately fully extended. To determine knee joint angle-dependent power-load curves during maximal voluntary explosive concentric knee flexions, a traditional knee flexor strength training machine (Edition-Line; Gym80 International MbH) was armed with the same setup that has been described above. All further methodical procedures were the same as for knee extensions.

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Data Analysis

Signals from the force sensor, displacement transducer, and electrical goniometer were synchronously recorded with a frequency of 10,000 Hz. Data were measured with the DasyLab software and stored on a PC. Final data calculation was done using the Matlab software (The MathWorks, Inc., Detroit, MI, USA). All data analyses were restricted to the concentric phase of the contraction cycles. Separation of the contraction phases was defined using the goniometer values. For each testing load, the trial with the highest average power output during the concentric phases of 3 consecutive concentric-eccentric contraction cycles was taken for analysis. In a first step, signals from the displacement transducer were used to calculate velocity (ms) of the contraction. Net mechanical power (Watts) was then calculated as the product of force and velocity values. In a further calculation, power values were averaged in 5° knee joint angle intervals to obtain knee joint angle-dependent power-load curves for QF and HAM. Afterward, power data were normalized to each subject's maximal value (% max.) that occurred for any 5° knee joint angle interval. Finally, a second-order polynomial curve fitting approach of the normalized (% max.) power values of each subject was used to determine the optimal knee joint angle (°) for power production in each training group. For each subject, the highest mean maximal power value that occurred for any 5° knee joint angle interval was calculated as peak power for the respective testing load.

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Experimental Training

Training was conducted 2 times a week during the initial 2 weeks and 3 times a week for the rest of the training period. The overall number of training sessions was 22, and subjects were not allowed to miss more than 1 session. Subjects were given at least 1-day rest between sessions. Contractions were performed as single-leg exercises in all training groups. Subjects always conducted 1 set of contractions with their stronger leg and afterward with the contralateral leg, respectively. A 2.5-minute recovery period was given between sets. Group 1 (isometric training group, Table 2) performed cyclic (hold and relax) isometric contractions at knee joint angle positions corresponding to long MTU lengths for QF and HAM. Training positions were 90° knee joint angle and 90° hip joint angle for knee extensions and 160° and 170° for knee flexions, respectively. Knee and hip joint angles of 180° correspond to full knee and hip extension. Isometric sessions for QF were performed at a testing and training apparatus developed for these kinds of investigation (30). Isometric training for HAM was conducted at the training machine that was also used for testing (Edition-Line; Gym80 International MbH). During isometric exercise, the training loads were fixed mechanically. Within an isometric contraction cycle, subjects had to reach the required target force (% MVC) as fast as possible, hold a 2-second force plateau, and relax for 1 second before starting the next cycle. This cyclic isometric exercise was performed using online feedback from a DasyLab program. At the beginning of every week, subjects performed 2 MVCs in the training positions to account for progressive strength increases.

Table 2

Table 2

Group 2 (dynamic training group, Table 3) performed concentric-eccentric contraction cycles that were restricted to knee joint angles corresponding to predominantly long MTU length for QF and HAM. Knee joint angle ROM was set from 80° (starting position) to 115° (final position) for knee extensions and from 170° to 135° for knee flexions, respectively. The hip joint angle was held constant at 105° for knee extensions and at 170° for knee flexions. Sessions were conducted at traditional training machines (Edition-Line; Gym80 International MbH) for knee extensors and flexors. Within one contraction cycle, subjects had to accelerate the load throughout the ROM as fast as possible (concentric phase), lower the load within 0.5 seconds (eccentric phase), and relax for 1 second before starting the next cycle. Throughout the course of training, loads corresponding to 40, 60, 80, and 90% of each subject's individual 1 RPM were used. Contraction cycles were controlled with a commercial available training software (Digimax-Biofeedback, Version 2.1; Mechatronik MbH) that presented direct online feedback to the subjects. At the beginning of every week, standardized (29) single-leg 1 RPM testing was conducted to account for progressive strength increases. Group 3 (combined training group, Tables 2 and 3) combined the training protocols of G1 and G2. During each week, days were broken into isometric training days and dynamic training days. During the initial 2 training weeks, subjects had 1 isometric and 1 dynamic training day each week. From week 3 on, subjects conducted 2 isometric and 1 dynamic training session for the first week and 1 isometric and 2 dynamic training sessions the next week. This resulted in 11 isometric and 11 dynamic training sessions throughout the course of the study for subjects of G3.

Table 3

Table 3

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Statistical Analyses

The initial level of statistical significance was set as α = 0.05 for all statistical procedures. For all testing loads, temporal changes in absolute peak power production (Watts) and optimal knee joint angle for power production (°) were examined separately in each training group using 1-way analysis of variance with repeated measures. Significant main effects were further examined with the Duncan test. The interday variability (measurements at pre-training testing days 1 and 2) of absolute power production and optimal knee joint angles for power production was estimated for all corresponding testing loads by the coefficient of variation (CV) and quantified with an intraclass correlation coefficient (ICC) with the testing session as the independent variable. Results are presented as mean ± SD in the tables. Notably, the figures presented in this study show curve-fitted knee joint angle-dependent power-load curves of knee extensors and flexors throughout the course of the study within each training group. In line with previous investigations that used similar curve-fitting approaches (6,30), values are presented as means. A post hoc power analysis (α = 0.05, 2 tailed) showed that, according to testing load and training group, statistical power for maximal gains in net power values varied between 24 and 79% for QF and between 35 and 89% for HAM, respectively.

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Results

Body mass remained unaltered during the course of the study in all training groups (Table 1). For all testing loads and training groups, no significant alterations occurred in absolute net peak power values of QF and HAM during the baseline measurements (Tables 4 and 5). Interday reliability measurements of absolute power production showed CV values between 8.1 and 10.2% and significant (p < 0.05) ICCs between 0.75 and 0.93 according to muscle group, training group, and testing load. In G1, QF peak power at 40 and 60% of 1 RPM increased significantly (p < 0.05) throughout the study (Table 4). However, QF peak power at 80% of 1 RPM remained unaltered in G1 (Table 4). In G2, significant (p < 0.05) gains in QF peak power were detected throughout the study for all testing loads (Table 4). In G3, QF peak power at 40% of 1 RPM was significantly (p < 0.05) higher after 5, after 8, and 4 weeks post training compared with both pre-training measurements (Table 4). Quadriceps femoris peak power at 60% of 1 RPM was significantly (p < 0.05) higher after 5 and 8 weeks of training compared with pre-training testing day 1 but not compared with pre-training testing day 2 in G3 (Table 4). Additionally, QF peak power at 60% of 1 RPM was significantly (p < 0.05) higher after 5 and 8 weeks of training compared with 4 weeks post training in G3 (Table 4). Quadriceps femoris peak power at 80% of 1 RPM was significantly (p < 0.05) higher at pre-training testing day 2 compared with 4 weeks post training in G3 (Table 4).

Table 4

Table 4

Table 5

Table 5

Hamstring peak power at 40 and 60% of 1 RPM showed significant (p < 0.05) gains throughout the study in G1 (Table 5). As detected for QF, HAM peak power at 80% of 1 RPM did not change significantly in G1 (Table 5). Also in line with QF, G2 reached significant (p < 0.05) increases in HAM peak power for all testing loads (Table 5). HAM peak power at 40% of 1 RPM was significantly (p < 0.05) higher after 8 training weeks compared with 4 weeks post training in G2 (Table 5). In G3, HAM peak power at 40 and 60% of 1 RPM displayed significant (p < 0.05) gains throughout the study (Table 5). However, despite significant (p < 0.05) higher values at pre-training testing day 1 compared with training 8 weeks, no significant changes occurred at 80% of 1 RPM in G3 (Table 5).

Most importantly, for each testing load and training group, no significant shifts in optimal knee joint angle for QF and HAM power production were detected at any testing day (Figures 1A-C and 2A-C). Interday reliability measurements of the optimal knee joint angle for power production showed CV values between 3.0 and 4.2% and significant (p < 0.05) ICCs between 0.54 and 0.63 according to muscle group, training group, and testing load. According to testing day, training group and testing load optimal knee joint positions varied between 115.7 and 121.8° for QF power production (Figures 1A-C) and between 117 and 126.2° for HAM, respectively (Figures 2A-C). For easier data presentation, Figures of curve-fitted power-load curves for QF and HAM are presented only for 60% of 1 RPM in every training group. Notably, the shape of the curve-fitted power-load curves was very similar for all testing loads.

Figure 1

Figure 1

Figure 2

Figure 2

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Discussion

This work examined if different length-restricted strength training regimens shift athlete's voluntary explosive concentric power-load curves of knee extensors and flexors in the direction of the training length. The present findings indicate that different strength training regimens with a common restriction to knee joint angles corresponding to predominantly long MTU lengths for QF and HAM failed to induce length-dependent alterations in athlete's voluntary explosive concentric power-load curves of knee extensors and flexors. Therefore, the muscle excursion range during loading seems to be an inappropriate trigger to cause length-dependent alterations in athlete's explosive power production. Additionally, our results indicate that only such athletes who used concentric-eccentric loading cycles with moderate- to high-resistance loads enhanced peak voluntary explosive power production throughout a range of testing loads.

Before discussing our results, we want to mention that this study is not without assumptions and limitations. The first assumption is that the adaptations were caused by our training regimen and not by exercises beyond the scope of this study. To avoid this problem as good as possible, the study was performed during the winter season when athletic training programs remained rather constant and all athletes gave their written consent to train QF and HAM exclusively within our study. The second limitation is that the different strength training regimens contained different contraction modes (i.e., isometric contractions and dynamic lifting and lowering of weights). Due to the force-velocity relation, the tensions of the working muscles are not comparable among these training exercises. Therefore, we examined the adaptation responses to different strength training regimen separately in each training group, and no between-group analysis was performed. Finally, one might argue that pure isometric training does not fully impact the applied aspect of strength training with competitive athletes. For example, isometric strength training is known to be highly time consuming, and furthermore, the mechanical specificity compared with dynamic athletic movements might be limited (16,10). However, there is also evidence on that isometric training resulted in significant improvements in both isometric and isovelocity strength (10). In the present study, isometric training was used to examine the effect of highly length-dependent mechanical loading on athlete's power-load curves of knee extensor and flexor muscles. To account for practical implications with competitive athletes, additional strength training regimens that used dynamic and combined isometric and dynamic training were also tested in this study.

Absolute gains in moment and power generation after high-resistance training show considerable variation with respect to training status of the subjects and intensity and duration of the training programs (1-3,20,25). In the present study, resistance loads used during power testing were not constant among different testing days as they were increased to remain constant relative to improving strength of the subjects. According to testing load and training group, mean absolute gains in peak power varied between 10 and 28% for QF and between 17 and 53% for HAM, respectively. These changes are not out of line with the named literature. As expected, mean peak power enhancements were higher for HAM compared with QF in every training group. This most likely reflects a higher baseline level of adaptation for knee extensors due to their wide use during daily activities and athletic exercise. For both muscle groups, peak power at 40 and 60% of 1 RPM increased significantly throughout the study in all training groups. Contrarily, significant gains for peak power production at 80% of individual 1 RPM exclusively occurred in training group 2 that used concentric-eccentric loading cycles with moderate to high resistances. Therefore, subjects who (at least partly) conducted high-intensity isometric training (training groups 1 and 3) enhanced peak power at low to moderate testing loads but not at heavy resistance. Kaneko et al. (15) and Moss et al. (24) reported load specificity in muscular power development, that is, the greatest power outputs were detected at the loads used during training. According to these findings, one might have expected peak power enhancements at heavy resistance in training groups 1 (isometric training) and 3 (combined training). However, the present findings suggest that concentric-eccentric loading cycles performed at moderate to high resistance were the best exercise to reach significant gains in muscular power production throughout a range of testing loads.

Increasing evidence exists on that strength training exercise affect human force-length properties of knee extensors and flexors (5,7,17). For example, shifts in isometric or isovelocity moment-angle relations were reported after eccentric exercise (6,7,17,30,32). Based on findings from animal studies (8,9,21), it was suggested that muscle architectural changes, namely, an increased number of serial sarcomeres, underpin these alterations in length-dependent force generation capacities (6,7,30). Also, supportive for this hypothesis were findings that electromyographic data indicated a lack of association of neural and force-length adaptations (26). However, despite considerable research, the key trigger mechanism for these adaptations remains unclear. Whereas some animal studies suggested contraction mode being a major trigger of architectural changes that underpin force-length alterations (9,21), others failed to prove this hypothesis (19). Additionally, a recent strength training study of knee extensors indicated that the predominant muscle excursion length during loading, and not contraction mode, has to be considered as a key trigger of human force-length adaptations (5). The hypothesis that human muscles can adapt to the length they are most frequently confronted with was also proposed by Herzog et al. (12) and more recently by Savelberg and Meijer (27). Encouraged by these findings, the present study examined if different length-restricted strength training regimens shift athlete's concentric power-load curves in the direction of the MTU lengths that were predominantly used during training. Training exercises were restricted to long MTU lengths (i.e., more flexed knee joint angles for QF and more extended knee joint angles for HAM) because explosive knee extensions at more flexed knee joint positions are important for many athletic movements (30), whereas HAM muscles are prone to injury when lengthened during rapid knee extensions (4,7,13). Therefore, shifting power-load curves of knee extensors and flexors to longer MTU lengths would be of direct practical relevance for many athletic activities such as cycling, ice skating, and team games that require kicking movements and change of direction tasks (12,13,30). However, independent of the testing load, such shifts were not detected in the present study. According to testing day, training group and testing load optimal knee joint positions varied between 115.7 and 121.8° for QF power production and between 117 and 126.2° for HAM, respectively. Similar optimal knee joint angles were reported by researchers who used isovelocity measurements (7,30). A high volume of concentric-eccentric contraction cycles (i.e. lifting and lowering of different loads) restricted to predominantly long MTU lengths of QF and HAM was performed in the training groups 2 (dynamic training) and 3 (combined training). Additionally, training group 1 (pure isometric training) and partly training group 3 executed high-intensity isometric contractions at fixed knee joint positions corresponding to long MTU length for both muscle groups. Therefore, a considerable amount of muscle loading at predominantly long MTU lengths of knee extensors and flexors was imposed for 8 weeks on our subjects. To our knowledge, this was the first study that examined the influence of length-restricted strength training exercise on athlete's voluntary explosive concentric power-load curves of QF and HAM. According to the present findings, it is suggested that knee joint angle-dependent explosive concentric power production of QF and HAM has already reached some kind of an adapted state in well-trained athletes. Restricting the muscle excursion range during loading was not found to work as a sufficient trigger for length-dependent alterations in voluntary explosive concentric power-load curves of knee extensors and flexors in this population.

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Practical Applications

This work examined if different strength training regimens with a common restriction to knee joint angles corresponding to long MTU length for the QF and hamstring muscle group (HAM) shift voluntary explosive concentric power-load curves of these muscles in the direction of the training length. Physical activities such as cycling, ice skating, and skiing and furthermore change of direction movements in many team games require high amounts of knee extensor power production at more flexed knee joint angle positions (12,30). Additionally, hamstring muscles are prone to injuries when lengthened during rapid knee extensions (4,13). Therefore, the approach to develop specific strength training programs that induce systematic shifts in knee joint angle-dependent power production of QF and HAM is of direct practical relevance for coaches who work with athletes from different disciplines. However, the present findings indicate that restricting the muscle excursion range during strength training exercises is not a sufficient trigger to cause length-dependent alterations in voluntary concentric power-load curves. The present results suggest that there is no rationale to limit the muscle excursion range during strength training exercises that aim to improve concentric power production of QF and HAM in competitive athletes. Possibly, knee joint angle-dependent explosive concentric power production of knee extensors and flexors has already reached a high level of adaptation in this population. Our findings also propose that only such athletes who used explosive concentric-eccentric loading cycles with moderate- to high-resistance loads enhanced peak voluntary explosive power production throughout a range of testing loads. Therefore, coaches are encouraged to use explosive dynamic exercises with different resistance loads to enhance athlete's muscular power production.

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Keywords:

muscle function; muscle mechanics; power; concentric muscle actions; ROM-restricted exercise

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