Muscle Plasticity after Weight and Combined (Weight + Jump) Training

Abstract

TOUMI, H., T. M. BEST, A. MARTIN, and G. POUMARAT. Muscle Plasticity after Weight and Combined (Weight + Jump) Training. Med. Sci. Sports Exerc., Vol. 36, No. 9, pp. 1580–1588, 2004.

Purpose: 

The purpose of this study was to compare the effects of jump training as a complement to weight training on jump performance and muscle strategy during the squat and countermovement jump.

Method: 

Twenty-two male handball players, between the ages of 17 and 24, and in good health, were randomly divided into three groups. Two were trained groups, weight training (WTG) and jump training combined with weight training (CTG), and one was a control group (CG). Maximal isometric force and maximal concentric power were assessed by a supine leg press, squat jump (SJ), counter movement jump (CMJ), and surface EMG was used to determine changes in muscle adaptation before and after the training period.

Results: 

After 6-wk training programs, the two training groups increased maximal isometric force, maximal concentric power, and squat jump performance. However, only combined training presented a significant increase in height jump performance during the countermovement jump (P < 0.05). EMG analysis (as interpreted through the root mean square values) showed that the SJ was performed similarly before and after the training period for the two training groups. However, during the CMJ, only the CTG group adopted a new technique manifested by a short transition phase together with an increase in knee joint stiffness and knee extensor muscle activation and rectus femoris ratio.

Conclusion: 

It was suggested that the central activities in knee joint during the transition phase, in conjunction with intrinsic muscle contractile properties, play a major role in the regulation of performance during a CMJ. Furthermore, our study suggests that a change in maximal strength and/or explosive strength does not necessarily cause changes in combined movement such as the stretch shortening cycle.

Several studies have attempted to identify mechanisms to which improvement in vertical jump performance can be attributed (^1,8,19,28,29). Researchers who investigate the transference of training programs (i.e., plyometric, resistance, etc.) to jump performance appear to be entrenched in two camps. One suggests that the training program should simulate the contraction characteristics of the vertical jump (i.e., plyometric training) and therefore needs to increase the degree of coordination and maximize the ability to use the muscle’s stretch-shortening cycle (SSC) (^8,19). A second theory proposes that there is no need for specificity with training and that plyometric exercises improve power and vertical jump performance because of increases in muscle contraction force (^1,7,29). It would seem important to verify the necessity of the specificity with training in order to determine optimal strategies for improvement in performance.

It has been recently demonstrated that improved strength does not necessarily transfer to better functional performance for training methods that improve dynamic strength and power in relation to a specific motor task (⁹). Furthermore, significant improvements in performance have been observed not only with training (^28,29) but also through mental practice and preparation (³²). It is possible therefore that the training or learning process is guided by the central nervous system, which constructs and shapes the motor program according to the aim and requirements of a specific task (⁹). Several studies reported that changes in reflex activity can occur after prolonged plyometric training (²⁷). Thus, the voluntary and reflex functions of the automatic motor program presumably interact so that reflexes are appropriately adjusted to the motor program required by the particular task (³⁰).

In the interest of developing optimal training programs, it would be valuable to know if the improvement in muscle jump performance with SSC training exercises is the result of: (a) a change in coordination (i.e., changing the neural input) (¹³); (b) a change in the functional capacity of the muscle to produce force and/or muscle properties (i.e., changes in stiffness without changing the neural input); or (c) the coordination of a multitude of factors (i.e., changes in stiffness, force, power, neural input) (^13,17,28,29). In other words, is the force is the criterion determinant of the CMJ performance? Moreover, will the gain in force and jump performance be the same after weight training compared with weight training supplemented with an extra volume of work (jump exercises)?

The purpose of this study was to compare the effects of jump training as a complement to weight training on jump performance and muscle strategy during the squat and countermovement jump. Two modes of training were used for this purpose. The first mode consisted of SSC movements in a leg press. The second mode consisted of SSC movements in a leg press combined with jump training. Our hypothesis was that weight (SSC movements in a leg press) training alone would facilitate the improvement in isometric and explosive strength (isometric and or concentric contraction), whereas weight training combined with jump training would facilitate and increase the synchronization between the eccentric and concentric contractions during a combined movement (i.e., SSC) which is necessary for countermovement jump movement.

METHODS

Twenty-two male handball players who were competing at the third national level in the French Handball Federation League participated in the study (Table 1). The study received approval from the Handball League (Clermont-Ferrand, France) before entry of any subjects into the training protocols. Each subject signed a university-approved human subjects informed consent. All players had trained and competed regularly in handball for at least 5 yr and were injury free for 2 yr before participation in the study. Participants began the study by performing squat and countermovement jumps. Based on these results, subjects were divided into three groups such that there were no significant differences between the mean performance scores of the groups. Two were trained groups, weight training (WTG) and weight training combined with jump training (CTG), and the third was a control group (CG).

TABLE 1: Physical characteristics for participants.

Training programs

Weight training: WTG.

Weight training was performed in a “leg press machine” (Multi-form/Type M11: Serial number: 402, DPS Company, Clermont Ferrand, France). Subjects performed six sets of 10 repetitions of stretch-shortening contraction (SSC) at 70% of the maximal isometric force output of the leg press with 3-min rest between sets.

Combined training: CTG.

Subjects performed two jump exercises along with the weight training exercises:

First crossover jumping.

Ten benches (height 35 cm and interval 50 cm) were used. Subjects jumped across the bench in one continuous movement and rested for 3 s between benches and then repeated the same movement. All jumps were a form of countermovement jump and not a type of drop jump. Subjects performed three sets of five repetitions with 3-min rest between sets.

Second crossover jumping.

Subjects jumped across the bench and continued jumping without rest on five benches (3-min rest between sets, height 35 cm and interval 50 cm).

Control group: CG.

The control group performed its standard handball training and no additional exercises over the 8-wk period.

For both training groups, the 70% load used on the leg press exercise was modified at 3-wk intervals by evaluating maximal isometric force of the knee extensors. Training was performed 4 × wk⁻¹ over the 6-wk period. At the beginning of the program, 2 h were dedicated to familiarizing participants with the events that would take place during the experimental tests and the training program. Subjects were tested before the training program (week 0), in the middle of the training period (week 3), once the program was completed (week 6) and after 2 wk of standard handball training (week 8). All three groups (CG, WTG, and CTG) performed their standard handball training that was supervised by the same coach (three sessions per week; 90–120 min per session) during the 8-wk period.

First test.

A leg press (Panatta Sport, DPS Company) was used to measure force and displacement. Force was recorded under both the heel and the sole by using two force transducers per foot (EM, 200daN, DPS Company). The total force was calculated by summing the signals from the four force transducers. For the dynamic concentric contractions, a fifth transducer (PSI/TRONIX/MOD DT-80A/SEN 12.20 mV/V DPS Company) was used to measure the displacement of the mobile part of the press.

The subjects first performed three maximal isometric (1RM) leg press extensions, but only the best result was recorded. The knee angle was standardized at 70° and was selected because pilot testing demonstrated that the force-angle relationship was maximum around 70°.

For the leg press test, dynamic concentric movements were performed at 40%, 50%, and 60% of each subject’s 1RM. The knee angle was standardized and measured with a goniometer (DPS Instruments, serial number 0145) at a starting position of 90°. Data were collected at 2000 Hz, stored in a PC computer (Intel Pentium, PSI Instruments, Clermont Ferrand, France), and later quantified for displacement, force, velocity, and power. All calculations were carried out using Matlab (Version 5.3.1.29215a, R11.1).

Second test.

A force plate (9981C, Kistler Instrument AG, Winterhur, Switzerland) was used to measure force, power, and height during the squat jump (SJ) and the countermovement jump (CMJ) on both feet. Data were collected at 100 Hz. The subject’s knee was flexed to 90° measured using a goniometer (EG, DPS Company). A cord (Fig. 1) was placed horizontally at the buttock level behind the jumper referring to this position (knee at 90°). The SJ started with the buttock at the cord level. A pure and maximal concentric knee extension was performed. In the CMJ, subjects from a standing position flexed the knee to the cord level and then immediately extended the knee by a jump to maximal height. For both SJ and CMJ, the force plate and the kinematic data were immediately plotted; if there was any initial countermovement observed and/or the knee ankle moved more than 90°, the subject was permitted to perform another jump. At the beginning of the jump tests, 2 h were dedicated to familiarizing participants with the events that would take place during this experimental test. Pilot testing demonstrated that after 2 h of familiarization the participants were able to repeat and develop their maximum jump (SJ and CMJ) performance according to method proposed.

FIGURE 1: Joint stiffness (K) for the knee joint calculated as an average value in the braking phase by dividing the change in joint moment (M) by the corresponding change in joint angle (θ). H, hip greater trochanter; K lateral femoral epicondyle of the knee; M, the ankle lateral malleolus. The direction of positive joint moments and indications of the joint angles are given in the inset to the figure.

Subjects performed the CMJ three times, and the best performance was recorded. Hands rested on the hips for the duration of the jump. In the jumping tests, jumping performance was determined by the vertical take-off velocity (V₀) of the center of gravity, which was calculated from the flight time (t_f) according to the equation: V₀ = 1/2 t_f × g (g = 9.81 m·s⁻²). Jump height (H) was then calculated using H = V₀ 2/2g (²).

Kinematic assessment.

The Saga-3 3-D system (Saga-3, Biogesta Denain, Clermont Ferrand, France) was used to analyze kinematic data for the most prominent protuberance of the hip greater trochanter (H), lateral femoral epicondyle of the knee (K), and the ankle lateral malleolus (M) during each exercise. Subjects were filmed (100 Hz), and the coordinated marker points were filtered using spline functions. Each jump was divided into three phases: eccentric phase (ECC), transition phase (TR), and concentric phase (CON) (Fig. 2). The initiation of the eccentric phase was calculated using the force plate data (corresponding to the instant at which vertical force began to decrease continually). The end of the CON phase was defined as the instant at which the subject lost contact with the force platform (corresponds to the instant at which the vertical component of the ground reaction forces was zero) (^3,26). Vertical velocity of the body mass center was calculated by integrating the vertical acceleration component. The end of the ECC phase and the beginning of the CON phase was defined as the first instant at which the vertical velocity of the body mass center was positive. The TR phase was determined according to Rodacki et al. (²⁵) as the period in which the knee angular velocity ranged between −30°·s⁻¹ and −30°·s⁻¹ in relation to the deepest knee flexion angle (Fig. 1).

FIGURE 2: Reaction force curve (a), knee joint angle (b), and EMG activity (c) for the vastus lateralis during the countermovement jump (CMJ) movement phase divided into eccentric phase (ECC), transition phase (TR), and concentric phase (CON) based on the force plate and kinematic data.

The knee angle (θ) was calculated as:

EQUATION

Joint moment of the knee was calculated using an inverse dynamics approach given segmental displacements, velocities, and accelerations obtained from the kinematic record and the force plate data (^6,31).

Joint stiffness (K) was calculated as an average value during the negative phase by dividing the change in joint moment (M) by the corresponding change in knee joint angle (²¹) (Fig. 1). The negative phase was defined between the initiation of the movement (the instant at which vertical force began to decrease continually) and the deepest excursion of the knee joint.

EQUATION

Electromyographic Recordings (EMG)

Myoelectric signals were recorded for the following muscles: vastus medialis (VM), vastus lateralis (VL), and biceps femoris (BF). Bipolar surface electrodes (sensors, 10 mm) were placed longitudinally over the muscle belly with an interelectrode distance of 20 mm. All recording points on the skin were shaved, rubbed with alcohol, and permanently marked for the duration of testing. A large strip of sticking plaster was applied to cover and fix the electrical contact with the skin. Finally, a reference electrode was placed on the patella. The signals were collected and preamplified, filtered using a band-pass filter (15–1000 Hz), and sampled at 2000 Hz. EMG signals were integrated in each corresponding period. In the SJ, the EMG signal was integrated during the whole movement duration (concentric phase). On the other hand, in the CMJ, EMG signals were recorded in the ECC, TR, and CON phases. Phases were defined according to the kinematic measurements (Fig. 2). Root mean square (RMS) values were calculated and normalized to the RMS of maximal values recorded during maximal isometric knee extension (1RM) pre- and posttraining for the VL and VM to account for differences in electrical impedance and electrode placement. The RMS values of the posttest data were normalized according to maximal activation of the posttraining data. For the BF, the RMS values thus obtained were normalized to the RMS of maximal values recorded during maximal isometric knee flexion. Mean and SD were calculated in each group for each phase.

Statistical Methods

Mean and SD were calculated from individual measurements for the three groups. Differences between pre- and posttraining results were tested for all parameters by a Student’s t-test for paired observations. Two-way ANOVA (repeated measures on one factor) was used for the statistical analysis. The three groups (CTG, WTG, and CG) and the four time points of measurements (weeks 0, 3, 6, and 8) were defined as factors 1 and 2, respectively. When significant treatment effects occurred, Fischer post hoc tests were used. In each case, the level of significance was established at P < 0.05. All statistical analyses were carried using Stat View software for Microsoft Windows.

RESULTS

Force plate test.

After 3 wk of training, both training groups showed no significant differences in vertical height jump performance compared with pretraining values for all the jumping modalities (Fig. 3). At the end of the 6-wk training programs, the two training models induced a significant increase in the SJ performance, 9.1% for the weight training and 11.3% for the combined training groups, respectively (P < 0.05). However, the countermovement jump results showed a significant increase in CMJ height only for the combined training group (13.2%, P = 0.02). As shown in Figure 3, the increases observed in SJ and CMJ jump performance were subsequently maintained after 2 wk of handball training.

FIGURE 3: Jump height during the squat jump (SJ) and counter movement jump (CMJ) at weeks 0, 3, 6, and 8 in weight training group (WTG), combined training group (CTG), and control group (CG). Mean values in cm ± SE; * significantly different from the preexercise value P < 0.05.

EMG analysis.

The SJ was performed similarly before and after the training period for the two training groups (Fig. 4). However, during the CMJ, only combined training produced a significant increase in RMS values for the VL and VM in the TR phase (P = 0.03). Similarly, a significant increase in RMS (VL + VM) values was observed in the positive phase (TR + CON) (P < 0.05). However, CTG resulted in no significant differences in RMS values during the ECC and CON phases of the CMJ. No significant differences were observed for the BF in RMS values during all jump modalities. However, for the CMJ a significant increase was observed in (vastus lateralis + vastus medialis)/biceps femoris ratio [(VL + VM)/BF], (P = 0.04) during the TR phase.

FIGURE 4: RMS values (VL + VM) for the eccentric phase (ECC), the transition phase (TR), and the concentric phase (CON) for groups WTG, CTG, and CG during the SJ and CMJ. Values are expressed as percentages of the preexercise value; means ± SE; * significantly different from the preexercise values P < 0.05.

Kinematic analysis.

No significant changes were observed in the CON duration phase for the two training groups in the SJ (Fig. 5). In the CMJ, only combined training presented a significant decrease in the ECC and TR, respectively 7.1% and 8.7% (P = 0.03). Also, the negative phase presented a significant increase in the joint knee stiffness (K), 8.2% (P = 0.03) (Fig. 6).

FIGURE 5: Duration of eccentric phase (ECC), transition phase (TR), and concentric phase (CON) for groups WTG, CTG, and CG. Values are expressed as percentages of the preexercise value; means ± SE; * significantly different from the preexercise values P < 0.05.

FIGURE 6: Joint knee stiffness during the eccentric phase (ECC) for groups WTG, CTG, and CG during the CMJ at weeks 0 and 6. Means values in N·m·deg⁻¹·10⁻³ ± SE; * significantly different from the preexercise value P < 0.05.

Leg press test.

Three weeks of combined training increased maximal isometric force (Table 2; 13.2%; P = 0.03). After 6 wk of training, maximal isometric force and maximal concentric power increased for both two training groups (Table 2;Fig. 7a; P = 0.02). No differences in maximal isometric force were observed for the CG throughout the study (Table 2). The gains in maximal voluntary isometric force and power were subsequently maintained after 2 wk of handball training for both training groups.

TABLE 2: Maximal isometric force (1RM) before (week 0), at weeks 3, 6 and 8.

FIGURE 7: Maximal power (a) and sum (VM + VL) RMS values (b) normalized with respect to the maximal activation recorded during the 1RM, in the leg press test at 40%, 50, and 60% of 1RM after 6 wk of training. Values are expressed as percentages of the preexercise value; means ± SE; * significantly different from the preexercise value P < 0.05.

Changes in EMG activity are shown in Figure 7b. Normalized RMS values are expressed as a percentage of the preexercise values. There was an increase in RMS values for the VM and VL in maximal concentric contraction at 40%, 50%, and 60% of 1RM for both training groups (P = 0.02).

DISCUSSION

The purpose of this study was to investigate the effects of jump training as a complement to weight training on jump performance and muscle strategy during the squat and countermovement jump.

The major finding in our study was that only combined training presented a significant increase in the combined movement (CMJ). The increase that occurred in the latter was accompanied by an increase in EMG activity of the knee extensors muscles (VL and VM) and a change in the jump strategy (as interpreted through the kinematic data findings).

On the other hand, our results also show that both training programs lasting 6 wk and incorporated into preseason handball training significantly increased maximal voluntary strength of the knee extensors and height of SJ. In addition, 2 wk of standardized handball training maintained the gains previously achieved with both training strategies. However, the data indicate that maximal voluntary strength increased more rapidly after weight training supplemented with an extra volume of work (jump exercises) than with weight training alone. Only combined training led to a significant increase in maximal isometric strength (1RM) after 3 wk of training.

Our study also showed that 3 wk of combined training was sufficient to produce a significant increase in maximal voluntary strength of the knee extensors, whereas no significant change was observed in the weight training only group. In the present study, the combined group performed the same amount of leg press exercise as the weight-training group, but in the former group, this was supplemented with the jump exercises. This extra volume of work performed by the combined training group could explain the increase in maximal isometric force after 3 wk with combined training rather than with weight training. However, the volume of work used in the weight training (3 wk of weight training at 70% of 1RM) was not sufficient to produce a significant increase in maximal voluntary strength of the knee extensors. On the other hand, the present data showed that 3 wk of combined training produced no significant change in vertical jump height (SJ and CMJ). These results lead us to believe that initial adaptations during the first stage of combined training appear to have a greater effect on maximal isometric voluntary strength than on maximal explosive strength. These observations are consistent with those observed by Maffiuletti et al. (²²), who reported that 2 wk of combined electromyostimulation-plyometric training were marked more for maximal voluntary strength than for explosive strength. Our results appear to support a previous study by Bobbert and Van Soest (⁵), who reported that stronger muscles do not necessarily result in greater jumping ability. In other words, our study showed that the change in maximal isometric strength after combined training occurred before the change in explosive power.

After 6 wk, the two training groups both showed improvement in maximal isometric force and maximal power output during concentric movements. It is well known that increases in maximal voluntary strength are usually associated with changes occurring in the central nervous system (i.e., neural drive) and/or at the muscle level (i.e., muscle volume). Although no muscle cross-sectional area measurements were taken, the EMG results support nervous system rather than muscular adaptations (i.e., hypertrophy). It has been previously documented that increases in maximal strength observed during the initial weeks of strength training can be attributed largely to the increased motor-unit activation of the trained agonist muscles (^16,23,24). This concept is well supported in the current study by the large increases observed in the maximum RMS values of the leg extensors (VL and VM) in both training groups. The present study confirms our previous reports (^28,29) and demonstrates the contributing role of the nervous system to maximal strength development. Six weeks of combined jump and weight training induced increases in the EMG activity, which could result from an increased number of active motor units or alternatively, an increase in their firing frequency (²⁷).

We also observed that weight training combined with plyometric training produced almost the same increase in maximal isometric voluntary strength as in maximal concentric power (as interpreted through the 1RM, maximal concentric power, and SJ performance findings) of the knee extensors compared with WTG. Previous studies concerning the relationship between the change in muscle force and jump performance have shown that quadriceps activation contributes 50% of the total work in a vertical jump (¹⁷). Similar findings have also been reported by Bosco et al. (⁸) for the SJ and the CMJ. A study reported by Häkkinen (¹⁵), which included male and female basketball players, has shown that both SJ and CMJ performance correlate with maximal leg extension isometric force. These findings are also consistent with recent reports from our laboratory (^28,29). In the latter study (²⁸), 8 wk of plyometric training composed of six sets of 10 repetitions with a barbell on the shoulders at 70% of the maximal isometric force (1RM) produced improvement in leg extension force, power, neural activation, SJ, and CMJ performance. However, the present data are not consistent with these findings. In the current study, weight training alone did not induce any change in CMJ performance, although increases in maximal strength and SJ were obtained. Although the contraction modality was the same (SSC), the difference between the present data and our previous study can be explained. Our previous study used a plyometric training compound of SSC exercise realized with a barbell on the shoulders. However, in the present protocol, the SSC training was performed on a leg press. Therefore, the differences observed among the results may be explained by the differences between the training modalities used. For example, the difference between the two training models used could result from the differences in work and muscles activations performed by the hip extensors. Further study is needed to compare the SSC exercise on a leg press to the SSC exercise realized with a barbell on the shoulders.

Several factors may have contributed to the change in muscle performance during the CMJ for the combined training group. These factors could be (a) an increase in functional capacity of the activated muscles as a result of the eccentric phase, (b) a change in the stretch (myotatic) reflex, and (c) an increase in muscle capacity to store and reuse elastic energy. One or the interaction of these factors may explain the difference in countermovement jump performance between the weight training group and the combined group.

An important limits in our study and may explain the difference in countermovement jump performance between the weight training group and the combined group is the training volume. The combined training did an extra volume of work (jump exercises) compared with weight training group. Therefore, the difference in countermovement jump performance between the weight training group and the combined group can be explained by the extra exercises imposed in the combined training (jump exercises). However, the present data also showed that in spite of the extra work (jump exercises) realized by the combined group, the gain in force and SJ was statistically not different compared with weight training alone. In other words, the jump exercises affected only the CMJ performance (Fig. 3) and technique. Consequently, the more likely explanation for such results is that the combined training group changed the modality of their jump technique (as interpreted through the changes in the TR). It appears that this group adopted a new technique manifested by a short transition phase correlated with an increase in the knee joint stiffness, muscle activation (VL and VM) and (VL + VM)/BF ratio. The relationship between muscle activation and stiffness during the TR support the suggestion that the high stiffness in the negative phase may be accomplished by a proper preprogrammed motor command (as interpreted through the increase in the iEMG during the TR phase). It is well known that, during the transition phase, such as occurred in the CMJ, the preactivity is involved in the centrally preprogrammed motor commands of the required motor task (^11,13). Moreover, another function of the preactivation is assumed to be an increased sensitivity of the muscle spindle via enhanced α-β co-activation to enhance the stretch reflexes (¹³). Therefore, the change that occurred in knee joint stiffness during the negative phase could be the result of a change in muscle activation. Such results could have a positive effect on the CMJ performance. Komi (¹⁹) has suggested that greater lower-limb stiffness during the eccentric phase of a stretch-shortening cycle task might provide an advantage by allowing for the greater storage and release of elastic energy. The same group confirmed these findings recently (¹⁸) and suggested that the increased prestretch intensity has considerable influence on the process of storage and the subsequent recoil of elastic energy during the stretch-shortening cycle action. However, this speculation requires further research and may not be optimal during the CMJ.

A previous study reported that the performance difference between SJ and CMJ might result from a difference in work performed by the hip extensors rather than from the effects of stored elastic energy (¹²). Therefore, one reasonable explanation could assume that the increase in knee joint stiffness and muscle activation during the TR suggests that the initial high stiffness and muscle intrinsic property would be transmitted to the concentric phase. Such phenomena could be a considerable benefit during the concentric phase (i.e., increase in force and power). For example, according to simulation results performed by Bobbert et al. (⁴), the explanation for the enhancement of performance in CMJ over that in SJ seemed to be that the countermovement allowed the muscles to build up a precontraction active state in which there is a high fraction of attached cross-bridges and force before the onset of shortening, so that muscles are able to produce more work over the initial part of their shortening distance.

Conclusion.

Jump training as a complement to weight training has a beneficial effect on muscle strategy and performance during the countermovement jump. Furthermore, it appears that maximal isometric force and maximal power performance are not related to combined movement performance, such as the stretch-shortening cycle. Taken together, weight training inducing gain in muscle force is necessary for improvement in SJ performance. However, CMJ exercises are indispensable to induce an increase in CMJ performance. Finally, central activities in the knee joint during the transition phase, in conjunction with intrinsic muscle contractile properties, play a major role in the regulation of performance during a CMJ.

The authors gratefully acknowledge Mr. R. Chandezon for technical assistance.