Muscle power is important for success in many sports and thus numerous studies have aimed to determine the characteristics of effective training programs that maximize adaptations in dynamic, multijoint movements (1). A wide range of training loads and methods have been shown to improve muscle power or rate of force development (RFD). These include high load (>85% of 1 repetition maximum [1RM]) to low repetition programs with intention to move fast (1,2), heavy-load constant pace resistance training (3), lower load (0%–50% of 1RM) ballistic programs (4), plyometric training (5), or a combination of these methods (1).
One of the main mechanisms for the enhancement of power is an increase in type II (IIa and IIX) muscle fiber cross-sectional area (CSA) (6,7). However, heavy resistance training protocols typically result in a decrease in the percentage of type IIX fibers, and thus improve mainly maximal strength rather than muscle power (4,8–10). For example, Andersen and Aagaard (8) have shown that 12 wk of heavy-load resistance training resulted in a decrease in myosin heavy chain (MHC) IIX content from approximately 9% to 2%, whereas MHC IIA content increased from approximately 42% to 49%. In that study, a significant increase in the CSA of type II fibers was observed after training, in parallel with an increase in maximal isometric quadriceps strength.
Training with eccentric muscle actions may be used as an effective alternative method to concentric actions to induce favorable neuromuscular adaptations, due to the greater maximal force developed (11). Gillies et al. (12) investigated the effects of long-duration (6 s) concentric and eccentric leg exercises on muscle strength and fiber composition and found that the participants increased maximal strength similarly, with slow eccentric actions inducing an increase in type I fiber proportion and CSA. However, there is evidence to suggest that fast isokinetic eccentric muscle actions may result in greater muscle hypertrophy and strength gains compared with slow eccentric and fast or slow concentric training (13–15). For example, Oliveira et al. (14) found that 8 wk of maximal unilateral knee extensors eccentric actions at 180°·s−1, improved both maximal isometric knee extensor torque and RFD, but did not provide evidence of the mechanism involved. Although neural mechanisms have been suggested to partially explain such adaptations (16), muscle fiber-specific hypertrophy in type II fibers may be possible after eccentric training, because there is some evidence for a preferential increase in type II fiber CSA after this type of training (17). Furthermore, muscle fiber composition is considered to be a major factor influencing RFD (18,19), as shown by the moderate to strong correlations between type II fiber type percentage and muscle fiber area versus RFD (19,20).
The loaded squat is a popular training exercise used in many sports, as well as in recreational training. This multijoint exercise activates the major leg extensor muscles and is biomechanically similar with the push-off phase during jumping and running (21). Thus, variations of loaded squats can be used to improve muscle power that may be transferred more effectively to sport activities. One previous study has shown that training-induced improvements in jump performance after both heavy strength and ballistic power training using loaded squats were attributed to changes in force and RFD during the eccentric phase (22). As peak ground reaction force and RFD during loaded and unloaded jump squat exercises are attained during the downward eccentric movement, it may be hypothesized that training using only the eccentric part of the loaded squat exercise may be more effective in inducing favorable adaptations to increase leg strength and power. Moreover, the addition of plyometric jumps after eccentric or ballistic squats forming a strength–power–potentiation complex may further enhance the training stimulus by taking advantage of postactivation potentiation (23,24). Thus, the purpose of the present study was to examine the effects of short-term fast eccentric and ballistic complex training on muscle power, RFD, muscle fiber composition, and CSA.
Sixteen male university students were randomly assigned to either a low-volume fast eccentric and ballistic resistance training group (TG) (n = 8; age, 21.3 ± 1.9 yr; body height, 1.78 ± 0.05 m; body mass, 78.4 ± 7.4 kg; body fat, 9.0 ± 2.4; half-squat 1RM, 146 ± 25 kg or 1.87 ± 0.28 kg·kg−1 body mass) or a control group (CG) (n = 8; age, 20.6 ± 1.6 yr; body height, 1.73 ± 0.06 m; body mass, 70.7 ± 9.7 kg; body fat, 8.6 ± 3.0; half-squat 1RM, 133 ± 21 kg or 1.88 ± 0.20 kg·kg−1 body mass). The participants were physical education students who were involved in practical lessons that included team sports, swimming, and jogging two to three times per week. None of these activities were structured as training sessions, but mainly included teaching and short practice drills. Also, all participants had experience in strength training, including squats with moderate to high loads for at least 2 yr, but none had taken part in systematic strength or power training for the last 6 months (25). Before data collection, a medical history questionnaire was completed by each participant, and informed consent was obtained after a thorough description of possible discomforts and risks. Participants were free of musculoskeletal injuries for at least 1 yr before the study and were not taking any drugs or nutritional supplements. The study was approved by the local institutional ethics review board, and all procedures were in accordance with the Helsinki declaration of 1964, as revised in 2013 (Code of Ethics of the World Medical Association).
A repeated-measures design was used to examine the effects of 6 wk of low-volume fast eccentric squat and ballistic complex training on maximal strength, explosive performance and muscle fiber characteristics. Participants were randomly assigned to either a TG, that trained three times per week, for 6 wk, or a CG that continued their recreational activities. Before testing, all participants took part in two familiarization and four preliminary measurement sessions, including maximum squat strength (1RM) measurement, jump squats at five different loads (0%–65% of 1RM) and determination of the optimal eccentric squat load. Also, a muscle biopsy from the vastus lateralis muscle was taken. After 6 wk of training or control period, measurements were repeated within 7 to 10 d. The dependent variables were 1RM strength, lean leg volume (LLV), muscle fiber composition and CSA, jump height, peak power, peak velocity, peak force, and RFD at different time windows (0–50, 0–100, 0–150, and 0–200) during jump squats at 0% to 65% of the 1RM measured before and after training, for the pretraining and posttraining assessments, respectively.
Participants visited the laboratory on six separate occasions before the start of training. On the first occasion, a muscle biopsy sample was obtained from m. vastus lateralis of the right or left leg, in a random and counterbalanced order. Five days and 7 d after the muscle biopsy, participants took part in two familiarization sessions, during which they performed half-squats, squat jumps and eccentric half-squats (more details given below), using light and moderate loads. To ensure reliable execution of the eccentric half-squat exercise, participants were further familiarized at the end of the next two preliminary visits. On the fourth preliminary visit, anthropometric measurements were taken and maximal half-squat (1RM) strength (knee angle, 90°) was determined. On the fifth preliminary visit, that took place 3 to 5 d after the 1RM test, participants performed jump squats against external loads of 0%, 15%, 30%, 45%, and 65% of 1RM on a force platform. On the last preliminary session that took place 4 to 5 d after the 1RM test, the load for the fast eccentric half-squat was determined, using seven different loads, that is, 30%, 40%, 50%, 60%, 70%, 80%, and 90% of the dynamic 1RM.
A standardized warm-up consisting of 5 min on a cycle ergometer against a light resistance (60 W) and 5 min of dynamic stretching of the lower limb muscles (gastrocnemius, gluteals, hamstrings, quadriceps, and hip extensors) preceded every pretraining and posttraining tests and every training session. All pretraining measurements were repeated within 7 to 10 d after the end of the 6-wk training period. The 1RM test was also performed at the end of week 3, to readjust training loads.
Body height was measured to the nearest 0.5 cm using a stadiometer (Charder HM-200P Portstad) and body mass was measured to the nearest 0.1 kg (TBF-300A Body Composition Analyzer-Tanita). Seven skinfold thickness measurements were obtained using Harpenden skinfold calipers (British Indicators Ltd., Herts, England), and estimates of body fat and lean body mass percentages were calculated (26). The intraclass correlation coefficient (ICC) for percent body fat was 0.97 (90% confidence interval [CI], 0.94–0.99; P < 0.01). Lean leg volume was estimated by measuring lengths and circumferences at different predetermined levels of the leg and modeling the lower limb as six truncated cones (27). The ICC for LLV was 0.98 (90% CI, 0.95–0.99; P < 0.01).
Muscle biopsies and histochemistry
Muscle samples (80–120 mg of wet muscle) were obtained using Bergstrom needles from the middle portion of the vastus lateralis under local anesthesia (1% lidocaine injection). Posttraining muscle biopsies were obtained from the same leg, 5 cm proximal to the pretraining incision point. Samples were aligned, placed in embedding compound, and frozen in isopentane precooled to its freezing point with liquid nitrogen. All samples were kept at −80°C until analyzed (within 30 d). Serial cross-sections 10-μm thick were cut at −20°C and stained for myofibrillar ATPase after preincubation at pH 4.3, 4.6, and 10.3 (28). All pretraining and posttraining biopsy slices from each participant were incubated at the same time in the same jar. A mean of 324 ± 25 muscle fibers from each preintervention and postintervention samples were classified as type I, IIa, or IIX (8,29). The percentage of each fiber type was calculated and the CSA of all the classified fibers from each sample was measured with an image analysis system (ImagePro; Media Cybernetics Inc, Silver Spring, MD) at a known and calibrated magnification. The ICC for the percentage fiber type composition and fiber CSA using this methodology have been previously found to range from 0.93 to 0.96, (P < 0.01) for all fiber types (30).
Measurement of ground reaction force
Ground reaction force during the eccentric squats and jump squats was recorded using a force platform (Applied measurements Ltd, Reading, UK), connected to a PC via a PCD-320A sensor interface (Kyowa, Japan) and recorded at 1000 Hz. A fourth-order reverse Butterworth low-pass digital filter with a cutoff frequency of 25 Hz and customized software were used to analyze the data. Instantaneous values of velocity, displacement, and power of the center of mass were calculated by digital integration with respect to time. Rate of force development was calculated during the eccentric phase of the countermovement jump (CMJ) as the slope of the force–time curve at the time windows of 0 to 50 ms, 0 to 100 ms, and 0 to 200 ms (RFD0–50, RFD0–100, and RFD0–200) (3). The onset of the RFD intervals (0 ms) was defined as the time corresponding to the lowest vertical ground reaction force during the eccentric movement (22). The length of the RFD time window for jump squats was limited to 200 ms, because this was the widest common RFD window containing the eccentric phase only, for all external loads. The ICC for peak force was 0.981 (90% CI, 0.955–0.992; P < 0.01), whereas for RFD, the ICC ranged from 0.870 (90% CI, 0.677–0.948; P < 0.01 for RFD0-50) to 0.961 (90% CI, 0.903–0.984; P < 0.01 for RFD0-300). Peak force is expressed in absolute, relative (N·kg−1) and allometrically scaled units (N·kg−0.67).
1RM half-squat strength
This test was carried out in a squat rack, using a standard Olympic barbell and weight plates. Five minutes after the standardized warm-up, participants performed a specific warm-up (31,32). After the warm-up, participants performed single repetitions against progressively heavier loads, until the 1RM was determined within three to four attempts, using a rest interval of 3 to 5 min (33) between attempts. Participants began in an upright position with the feet at shoulder width. They were then asked to lower the bar to the point where knee angle was 90° and were assisted by a “beep” which sounded when a photocell beam was interrupted by the posterior portion of their thigh when knee angle was 90°, and return to the starting position. The ICC for the half squat 1RM measurement was 0.98 (90% CI, 0.951–0.992; P < 0.01).
Determination of optimal load for the fast eccentric squat training
The load for the eccentric training was determined on the last preliminary visit. The vertical component of ground reaction force and eccentric RFD at the time windows of 0–50 ms, 0–100 ms, 0–200 ms, and 0–300 ms (RFD0–50, RFD0–100, RFD0–200, and RFD0–300) were measured during eccentric half-squats, against seven different external loads (30%, 40%, 50%, 60%, 70%, 80%, and 90% of 1RM). The onset of RFD intervals (0 ms) for the eccentric squat exercise was defined as the time corresponding to the lowest vertical ground reaction force during the eccentric movement (Fig. 1). Total movement time was defined as the time interval from the initiation of the downward movement (i.e., initial change in velocity), until the velocity became zero as the lowest displacement of the center of mass was reached. Peak downward velocity of the center of mass and the time to reach it from the start of the movement (absolute and relative to the total movement duration) were also calculated.
The eccentric half squat was performed in a squat rack, and started with the participant standing upright on a force platform with the loaded bar on their shoulders. Participants were then instructed to start the movement by bending their knees and to squat fast, aiming to come to a complete stop when the half-squat position was reached (knee angle, 90°). The direction given to participants was: “squat fast and stop rapidly at a half-squat position.” To ensure that the desired range of movement was achieved during each repetition, a “beep” sound was heard when a photocell beam was interrupted by the posterior portion of the thigh reaching a position corresponding to a knee angle of 90°. When the final position was reached, the bar was placed on the adjustable stops of the squat rack that were fixed approximately 10 cm below that point and was subsequently lifted by two assistants to the starting position for the second repetition. Thus, only the eccentric part of the movement was performed under load. Two trials were performed at each load, and the results were averaged. The rest interval between each trial was 3 min. Consistency of the eccentric phase was evaluated by measuring the peak downward velocity of the center of mass from the force plate data and providing feedback to the participant. The reliability of the eccentric half-squat exercise was confirmed by the high ICC values for all parameters examined. The ICC for peak force, peak downward velocity, and time to reach peak downward velocity were 0.970 (90% CI, 0.929–0.988; P < 0.01), 0.912 (90% CI, 0.746–0.966; P < 0.01), 0.938 (90% CI, 0.850–0.974; P < 0.01), respectively. The values of ICC for RFD ranged from 0.861 (90% CI, 0.666–0.942; P < 0.01 for RFD0–50) to 0.947 (90% CI, 0.873–0.978; P < 0.01 for RFD0–300). Similar high reliability values (≥0.90) for similar variables evaluated during the eccentric phase of jump squats were reported by Cormie et al. (22).
To obtain the optimal load for the eccentric exercise for each individual, peak force and RFD for the time windows (0–100, 0–200, and 0–300 ms) at each external load were multiplied to identify the highest product. Analysis showed that for each individual, the highest product of RFD and peak force was independent of the RFD time window, suggesting that any choice of time window would give the same result. Thus, RFD0-200 was chosen to determine the highest product for each individual.
Jump squat test
Five minutes after the standardized warm-up, participants performed a series of jump squats on a force platform. Vertical jump performance was assessed across five different loads: no external load or 0%, 15%, 30%, 45%, and 65% of 1RM measured before and after training, for the pretraining and posttraining assessments, respectively. The jump squat exercise was performed in the squat rack, with the participants standing on the force plate and holding a loaded barbell behind the neck across their shoulders. The jump squat with no load was performed, with a wooden stick on the shoulders. Participants were instructed to perform a downward countermovement to a knee angle of 90° and then jump as high as possible, keeping a constant downward pressure on the barbell throughout the entire jump. All jump squats were performed as explosively as possible to achieve maximal height (34,35). Participants maintained the same body position during take-off and landing. The depth of the squat was controlled by a “beep” sound which was activated when a photocell beam was interrupted by the posterior portion of the thigh when knee angle was 90°. Two maximal trials were performed against each load and the data for each were averaged. Recovery between jumps was set to 2 min and between loads, 3 min. Ground reaction force was recorded during each trial using the force platform. Jump height was defined as the highest point reached by the center of mass, calculated by takeoff velocity. The ICC for jump height was between 0.96 (90% CI, 0.904–0.983; P < 0.01) and 0.99 (90% CI, 0.977–0.996; P < 0.01) for all loads.
Participants trained on three nonconsecutive days per week, with a low-volume high-intensity strength–power–potentiation complex training program, including a combination of fast eccentric loaded squats and plyometric jumps (sessions 1 and 3), and ballistic loaded jump squats and plyometric jumps (session 2). The inclusion of midweek training with ballistic jumps against 30% 1RM, with sessions 1 and 3 consisting of either power or heavy strength training has been used in previous studies reporting improvements in power and ground reaction force during jump squats (22). This weekly schedule makes the training program more realistic and applicable to athletes aiming to improve leg muscle power.
Eccentric half-squat training
During sessions 1 and 3 of each week, participants performed the same standardized warm-up as testing followed by six sets of two repetitions of eccentric half-squats against the predetermined individual optimal eccentric load. The time between repetitions in the same set was 30 s, whereas sets were separated by 4 min of recovery. On the first, second and third minutes of the recovery interval between sets, participants performed a maximal-effort unloaded countermovement jump. Consistency of the execution of the eccentric exercise was ensured by controlling the depth of the squat using a photocell, as described above, and also by measuring the peak downward velocity of the center of mass from the force plate data and providing feedback to the participant.
During session 2 of each week, participants performed six sets of four consecutive maximal jump squats against a load corresponding to 30% of 1RM, separated by 4 min of recovery. In the first, second, and third minutes of recovery participants performed a maximal-effort unloaded countermovement jump.
Multiple three-way mixed factor ANOVA (2 TG × 5 loads × 2 time points [pre–post]) were conducted to examine the effect of training on jump squat performance variables (CMJ height, RFD at selected time-windows, peak force and time to peak force). Changes in muscle fiber distribution and CSA for each fiber type were examined by two-way mixed factor ANOVA (2 TG × 2 time-points). When a significant two-way (group–time) or three-way interaction (group–load–time) was found, Tukey test was used for post hoc analyses. Force-time variables (peak force, time to peak force, and RFD at selected time windows) obtained during the fast eccentric exercise against seven different external relative loads (i.e., same percentages of the pretraining and posttraining 1RM, respectively) were compared using one-way repeated-measures ANOVA and Tukey post hoc test when a significant effect was observed. Partial eta squared (η2) values were used to estimate effect sizes (small, 0.01–0.059; moderate, 0.06–0.137; large, >0.138). For pairwise comparisons, the magnitude of effect size (ES) was determined by Hedges g (small, <0.3; medium, 0.3–0.8; large, >0.8). Test–retest reliability for all dependent variables was assessed by the intraclass correlation coefficient (ICC, including 90% CI), using a two-way random effect model. Relationships between variables were examined by calculating the Pearson product–moment correlation coefficient (r). All statistical analyses were performed using SPSS (IBM SPSS Statistics Version 23). Data are presented as mean ± standard deviation. Statistical significance was set a priori at P < 0.05.
Optimal load for the fast eccentric squat exercise
Peak ground reaction force, increased with external load (P = 0.0001; η2 = 0.87) up to 70% 1RM and remained unchanged thereafter (Table 1, Fig. 2). A significant main effect was found for RFD at all time windows examined (P = 0.049; η2 = 0.23; P = 0.010; η2 = 0.32; P = 0.015; η2 = 0.30; P = 0.006; η2 = 0.33; P = 0.011; η2 = 0.32, for RFD from 0 to 50, 100, 150, 200 and 300 ms, respectively). Post hoc tests showed a significant decrease in RFD at the heaviest load (ES = 0.61 to 1.07, see Table 1). Total movement time was increased with external load (P = 0.0001; η2 = 0.49), with the 90% 1RM load showing a significant decrease compared with all other loads (Table 1). Peak downward velocity was similar for the 30% to 50% 1RM loads, but decreased significantly at higher loads (Table 1). However, the time to reach peak downward velocity remained unchanged across all external loads (Table 1). The downward vertical displacement of the center of mass was similar across all external loads and ranged from −0.30 ± 0.05 m to −0.32 ± 0.04 m.
The load that maximized the product of RFD and peak force ranged between 60% and 80% (four individuals at 80%, three individuals at 70% and one individual at 60% of 1RM) averaging 74% ± 7% 1RM.
Anthropometric characteristics and maximal half-squat strength
Body mass and percent body fat were similar in the two groups and were unaffected by the intervention, as shown by the lack of main effects or interaction (P > 0.42, TG vs CG: 8.0% ± 1.3% vs 8.1% ± 2.9%). However, a group–time interaction was found for LLV (P = 0.007, η2 = 0.41) and the post hoc test showed an increase from pretraining to posttraining in TG only (8077 ± 682 vs 8467 ± 794 mL, P = 0.04; ES = 0.50). The absolute change in LLV was highly correlated with the absolute change in 1RM performance (r = 0.90, P = 0.002). There was no change in LLV for the CG.
A significant group–time interaction was observed for half-squat strength (1RM), expressed both in absolute (P = 0.008, η2 = 0.40) and relative to body mass (P = 0.02, η2 = 0.31). Post hoc tests revealed that posttraining 1RM increased in the TG by 14.4% ± 9.3% (from 1.87 ± 0.28 to 2.14 ± 0.31 kg·kg−1 body mass; P = 0.001; ES = 0.83), but there was no significant change in the CG.
Muscle fiber type characteristics
The percentage of type I, IIA, and IIX fibers were similar in the two groups at pretesting and did not change after the intervention period, as shown by nonsignificant main effects or interaction (P = 0.53–0.89) (Fig. 3). However, significant interactions were found for type I, type IIA, and type IIX fiber CSA (P = 0.002, η2 = 0.58; P = 0.003, η2 = 0.53; P = 0.03, η2 = 0.33, respectively). Post hoc tests revealed that CSA increased in all fiber types by 8.3% to 11.6% (Fig. 3) for the TG only. Specifically, type I fibers increased from 4705 ± 643 μm2 to 5132 ± 596 μm2 (P = 0.001, ES = 0.65), type IIA fibers from 5939 ± 697 μm2 to 6421 ± 717 μm2 (P = 0.001, ES = 0.65), and type IIX fibers from 5004 ± 669 μm2 to 5568 ± 860 μm2, (P = 0.02, ES = 0.69). The sum of increases in CSA of all fiber types after training was correlated with peak power generated against different external loads (from r = 0.60, P = 0.015 to r = 0.83, P = 0.001) and with RFD for all time windows except the 0 to 50 ms (from r = 0.54, P = 0.029 to r = 0.88, P = 0.001).
A significant three-way group–load–time interaction and a two-way group–time interaction were found for CMJ height (P = 0.004, η2 = 0.23 and P = 0.0001, η2 = 0.82, respectively). For peak power, there was a significant two-way group–time interaction (P = 0.0001, η2 = 0.89), but not a three-way interaction (P = 0.32, η2 = 0.07). Post hoc tests revealed that CMJ height and power increased only for the TG, across all external loads by approximately 20% to 36% (P < 0.01, ES = 1.24 to 2.12) and by approximately 16% to 22% (P < 0.01, ES = 0.97 to 1.06), respectively (Fig. 4).
For peak force, the three-way interaction was not significant (P = 0.77, η2 = 0.03), but there was a two-way group–time interaction (Table 2). However, the post hoc test failed to reveal a significantly higher force for either the training (P = 0.12, ES = 0.28) or CG (P = 0.81, ES = 0.15). Peak force scaled allometrically or divided by body mass showed similar results to the absolute peak force (Table 2). There was only a two-way group–time interaction for time to peak force (Table 2), but the post hoc test did not show significant differences for either the training (P = 0.058, ES = 0.28) or CG (P = 0.74, ES = 0.16).
For RFD values up to the 0 to 200 time window, there was no significant three-way interaction (P > 0.49 to 0.99). However, a significant two-way group–time interaction was found for all RFD time windows (Table 2). The post hoc tests showed higher posttraining RFD values only in the TG (P = 0.001 for all time windows and ES = 1.06, 1.77, 1.77 for RFD50, RFD100, and RFD200, respectively). The typical increase in RFD in the TG is depicted in Figure 4 for RFD150 (P = 0.001, η2 = 0.65, ES = 1.95).
The main finding of this study was that low-volume, fast eccentric, and ballistic half-squat resistance training, in combination with unloaded jumps resulted in large improvements in leg explosive force (RFD), power and CMJ across a wide range of external loads. This was accompanied by increased CSA in all fiber types, while the percentage of fiber types remained unchanged. These adaptations occurred after only 6 wk of low volume training and may be attributed to the high ground reaction forces and RFD produced with fast eccentric squat training.
In the present study, both unloaded and loaded jump squat (0% to 65% of 1RM) performance was improved by approximately 20% to 36% following only 6 wk of fast eccentric training. A previous study by Cormie et al. (34) compared the effects of repeated unloaded jump squat training and a combination of jump squat and heavy strength squat training (90% 1RM). Interestingly, only the combination of heavy strength squat training (90% 1RM) and unloaded jumps improved jumping performance across a range of external loads (0–80 kg) after 12 wk of training. In contrast, when training was restricted to unloaded squat jumps only, CMJ height and power improved only when the external load was low (0 and 20 kg), and this was achieved after 21 training sessions performed over 12 wk (34). In the present study, the use of a low volume training protocol, consisting of a total of 12 fast eccentric squats with a submaximal load (≈74% 1RM) plus 18 CMJ in sessions 1 and 3 and 24 jump squats plus 18 CMJ in session 2 of each week resulted in a substantial shift of the relationship (function) between load versus jump height and power relationships and RFD (Fig. 4) in only 6 wk of training. This would suggest that short-term improvements in muscle power and explosive force, accompanied by hypertrophy of both fast and slow muscle fibers, may be attained without using heavy loads. The effectiveness of the training program used in the present study may be due to the high training intensity achieved using a fast plyometric squat exercise and a submaximal external load (≈74% 1RM). As seen in Figure 2 and Table 1, high peak ground reaction forces, as well as high RFD values were achieved. The fact that the optimal load was achieved at ≈74% 1RM makes this exercise both effective and possibly safer than other modes of eccentric training, such as drop jumps, where high impact forces increase the risk of injury (36) or very heavy traditional squats. Longer-term studies should examine the time course and sustainability of adaptations caused by this mode of eccentric training.
Another main finding of the present study was that the decrease in the percentage of type IIX fibers, commonly observed during heavy strength (3,8,9) or power training with loads from 30% to 60% 1RM (10), did not occur in this 6-wk training program. There is some evidence in the literature suggesting that low volume power training may enhance performance variables without decreasing the percentage type of IIX fibers. One study comparing low volume bench press training (five sets of three repetitions with a 3RM load and ballistic bench press throws) with maximal strength training, showed no significant change in the percentage of type IIX MHC, but a 6.0% improvement in 1RM and a 3.4% increase in maximum movement velocity only after the low volume training (37). In contrast, in the study by Lamas et al. (10), where the total volume was significantly greater than that used in the present study, maximum strength in the squat exercise increased 16.5% but there was a 48% decrease in the percentage of type IIX fibers after training. The improvement in maximum strength was attributed to the enlargement of type IIa and IIX fiber areas. In a recent study, Zaras et al. (4) examined the effects of 6 wk of strength or ballistic-power training against loads of 30% and 85% of 1RM, respectively, on maximal strength and unloaded jump squats and found that the percentage of type IIX fibers was reduced in the heavy strength TG by about 45%. In contrast, the percentage of type IIX fibers remained unchanged in the ballistic-power TG accompanied by a 25.8% increase in type IIx fiber CSA (4). In that study, maximal leg press strength was improved twice as much in the strength group (43% vs 21%), whereas CMJ performance was increased by about 10% only in the ballistic power group. In the present study, the improvement of maximal strength was 14.4%, whereas the improvement in CMJ performance and power during jumping was between 20% and 36%, across all external loads (Fig. 4). The fact that there was hypertrophy in all fiber types in the present study (Fig. 3), as opposed to only type II fibers in the study of Zaras et al. (4), may explain the relatively large increase in leg muscle power and RFD across the wide range of external loads examined in the jump squat exercise (18). Similar findings to the present study were recently reported after short-term eccentric squats and plyometric training in women (15), which resulted in an increase in CSA of the vastus lateralis in all fiber types and an improvement in unloaded CMJ by 10.7%. However, Kyrolainen et al. (38) reported that 15 wk of power training using a variety of stretch shortening cycle exercises (loaded and unloaded jump squats, sledge jumps, hopping and hurdle jumps) that had a fourfold higher training volume compared to the present study (80–180 vs 30–42 muscle actions) increased drop jump height performance by 23.3% but did not change muscle fiber distribution or muscle fiber area. In another study, a jump squat training protocol with loads ranging from 26% to 48% of 1RM was used three times per week for 8 wk, and similar results to our study were reported in terms of performance, that is, improvements in peak power and RFD during an isometric midthigh pull, as well as unchanged muscle fiber composition (39). However, there was no increase in squat strength, possibly due to the low load used for most of the training duration. Furthermore, there was no change in muscle fiber distribution in that study, possibly due to the low training volume, but CSA was not measured. In contrast, the present study shows that low-volume ballistic and fast eccentric training, combined with plyometric jumps, results in not only increases in muscle power and explosive performance, but also in squat strength and vastus lateralis muscle CSA, in relatively strong individuals who had not taken part in systematic strength or power training for the last 6 months before the study.
A novel finding of the present study was large increases in RFD (by 40%–107%) at all time windows across all loads used during the jump squat test (Table 2 and Fig. 4). Increases in RFD of similar magnitude have been previously reported for unloaded jump squats, after 12 wk of squat jump and light load jump squat training in individuals with similarly high 1RM values as in the present study (22). These improvements were accompanied by an increase in the rate of rise in average integrated EMG of the vastus medialis muscle after training (40). Interestingly, comparable gains in RFD were also observed during unloaded jump squats following a 12-wk heavy-resistance strength training intervention by Jakobsen et al. (3), which were also accompanied by gains in neuromuscular EMG activity and a large (34%) increase in mean muscle fiber CSA. RFD depends on both neural activation and the contractile properties of the muscles involved (19,41). The magnitude and rate of increase in EMG amplitude is considered as a major factor influencing early RFD, for example, in the first 30 to 75 ms (19). However, for muscle contractions that last longer than 75 ms, voluntary RFD may be influenced more by the contractile properties of the muscle and the magnitude of maximal voluntary force (41,42). This is in accordance with the findings of the correlation analysis of the present study, showing moderate to high correlations between changes in CSA and RFD across all time windows, except RFD in the initial 50 ms. Thus, a significant part of the improvement in explosive force and power found after the fast eccentric squat training in the present study may be due to an increase in the CSA, especially of the fast-twitch type IIa and IIX fibers (18). Similar to our results, Hvid et al. (20) reported a significant correlation (r = 0.70) between vastus lateralis type II area and knee extensor RFD measured in the first 50 ms in young men. The importance of muscle contractile properties for training-induced changes in explosive muscle performance has been previously implied in studies where there were no changes in EMG activity of the muscles involved, despite explosive performance increases (43). One limitation, common to all studies examining changes in muscle morphology during multijoint movements, is that the biopsy is obtained from a single muscle. Although the knee extensors are major contributors to jumping and squatting movements, the hip and ankle extensors generate torque and power and are stimulated during training. However, the vastus lateralis muscle is easily accessible, and the histochemical changes observed may be taken as representative of the muscle adaptations due to training.
An interesting observation was the relative constancy RFD values across all loads examined, in both the fast eccentric squat and the jump squat exercise (Table 2, Fig. 4). This would suggest that RFD during these dynamic squat exercises represents intrinsic contractile properties of the muscles involved and may be accurately examined irrespective of external load. Despite the relative similarity of RFD values from light to heavy loads during the fast eccentric squat exercise, RFD decreased when the external load was near maximal (i.e., 90% of 1RM, see Fig. 2 and Table 1). This may be due to a possible modification of the movement pattern when external loads were very high. Thus, it may be concluded that RFD determination during the jump squat exercise may be reliably performed using loads that range from 0% to 80% 1RM, but caution should be taken when heavier loads are used.
In conclusion, the findings of the present study demonstrate that a combination of fast eccentric and ballistic strength training with plyometric jumps in a strength–power–potentiation complex format, may lead to substantial increases in maximal leg muscle power, RFD and maximal strength, accompanied by gains in CSA of all muscle fiber types, without a reduction in fast twitch fiber composition. The relatively large improvements in power and RFD over a relatively short training period (18 training sessions across 6 wk) lends credence to this program’s effectiveness and makes it attractive for use by athletes and other populations who aim to achieve rapid short-term gains in muscle strength and power, with presumably low injury risk compared with heavy squat training and drop jumps.
There were no funding sources for this article. The authors do not have conflicts of interests. The results of the present study do not constitute endorsement by the American College of Sports Medicine. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
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Keywords:© 2018 American College of Sports Medicine
RESISTANCE TRAINING; PLYOMETRIC TRAINING; FIBER TYPE; CROSS-SECTIONAL AREA; RATE OF FORCE DEVELOPMENT; JUMP HEIGHT