Much research has been directed toward determining the effects of various training protocols on vertical jump (1,8-10,17,23,29,33). Wilson et al. (33) compared the effects of 10 wk of training using loaded jump squats, traditional back squats, or plyometrics in the form of drop jumps on vertical jump performance. All the training groups produced increases in vertical jump performance except the plyometric group which did not increase concentric-only squat jump height. This study demonstrates many aspects of training for improved vertical jump performance. Several studies (5,29,31) have observed that plyometric training increases primarily counter movement jump (CMJ) performance but not necessarily squat jump (SJ) height. This may be because the plyometric training enhances the ability of the subjects to coordinate the neural control of stretch shorten cycle (SSC) movement more effectively but does not produce significant increases in leg strength and contractile power.
Traditional squat training increases vertical jump ability but not to the same extent as squat jump training (33). This may result from an inherent problem with traditional weight training when one is attempting to increase power output rather than strength (26). It has been observed that the load is decelerating for a considerable proportion (24%) of the concentric movement (11) during traditional weight training exercises. This percentage increases to 52% when performing the lift with a lower percentage (81%) of 1RM lifted (11) or when attempting to move the bar rapidly in an effort to train more specifically to the movement speed of the target activity (26). Plyometric and weighted jump squat training avoids this problem by allowing the athlete to accelerate all the way through the movement. Newton and Kraemer (25) have described this form of training as "ballistic" resistance training.
Several studies have compared the effectiveness of plyometric, weight training, and a combination of plyometric and weight training (1,3,8,12,23,28,30). Although specific training protocols vary, in general, plyometrics have been effective for increasing vertical jump (1,8-10,17,29,33). Traditional weight training has resulted in increases in vertical jump by the majority of the research (1,2,27,32-34) with a limited number of papers finding only minor changes in already strength trained subjects (15).
When weight training is combined with plyometrics, vertical jump is increased, (2,3,8,23) and this is perhaps a greater stimulus to vertical jump performance than either weights or plyometric training alone (1). These findings highlight the multi-faceted nature of vertical jump performance with a mixed methods approach being most effective as it develops more components of the vertical jump (25).
Although research has demonstrated the efficacy of ballistic resistance training (16,23,33), the subjects used in these studies were not at the elite level. It remains to be determined whether a ballistic resistance training program will be effective for highly trained jump athletes. In addition, should significant changes in functional jump performance result, what characteristics of muscle function, i.e., maximal strength, SSC capability, maximum rate of force development (mRFD), and power output have exhibited adaptations that contribute to this performance improvement. Further, preseason preparation for competition involves a short-term, intense training program with multiple goals of increasing strength, endurance, and game skills. This may interfere with vertical jump development because of the time devoted to improving the other fitness components and the interference effects particularly of the endurance training (22). Therefore, the aim of this study was to determine the effects on vertical jump performance of an 8-wk ballistic resistance training program incorporated into the preseason preparation of elite volleyball players.
Sixteen male volleyball players from a NCAA Division I team were informed of the benefits and risks of the study. Subsequently each signed an informed consent document in accordance with the guidelines of the University's Institutional Review Board for use of human subjects. The subjects were medically screened and had no medical or orthopedic problems that would compromise their participation and performance in the study. At the conclusion of the subsequent playing season, the team was ranked in the top four in the United States for men's NCAA volleyball. The subjects were randomly allocated to either the treatment or control with eight subjects in each group. Subject characteristics were as follows: age, 19 ± 2 yr; height, 189 ± 7 cm; weight, 84 ± 6 kg. All subjects had a minimum of 2 yr resistance training and 5 yr volleyball training background.
Experimental Design and Procedures
This study was a longitudinal training experiment with testing completed before and after the 8-wk training period.
Training program. All subjects completed the same resistance training program for the upper body plus leg curl and leg extension exercises. This program consisted of free weight and machine exercises performed at an intensity of 10RM with sets of 15 repetitions completed for the abdominal and lower back exercises. In addition, the control group completed three sets each of squat and leg press exercises using a load of 6RM while the treatment group completed ballistic resistance training consisting of six sets of jump squats with a counter movement performed on a Plyometric Power System (PPS) (Optimal Kinetics, Lismore, Australia). Because of the difficulty of comparing training volume between the ballistic and traditional resistance training based on work done, the amount of training was balanced between the two groups based on total number of repetitions completed. The ballistic training program consisted of six sets of six repetitions performed with two sets at each load of 30%, 60%, and 80% of the subject's pretest 1RM squat. The eccentric brake system previously described by Humphries et al. (18) was used to remove approximately 75% of the weight of the bar on the downward or eccentric phase. All subjects trained for 8 wk with two sessions of lower body and two sessions of upper body resistance training per week. The test measurements were taken immediately before and again after the training period. In addition, all subjects completed the usual preseason volleyball preparation that included aerobic training in the form of jogging, on-court skills training, and actual game play; however, they did not complete any specific plyometric training. In general, the players completed four to five training sessions per week not including the four resistance training sessions.
Testing protocols. Jump and Reach tests: Jump and reach performance was measured using a Vertec (Questtek Corp., Northridge, CA). Reach height was established by having the subject stand flat-footed and reach up to displace the marker on the Vertec. The subject then performed two types of jumps: a) the standing vertical jump and reach (SJR) for which he dipped to a self-selected depth and then jumped and reached with his preferred hand to displace the marker on the Vertec; b) a three-step approach followed by a takeoff from one leg to reach and displace the marker on the Vertec (AJR). Three trials were permitted for all jumps with the highest jump being used in subsequent statistical analysis.
Maximal squat strength test. Maximal squat strength (1RM) was assessed by having the subjects perform a concentric-only squat from a position of 110° knee flexion using the Plyometric Power System (PPS) following methods similar to that described by Wilson et al. (33). The load lifted in the best successful attempt was recorded in kilograms.
Plyometric Power System squat jump tests (PSJ). Squat jump performance under loads of 30%, 60%, and 90% of 1RM was assessed using the PPS. Subjects were placed in a position of 110° knee flexion (measured by goniometer) with the heels directly under the bar of the PPS. They were then instructed to explode upwards attempting to jump for maximal height. Bar displacement and mass was recorded and various velocity, acceleration, work, and power variables were calculated.
Force plate tests. Force plate testing consisted of drop jumps (DJ), counter movement jumps (CMJ), and squat jumps (SJ) with both takeoff and landing performed on a triaxial force plate (AMTI, MA). The vertical ground reaction force was recorded and measures of flight time, contact time, force, mRFD, velocity, and power output calculated. The various jump types were used to assess specific aspects of vertical jump performance.
- Drop jumps: The subject dropped from a 30-cm height, landed on the force plate, and then attempted to jump for maximum height, landing back on the force plate.
- Counter movement jumps were performed by dipping down to a knee angle of 110° (visually checked by an observer) and then jumping for maximum height landing back on the force plate. The counter movement jump was performed with body weight alone.
- Squat jumps involved the subject flexing the knees and hips such that the angle at the knee was 110° (measured by goniometer) and holding this position for 4 s. They then jumped vertically upwards from this position attempting to attain a maximum height. If a preparatory dip was observed on the force time graph, then the trial was discarded and the subject made a further attempt. Squat jumps were performed with body weight alone, and with the addition of 20 kg and 40 kg strapped to the subject's torso.
Two trials were recorded for each condition with the trial resulting in the greatest jump height used in further statistical analysis. The vertical force output was integrated with time to produce velocity and displacement data for the subject center of gravity. Summary kinetic and kinematic data were then calculated using standard methods.
As a number of novel tests and calculated variables were used in this study, the test-retest reliability was determined for all variables by comparing two trials from the pretraining testing sessions. The intraclass correlation coefficient (ICC), technical error of measurement (TEM), and technical error of measurement as a percentage of the mean measurement (TEM%) was calculated according to the methods of Knapp (20) for each variable. The results of this analysis appear in Table 1.
Means and SD were calculated. Multivariate ANOVA was performed with two independent variables of group (control and treatment) and test occasion (pre- and post-training period). In the event of a significant interaction, univariate F-tests were applied to each dependent variable followed by Newman-Keuls post-hoc analysis. A criterion alpha level of P < 0.05 was used to determine statistical significance. Statistical power for the various comparisons ranged from 0.05 to 0.99 (Table 2); however, for the SJR prepost and prepost by group statistical power was 0.99 and 0.60, respectively. Analysis of the AJR results revealed prepost and prepost by group statistical power of 0.88 and 0.65, respectively.
Jump and Reach tests
The results of the standing and approach jump and reach tests are presented in Table 3. The control group's performance did not change over the training period for either jump. The treatment group increased jump height significantly over the training period by 5.9 ± 3.1% and 6.3 ± 5.1% for the standing and approach jump, respectively. Both these percentage increases were significantly greater than for the control group.
Maximal Squat Strength
There were no significant changes in 1RM squat strength in either group between the pre- and post-training tests (Table 4).
Plyopower Squat Jump Tests
MANOVA revealed significant group by prepost by load and group by prepost interactions. Subsequent F-tests showed significant group by prepost effects for bar displacement, velocity, and power. Figure 1 compares the two groups pre- and post-training. Specifically, the treatment group increased bar displacement, velocity, and power out-put over the training period at all the loads tested. The control group produced increases in displacement, velocity, and power output only at the 30% of 1RM load. No other significant changes were recorded for the control group. A comparison of the percentage changes over the training period revealed that only power output at the 90% of 1RM load was increased significantly more for the treatment compared to control groups.
Force Plate Tests
Counter movement jump tests. There were no significant interactions of group or test occasion for any of the variables measured during the counter movement jump test. The control group significantly increased maximum velocity by 5.2 ± 4.2%. The treatment group significantly increased mean force by 2.1 ± 2.5% and peak power by 8.0 ± 8.9%. There were no significant differences between the groups for percentage change pre- to post-testing for any of the measured variables. In particular, there was no change in flight time or jump height by either group.
There were significant group by prepost interactions for the variables measured during the squat jump tests on the forceplate. Figures 2 and 3 contain the results of these tests. Notably, between the pre- and post-tests the treatment group produced significant increases in peak force production of 11.3%, 5.4%, and 5.4% when performing squat jumps with BW, BW+20 kg, and BW+40 kg, respectively (Fig. 2). The control group only increased peak force at the highest load of BW+40 kg by 2.5%. The 11.3% increase by the treatment group when jumping with BW only was significantly greater than the percentage change by the control group. When examining the average force produced over the concentric phase of the squat jump, the treatment group exhibited 9.7% and 4.7% increases for the BW and BW+40 kg trials. The control group did not produce any change in average force over the training period. In addition, the percentage increase in force was significantly greater for the treatment group than the control group for the BW only trial.
Peak power output was not significantly changed pre- to post-testing for the treatment group. The control group produced a significant increase in peak power output of 7.7% for the BW+40-kg load; however, this was not significantly different from the percentage change for the treatment group. The only significant change in the average power produced during the concentric phase of the squat jumps was exhibited by the treatment group for the BW only trial (18.9%); however, this was not significantly different from the percentage change for the control group.
The treatment group produced a 47% increase in mRFD over the course of the training period, and this was significantly greater than the control group that did not change mRFD pre- to post-testing. The control group did, however, produce a significant increase in mRFD (9.2%) for the BW+20-kg load but this was not significantly different to the treatment group.
The results for the drop jump tests appear in Table 5. There were no significant differences between the control and treatment groups for any of the measured variables either before or after the training period. The treatment group significantly decreased contact time by 14.6 ± 9.7%, and this was significantly greater than the control group that had no change. Flight time increased by 4.7 ± 3.4% for the treatment group and 1.9 ± 2.2% for the control group; however, there was no significant difference between the groups for the percentage change. The treatment group increased their flight to contact ratio by 24.4 ± 19.6%, and this was significantly greater than the control group which did not change over the training period.
The primary results of this study indicate that an 8-wk program of ballistic resistance training is effective for increasing the jump and reach performance of elite volleyball players. These performance gains were associated with improvements in force, velocity, displacement, power output, and rate of force development during jumping on the force plate and PPS but not maximal strength as measured by a concentric-only 1RM. Interestingly, jump height as measured by flight time did not change over the training period for either CMJ or SJ performed on a forceplate.
Ballistic resistance training has been effective for increasing explosive performance of resistance trained though nonelite subjects (23,33). The findings of the present study are that such training will also result in performance improvements in elite volleyball players. These subjects had a considerable training history in terms of traditional resistance training, plyometric training, and on court drills; however, the addition of ballistic training for an 8-wk period resulted in a 5.9% and 6.3% improvement in standing and approach jump and reach height, respectively. This would certainly be considered meaningful as well as statistically significant given the level at which these athletes are competing. The results are comparable with those of Lyttle et al. (23) who found 7.9% and 5.8% increases in performance of similar jump tests following 8 wk of ballistic resistance training in nonelite athletes. Fry et al. (13) has also reported a 7.4% improvement in approach jump and reach performance by women collegiate volleyball players following a training program combining traditional weight training and plyometrics.
The control group completed traditional, slow velocity resistance training as well as on-court practice involving a large volume of jumping with body weight alone over the same period, and it is notable that their vertical jump performance did not change. Lyttle et al. (23) found a combination of heavy resistance training and plyometrics to be equally effective as ballistic resistance training. Perhaps because the population used had never completed any explosive-type training, both protocols were effective because each represented a novel stimulus to the neuromuscular system. Komi and Häkkinen (21) have reported that in relatively untrained subjects a wide range of interventions will produce adaptations. The subjects used in the current study were familiar with both strength training and the on-court volleyball preseason preparation, and so little if any improvements resulted. However, the addition of the ballistic resistance training was a novel stimulus that was very specific to vertical jumping and resulted in performance improvement.
The increase in jump and reach performance by the treatment group is the most relevant even though changes in some of the forceplate tests did not realize increases. In terms of on court performance, the jump and reach test is the most specific and as such the standard by which improvement should be gauged. The other tests were performed in an attempt to explain or identify specific adaptations in jumping performance that contributed to the overall increase in jump height.
Given the specificity of the ballistic resistance training to the squat jump tests on the PPS it is not unexpected that the treatment group produced increases in jump height, velocity, and power output over the full spectrum of loads tested. Based on previous research (15,33) it would be expected that the control group would have increased strength in the leg press and squat exercises when training with a load of 6RM. Therefore it could be hypothesized that the heavy resistance training by the control group would result in improvements in jump performance, particularly for the heavier (90% 1RM) load, but this was not apparent. Bobbert and Van Soest (4) completed a simulation study of the effects of increased muscle strength on jump height. Interestingly, this initially resulted in a decrease in height jumped. The authors had to modify the control of the neuromuscular system, commonly referred to as coordination, timing, or technique, to actually produce an increase in jump height. The control group in the current study may have increased their muscle strength in the training exercises but required further movement specific training on the PPS to realize these strength improvements in enhanced squat jump ability.
Although the treatment group was training with loads up to 80% of 1RM, their 1RM strength was not improved. This may have been the result of using a concentric-only 1RM test while all ballistic resistance training was performed using SSC movements. These results may further support the test specificity of strength and power measures described by Murphy et al. (24). Alternatively, the neuromuscular adaptations resulting from ballistic resistance training even with heavy loads may be different from that resulting from traditional slow velocity resistance training. A further plausible explanation may be the low volume, only two sets of 80% 1RM jump squats, resulting in limited stimulation toward increases in maximal strength. As such, no improvement in 1RM strength was realized. Research has found load-and velocity-specific training effects (19) following training using ballistic movements, and so it is reasonable to assume that no gains in slow velocity maximum strength would result from the ballistic training program used in this study.
Possible factors contributing to the increased performance. The 1RM test results (Table 4) indicate that the strength of the subjects did not change over the training period; therefore, this is unlikely to have contributed to the improved jump and reach results. However, jump height, peak movement velocity, and peak power output increased for all loads during the squat jumps performed on the PPS (Fig. 1). It is possible then that although strength at slow velocities was not improved, the ability of the neuromuscular system to maintain tension while the muscles are rapidly shortening (fast velocity strength) may have been enhanced as well as explosive strength. This hypothesis is supported to some extent by the relatively large increases in average force and power output during the light load (BW only) squat jumps performed on the forceplate.
Perhaps the most significant adaptation was in terms of mRFD which increased some 47% (Fig. 3) for the treatment group performing a squat jump on the forceplate. This was a dynamic measure of mRFD, which has been reported by Murphy et al. (24) as being closely related to maximum power performance. This interesting finding is strongly supported in research by Häkkinen et al. (14) who found that explosive-type resistance training resulted in significantly greater increases in mRFD than heavy resistance training and this was related to greater improvements in vertical jump performance.
The evidence as to the relative importance of improvements in SSC performance is contradictory. The drop jump test (Table 5) emphasizes the SSC and its influence on jump performance. In this study, the treatment group produced a 4.7% improvement in flight during the drop jump test. In addition, they significantly reduced their contact time (−14.6%), resulting in a markedly increased (24.4%) flight-to-contact ratio. This would appear to suggest an improvement in SSC performance; however, this cannot be adequately confirmed other than to say that the ballistic-trained subjects appeared to spend less time in contact with the ground and produce a better subsequent jump height. Such performance changes have been reported to result from improved SSC capability (6,7).
The other test that involved a SSC movement was the CMJ test. Interestingly, neither group produced a change in flight time for this test. This would seem to indicate that SSC performance was not improved by the training; however, the result may have been caused by problems in using flight time as an indicator of jump height. Also, it is possible that the stretch load involved in a CMJ is much less than that used in the training, and there may be load-specific adaptations in terms of SSC as there are for muscle strength (19). The fact that DJ performance was enhanced by the higher resistance ballistic training supports this contention. Further research is required to determine whether the amount of eccentric loading during training results in load-specific SSC enhancement.
Wilson et al. (33) found highly SSC load-specific training adaptations. Plyometric training involves a considerable SSC component and resulted in a 10% increase in CMJ performance but only a 7.2% increase in SJ performance. In the same study, a weight-trained group produced improvement in SJ performance of 6.8% and improved CMJ ability by only 5%. These effects presumably reflect contractiontype specific training adaptations.
Lyttle et al. (23) found the combination of plyometric and weight training to be relatively superior to ballistic resistance training for increasing SSC performance. However, the load used in this training study was limited to 30% 1RM and no eccentric braking was used so comparison with the results of the current study is difficult. Given the findings of prior research (5,29) the use of the eccentric braking in the current study may also have reduced the SSC training adaptations. Future research should address the specificity of concentric-only and SSC movements used in training and subsequent improvements in explosive concentric-only and SSC performance.
The measurement of ground reaction force revealed many interesting aspects of the jump performance; however, they did not reflect training improvements as well as the jump and reach tests. This may have been a result of the restriction of technique in the forceplate tests. The increases in neuromuscular performance resulting from the training may be better realized in a familiar skill like jump and reach rather than the hands on hip, prescribed depth, and limited trunk flexion required of the CMJ, SJ, and DJ as used in this study.
Although this study has increased our knowledge of the effects of ballistic resistance training for elite athletes, there remain a number of avenues for further research. 1) What are the specific effects of heavy versus light load ballistic training and the velocity-specific changes occurring in terms of neural activation and motor unit recruitment patterns, as well as histochemical changes such as calcium activity and myosin heavy chain isoform profile? The ballistic training in the current study was performed over a range of loads. Therefore, it cannot be determined whether the differential effects between the two groups resulted from the different stimulus to the neuromuscular system of a) ballistic versus slow controlled movements and/or b) the inclusion of lighter load, high velocity movements when jump squat training with the 30% and 60% loads. 2) What would be the optimal number of repetitions per set at a given load? 3) Would periodization of aerobic, strength, power, and skills training during the preseason preparation be more effective and what are the interference effects of training several components of volleyball performance concurrently? Kraemer et al. (22) has demonstrated interference effects on strength improvement when aerobic training is done concurrently. Is a similar effect evident for concurrent aerobic and ballistic training in terms of maximal power development? 4) What was the effect of using the braking system and the role of eccentric loading on maximal power development? This question requires further investigation by a specific comparison of eccentric braking with no braking in terms of training adaptation.
In summary, ballistic resistance training is effective for increasing the vertical jump performance of elite volleyball players in a sport specific jumping performance. The improvements result primarily from an increased ability to produce force and power throughout the concentric phase, an increased maximum rate of force development, and possibly improved SSC capability.
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