The effects of plyometric training, also referred to as ballistic training or stretch-shortening exercise, have been studied rather extensively in both athletic and nonathletic populations. Benefits from this type of training include improved measures of muscular strength and power (4,8,14,18,23,27,29,30), joint function and stability (6,14,29), reduced incidence of serious knee injuries (13,14), and running economy (26). Furthermore, multiple studies (5,9,11,12,14,17,18,20,23,27,30), but not all (6,7,26) that have employed jump-specific plyometric exercises (i.e., depth jumps or drop jumps) have reported significant improvements in vertical jump (VJ). These increases in VJ have been associated with factors such as increased power output and maximum rate of force development (20) as well as increased muscle fiber size (23); characteristics likely attributed to the stretch reflex, high eccentric loading, and explosive nature of plyometric exercises (20).
Despite the numerous benefits associated with high-impact, high-intensity land-based plyometric training, the possibility exists for this type of training to induce acute muscle soreness, muscle damage, or even musculoskeletal injuries (1,11,15). Furthermore, for individuals undergoing current rehabilitation of an injury, the use of land-based plyometric training would most likely need to be delayed until such time that it could be safely incorporated into the training regimen. Thus, a relatively simple method of reducing impact forces and eccentric loading while still providing sufficient stimulus for physiological and sports-related improvements would be to perform plyometric training in a swimming pool, or aquatic plyometric training (APT) (25,28).
Although buoyancy reduces the stretch reflex and amount of eccentric loading during aquatic plyometric exercise, athletes encounter greater than normal resistance during concentric movements because of the viscosity of water. Thus, APT could provide a stimulus for improvement in a slightly different manner than land-based plyometrics.
Surprisingly, we could locate only two studies in peer-reviewed journals that examined APT (19,24). Miller and colleagues (19) compared the effects of 8 wk of land-based plyometrics versus APT on VJ, muscle power and torque, muscle soreness, and range of motion in college-aged men and women. Interestingly, the authors reported an increase in muscle power only in the APT group, whereas both the land-based and APT groups had increases in knee peak torque during knee flexion at only one of three speeds at which they were tested (6.28 rad·s−1). Furthermore, none of the groups displayed significant increases in VJ. One possible explanation for these blunted training responses could be the prescribed training intensity, which the authors characterized as “low” to “medium” intensity for the first 4 wk of the study (19).
On the contrary, Robinson et al. (24) compared the effects of 8 wk of land-based versus APT on VJ, muscle strength, sprint velocity, and muscle soreness in healthy college-aged women, reporting significant improvements in VJ (greater than a 30% increase in VJ for both groups), isokinetic torque production, and sprint velocity in both groups.
Although there were no significant differences in the magnitude of improvements between the two modes of training, the level of reported muscle soreness was significantly higher in the women performing land-based plyometrics as compared with those exercising in the water (24). Thus, it appears that APT has the potential to provide similar improvements in skeletal muscle function and/or sport-related attributes as land-based plyometrics, but with less soreness.
However, the findings of Robinson et al. (24) should be interpreted with caution, as the authors did not compare the training groups with a separate control group that was not involved in plyometric activities. Thus, it is difficult to determine the possible impact that a learning effect may have had on their VJ data.
Despite the important findings of Robinson et al. (24), it is not known if APT can provide increases in VJ and/or muscular strength for athletes undergoing concurrent sport-specific training. Thus, the purpose of the present study was to examine the effects of combined APT and traditional preseason volleyball training on muscular strength and VJ in high school–aged female volleyball players as compared with a control group performing a combination of flexibility exercises and volleyball training (CON). We hypothesized that the addition of APT to traditional volleyball training would lead to greater enhancements in VJ and muscular strength of the legs as compared with the CON group.
Approach to the problem.
The present study was conducted to determine whether adding APT to traditional volleyball training leads to larger increases in VJ and muscular strength as compared with a combination of flexibility exercises and traditional volleyball training (CON). We decided not to utilize a land-based plyometric comparison group for three reasons: 1) numerous previous investigations have substantiated the effectiveness of land-based plyometrics for increasing VJ and muscular strength; 2) only two previous studies have examined the efficacy of APT, reporting opposing findings with regard to VJ; and 3) to minimize the effects of outside physical activity on our findings, we studied an intact volleyball team of 19 subjects that were about to undergo the same program of preseason volleyball training; thus, dividing the team into three smaller groups would have reduced our sample size and statistical power. We decided on a 6-wk APT period to allow for the completion of all aspects of the study without interfering with the beginning the competitive season, but still allowing sufficient time for potential training adaptations. Also, due to the time constraints of the length of preseason, we were required to utilize the same order of testing before and after the study for each subject and, because APT reduces the stretch reflex and eccentric loading but increases resistance to concentric actions, we chose to only examine concentric isokinetic peak torque as a measure of muscular strength.
Nineteen female volleyball players (15 ± 1 yr, 61 ± 11 kg) (Table 1) were selected as a sample of convenience from a local high school volleyball team. The subjects and their legal guardians were informed of the potential risks and benefits of the study. The subjects then provided written informed consent to participate in accordance with the guidelines of the institutional review board of the University of Maryland Eastern Shore for use of human subjects. All subjects reported ≥2 yr of past experience in the sport of volleyball and were healthy as determined through a university-designed health history questionnaire and routine physical examinations performed by a physician. Subjects were excluded from the study if they had current or recent past musculoskeletal injuries, cardiovascular disease, uncontrolled metabolic disorders such as diabetes mellitus, or a fear of water. Although all subjects reported participation in submaximal aerobic exercise training for approximately 30 min·d−1, 2–3× wk−1 during the 3 months before the study, none had participated in strength training or plyometric training involving the legs during that same time period, and none of the subjects had ever participated in APT.
Submaximal cycle ergometry.
To estimate baseline fitness of the subjects before beginning the study, maximal aerobic capacity (V̇O2max) was predicted for all subjects on the same mechanically braked cycle ergometer (Monark, Stockholm, Sweden) (2,3,10). All subjects were asked to arrive at the laboratory without eating or drinking for at least 3 h, and without exercising for at least 8 h before testing. The subjects were also asked to refrain from taking any medications known to affect HR for at least 8 h before testing. At rest, and during each stage of the test, HR was monitored with a telemetry unit (Polar; Port Washington, NY), blood pressures with a standard stethoscope and sphygmomanometer, and ratings of perceived exertion with the 20-point Borg scale. All subjects were asked to sit quietly on the cycle ergometer for 5 min before obtaining resting HR and blood pressures. All subjects pedaled at a rate of 50 rpm with an initial resistance of 0.5 kp. Towards the end of the initial 3-min stage, a HR was recorded and, based upon the magnitude of the HR response to this initial work rate, the resistance was adjusted upward for a second 3-min stage while the pedal rate was kept constant (10). The same procedures were followed for a third stage and, in cases where there were not at least two stages with HR greater than 110 bpm, a fourth stage (10).
Vertical jump height, defined as the difference between standing reach height and the maximal jump height, was measured to the nearest 0.64 cm (0.25 inches) in all subjects at baseline and after 2, 4, and 6 wk by the same investigator (19,24). Briefly, the initial reach height of each subject was determined by having them stand, with feet flat, in a designated area adjacent to the wall with their dominant arm raised as high as possible. Each subject was then given an opportunity to perform two to three submaximal practice countermovement jumps. After a 2–3-min recovery, each subject performed three separate maximal VJ attempts. Although subjects were allowed to squat and swing their arms during each maximal attempt, they were required to maintain their feet within the designated area for all prejump movements. The highest of the three VJ attempts for each subject was utilized for data analysis. The intraclass correlation coefficient for the VJ test was 0.9293 (α = 0.9633) and the test-retest reliability was r = 0.93 (P < 0.001). For determination of sample size and statistical power, using an estimate of effect size of 0.75 cm, it was determined that seven subjects per group (14 total) would be sufficient to provide a power of 80% at α = 0.05.
Isokinetic peak torque.
To assess the effects of APT on nonspecific measures of leg strength, concentric peak torque (N·m) was measured unilaterally in both legs of all subjects during knee extension and flexion at 60 and 180°·s−1 by the same investigator before and after the study with an isokinetic dynamometer (Biodex, Inc., Shirley, NY) (16,21). The dynamometer was calibrated according to manufacturer specifications before each testing session, including the correction of gravitational effects on torque. All subjects participated in a brief warm-up consisting of light stationary cycling followed by static stretching of the hamstrings, quadriceps, and calf muscle groups. The subjects were then seated on the dynamometer with the hip and knee flexed to 90°, the foot hanging in a comfortable resting position, and the knee aligned with the dynamometer axis of rotation. Before testing, all subjects were stabilized using chest, thigh, and hip straps to reduce involvement of accessory muscle groups, and underwent establishment of a comfortable range of motion.
Beginning with a speed of 60°·s−1, each subject performed three to five submaximal practice lifts with the dominant leg, followed by five maximal extension and flexion efforts at the same speed. Rest periods of at least 30 s were given between each maximal lift. Upon completing the maximal lifts at 60°·s−1, a rest period of 2 min was given, followed by five submaximal practice lifts at 180°·s−1. Each subject then completed five maximal extension and flexion maneuvers at this faster speed, utilizing the same procedures. After completion of dominant leg testing, a rest period of at least 2 min was given before following the exact testing protocol for the nondominant leg. Peak torque values were determined to be the highest values recorded in each leg for both knee extension and flexion, at both speeds.
All baseline testing of leg strength (isokinetic peak torque) and VJ was completed less than 1 wk before the beginning of preseason volleyball training. The APT program was conducted concurrently with preseason volleyball training, and was completed before beginning the competitive season. Upon completion of all baseline testing, the subjects were randomly assigned to perform either APT combined with preseason volleyball training, or flexibility exercises combined with preseason volleyball training (CON); therefore, all subjects participated in the preseason volleyball training program, and APT was incorporated into the preseason volleyball training for the experimental group only (APT). The preseason volleyball training was conducted 5–6 d·wk−1, with each session lasting approximately 120 min. Typical preseason volleyball training sessions consisted of 10–15 min of submaximal warm-up exercises, followed by oncourt skills training, tactical situations, and actual game play. All volleyball sessions were directly supervised by the high school volleyball coaches who were informed about the study procedures, but were blinded to group assignment of the subjects. Thus, the coaches did not include any specific strength training or high-intensity plyometric exercises as part of the preseason volleyball training, other than volleyball drills that required the players to jump as part of the activity.
Aquatic plyometric training (APT) program.
The APT program was conducted twice a week for 6 wk in a swimming pool with a depth of approximately 122 cm and a temperature of 28°C. All APT sessions were begun within 30 min after cessation of preseason volleyball training sessions. Each APT session lasted approximately 45 min, and consisted of a warm-up, APT, and cool-down, all performed in the water. The warm-up consisted of approximately 5 min of light jogging in the water. The APT exercises included power skips, spike approaches, single- and double-leg bounding, continuous jumping for height, squat jumps with blocking form, and depth jumps (22). The subjects were encouraged to perform all APT exercises in an explosive manner, and to apply their maximal effort on all maneuvers.
The power skips, spike approaches, single- and double-leg bounding, and squat jumps were all performed with maximal effort along the width of the pool (12.2 m) two times per session during the first week.
These exercises were then performed along the width of the pool three times per session during the second week, four times per session for the third and fourth weeks, and five times for the fifth and sixth weeks. Bouts of continuous maximal squat jumps were performed three times (10 s of continuous jumps per bout, separated by 30-s recovery periods) per session during the first week, four times per session during the second week, four times per session during weeks 3 and 4 (increased from 10 to 20 s for each bout, separated by 30-s recovery periods), and four times per session during weeks 5 and 6 (increased from 20 to 30 s for each bout).
A series of depth jumps were performed involving three submerged boxes (61 cm in height) two times per session during week 1, three times per session for week 2, four times for weeks 3 and 4, and five times for weeks 5 and 6. The subjects began the depth-jump circuit by squat jumping from the pool floor onto the first box, then squat jumping without hesitation as high as possible and landing on the floor between the first and second box, at which point they immediately squat jumped as high as possible, landing on the second box. The subjects continued this pattern over the third and final submerged box, and recovered while walking back to the beginning of the circuit. After recovering for approximately 30 s, the subjects began the next interval. The cool-down period consisted of approximately 5 min of walking in the water followed by static stretching of the major muscle groups of the legs. Every APT session was directly supervised by two of the investigators.
To prevent CON subjects from dropping out of the study due to dissatisfaction about group assignment, we provided an investigator-supervised whole-body flexibility program twice a week for 6 wk. The program consisted of three sets of 8–10 static stretches for the major upper and lower body muscle groups, with each stretch being held for approximately 30 s. All flexibility sessions were directed by the same investigator, and were performed simultaneous to the APT sessions, but at a different location.
Independent samples t-tests were utilized to compare the physical characteristics of the APT and CON groups at baseline. Analyses of variance (ANOVA) with repeated measures (2 × 4 for VJ; 2 × 2 for peak torque) were used to examine changes in VJ and isokinetic concentric peak torque as a result of the study, as well as to compare group means at each time point (SPSS Inc., Chicago, IL). When significant time-by-group interactions were observed, post hoc paired t-tests corrected for alpha inflation (Bonferroni correction) were utilized for identifying the specific differences. All data are mean ± standard deviation (SD) with a significance level of P ≤ 0.05.
Physical characteristics of the subjects are presented in Table 1. There were no significant baseline differences between the APT and CON groups for age, height, body mass, resting HR, or predicted V̇O2max. All enrolled subjects completed the study, and there were no reports of significant muscle soreness or injuries resulting from the APT program.
The VJ heights were similar in both groups at baseline and after 2 wk of the study, and there were no significant increases in VJ for either group after 2 wk (Table 2). However, there were similar, significant increases in VJ after 4 wk in both the APT and CON groups (3 and 5%, respectively; both P < 0.05). A comparison of baseline VJ to values obtained after 6 wk revealed that the CON group had a higher VJ than at baseline, but was no different from the increase noted after 4 wk. On the contrary, the APT group improved their VJ by an additional 8% (P < 0.05) when comparing values obtained after 4 wk with those obtained after 6 wk, for a total increase in VJ from baseline of 11% (P < 0.05).
Isokinetic peak torque.
There were no significant differences in concentric peak torque in either the dominant or nondominant leg between the APT and CON groups at baseline (Table 3). Similar significant improvements in concentric peak torque were observed in the dominant leg of both groups when comparing baseline values with those obtained after 6 wk (all P < 0.05). More specifically, the improvements in both groups were similar for knee extension and flexion at both 60 and 180°·s−1. Testing of the nondominant leg for both groups revealed the same pattern of improvements as the dominant leg, except that the APT group displayed significantly larger increases than the CON group for knee extension at 180°·s−1 (P < 0.05).
Our hypothesis that larger increases in VJ height would occur in the APT group as compared with the CON group was supported, as APT resulted in a significantly larger improvement in VJ by the sixth week (11 vs 4% for APT and CON, respectively). With regard to concentric peak torque, both the APT and CON groups demonstrated significant improvements after the 6-wk study; however, the APT group had a significantly larger increase than the CON group for torque production in the nondominant leg during maximal knee-extension exercise at 180°·s−l. Thus, our study indicates, for the first time, that APT can produce significant increases in VJ in high school–aged female athletes.
Whereas Robinson et al. (24) examined the effects of 8 wk of land-based plyometrics and APT on VJ and torque production in physically fit college-aged women, not all of the women were currently involved in sports requiring the presence of significant leg power and jumping ability, such as volleyball or basketball. Although the subjects in the present study were significantly younger than in the study by Robinson et al., our findings coincide with and extend the findings of Robinson et al. (24) in that APT can induce significant improvements in VJ for young female athletes who are undergoing concurrent sports training.
Despite the reported success of land-based plyometrics for reducing sports-related injuries (6,14,29), the possibility of land-based plyometrics resulting in acute muscle soreness and musculoskeletal injury should not be ignored (1,15). Robinson et al. (24) reported that women performing 8 wk of APT had significantly less muscle soreness than those performing a comparable program of land-based plyometrics, especially after increases in training intensity. Although the present study did not specifically assess muscle soreness, it is important to note that there were no occurrences of injury, no complaints of significant muscle soreness, and no dropouts as a result of the APT program, but there were significant increases in VJ and leg strength.
In further support of the possible advantages of APT, Luebbers et al. (17) compared the effects of 4- and 7-wk land-based plyometric training programs on VJ height and power in physically active college-aged men. Despite both high-intensity training programs being performed 3 d·wk−1 and equalized for training volume, Luebbers et al. (17) observed significant decreases in VJ height shortly after completion of both programs. However, when VJ height was reexamined after both groups underwent a 4-wk recovery period, both groups displayed significantly higher VJ values than those obtained at baseline and immediately after the plyometric programs. These findings suggest that high-intensity land-based plyometrics may lead to an overtraining effect and may require a recovery period before competition (17). In contrast, neither the present study nor the study by Robinson et al. (24) incorporated a recovery period, but both observed significant increases in VJ shortly after training, providing evidence that APT can induce performance related benefits (VJ height) without the need for a long recovery period.
Despite the unique findings of the present study, there are factors that could be expanded upon by future investigations. Assessment of the effects of APT on muscle soreness, muscle damage, skeletal muscle biochemistry, neuromuscular characteristics, and biomechanics could provide information regarding possible mechanisms underlying training-induced adaptations. In addition, there is a need to compare the effects of land-based and APT programs on different athletic populations, and to compare APT programs of varying intensity, duration, and volume. Furthermore, the assessment of training-induced changes in muscular strength may be enhanced by including measurements that are highly specific to the mode of training. For example, the measurement of isokinetic peak torque in a seated position, as in the present study as well as numerous others, may not fully represent the extent of muscle adaptations that may have occurred as a result of APT or land-based plyometric training. Finally, because athletes would be exposed to greatly reduced ground impact forces during APT due to the buoyancy of water, investigations involving the efficacy of APT as a training mode for individuals currently in the rehabilitation process as well as for secondary prevention of musculoskeletal injuries seem warranted.
In summary, the present study indicates that APT can produce significant increases in VJ and, to some extent, isokinetic peak torque in young female volleyball players. In addition, because athletes can perform high-intensity plyometric exercises in water, it is proposed that APT could provide similar benefits as land-based plyometrics, but with lower risk of muscle soreness and/or overtraining.
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