The ability to generate lower body explosive power can be considered an important factor in many athletic activities. Speed-strength, also known as power, is crucial for the performance of different sports actions especially those involving changes in direction, accelerations, jumping, and sprinting (10). Vertical jump performance is considered an effective field evaluation of lower body power because the height of the jump correlates significantly with maximal power relative to body mass (W·kg−1) (18,19). Also, training that uses jumping movements has been shown to produce significant gains in lower body muscle power (13,22).
The VertiMax, a jump training apparatus using rubber bands, would appear to improve lower body power and vertical jump performance. Although this jump training modality has shown improvements anecdotally, no controlled studies have been performed to determine its effectiveness. The jump trainer features a 6 × 6-ft platform on which an athlete may perform 1 and 2 foot sport-specific movements. Four elastic cords retract down through the platform and coil around pulleys located underneath the platform. The VertiMax includes a belt that the athlete must wear around the midsection of their bodies while training. When the belt is in its proper space, the athlete can strap 2 or all 4 cords to the belt for the desired level of resistance. Due to the pulley system, nonvarying force is applied throughout the entire jumping movement. A more traditional form of training to improve the production of lower body power is plyometrics.
The most widespread approach to exercise prescription in resistance training is based on the concept of specificity (15). The universal thought in this theory holds that exercises should replicate a movement as closely as possible in the type of muscle action and contraction forces (3,6,11,15). This theory postulates that muscles should be taught to work (neuromotor learning) in training to improve power production in competition.
Plyometrics are used to improve lower body power and increase explosiveness by training the muscle to do more work in a shorter time (10). This has been accomplished by optimizing the stretch-shortening cycle, which occurs when the active muscle switches from a rapid eccentric muscle action (deceleration) to a rapid concentric muscle action (acceleration) (10,12). The rapid eccentric movement creates a stretch reflex producing a more forceful concentric muscle action than could otherwise be generated from a resting position (14). Therefore, the faster the muscle is stretched, the greater the force produced, and the more powerful the muscle action (20). Plyometric exercises that exploit the stretch-shortening cycle have been shown to enhance the performance of the concentric phase of the movement (8) and increase power. Traditionally, plyometric exercises include variations of bounding, hopping, and jumping drills. However, true plyometric training requires the rapid prestretch (eccentric muscle action) of the muscle and maximal effort of the athlete during the concentric muscle action. This type of plyometric training can be in various forms of depth jumps and box jumps (12).
This investigation attempted to determine the most effective form of jump training to increase vertical jump performance as well as expand current knowledge by providing insight into new training techniques being used in the field. The focus of this study was short-term training only. The program discussed here might be applicable to the power phase of a periodized training program. Since box jumps and VertiMax are both plyometric exercises, when done with proper technique and effort, both movements should have positive effects on the stretch-shortening cycle and result in an increase in lower body power and improved vertical jump. Therefore, the purpose of this study was to compare the effects of a twice-weekly, 6-week depth jump training program vs. a VertiMax basic training program.
Experimental Approach to the Problem
To measure the effects of VertiMax jump training vs. depth jump training on vertical jump performance, men and women kinesiology college students were randomly assigned to 3 different groups (control, VertiMax, and depth jump) with the VertiMax and depth jump groups participating in a 6-week training program with almost identical volumes.
Thirty-one men and women, recreationally trained kinesiology students, gave written informed consent to participate in this study. The subjects were not currently athletes and were asked to not take part in any other plyometric or jump training program or lower body strength training while participating in this study. There was no significant difference in any demographic measure between the 3 groups before testing (Table 1).
Each subject was measured for vertical jump height both pre- and post-training. Pre-testing was conducted the week prior to the initiation of training. Subjects were instructed to refrain from exercise for 48 hours prior to testing. The pre- and posttest procedures all followed the same routine with measurement of body weight, then a 5-minute warm-up on cycle ergometer, then an active-dynamic stretching routine including forward walking lunges, backward walking lunges with arms overhead, lateral lunges, Spidermans, Frankensteins, walking quadriceps stretches, inverted toe touches, and high knee butt kicks (15 yd of each). Each subject then had a 5-minute rest period before completing 2 practice vertical jumps and 3 test jumps.
Vertical jump height was measured by the stand and reach method using a Vertec (Sports Imports, Columbus, OH). Subjects completed 3 test jumps with a 30-second recovery between each jump. If a subject improved their jump height on the third jump, they were then allowed an additional jump. The subject's highest jump was used to quantify lower body power.
Each group (control, n = 10, with 4 women; VertiMax, n = 11, with 3 women; and depth jump, n = 10, with 4 women) completed a twice-weekly 6-week training program and the depth jump group completed a total of 137 jumps, while the VertiMax group completed 139 jumps. No subject missed more than 1 training session.
Depth jump subjects began by standing on a 50-cm plyometric box and were instructed to lead with 1 foot as they stepped down from the box and land with 2 feet on the ground. Instantly upon ground contact, subjects were instructed to “explode” off the ground by jumping as quickly and as high as possible. The volume began low and increased every week to accommodate training adaptations as well as increases in the plyometric box height (Table 2). The plyometric box height began at 50 cm and increased by 10 cm every week until the sixth week at 100 cm.
The VertiMax (Model V6, Genetic Potential, Tampa, FL) group training followed an altered basic training program. The manufacturer suggests a cord intensity that will not significantly alter jumping and landing kinematics. The manufacturer defines 3 jumps: quarter quick jumps in which the subject has resistance and dips until the thigh is 45° to the horizontal and then explodes upward with no pause between reps, squat jumps in which the subject has resistance and squats until the thighs are parallel with the floor and then explodes upward with a pause and reset between reps, and contrast jumps in which the subject performs multiple squat jumps with no resistance but a pause between reps. Resistance (through the use of the elastic cords) was increased 3 lb each week from 0 to 15 while volume was decreased each week (Table 2).
A 3-way mixed-factor repeated-measures analysis of variance (ANOVA) [2 (times) × 2 (genders) × 3 (groups)] was used to determine statistical differences between the 3 groups. The purpose was to ascertain whether vertical jump performance changed as a function of 2 different interventions when compared to each other and a control group and whether there was an effect for gender. Significant interactions were followed up by simple 1-way ANOVAs to determine the result of interest. A priori α was set at 0.05.
SPSS 14.0 was used to perform the vertical jump height statistical analysis, which included a 2 × 2 × 3 (time × sex × group), 3-way mixed-factor repeated-measures ANOVA. There were no 3-way interactions; however, there was a 2-way interaction of time × group. This was followed up by 3 simple ANOVAs for time for each group. Results demonstrated that the depth jump group significantly (P < 0.05) increased their vertical jump (pre: 20.5 ± 3.98; post: 22.65 ± 4.09) while the VertiMax group (pre: 22.18 ± 4.31; post: 23.36 ± 4.06) and control group (pre: 15.65 ± 4.51; post: 15.85 ± 4.17) did not change (Figure 1). Reliability analysis resulted in an intraclass correlation coefficient of 0.97 for the control subjects between pre- and post testing.
This study was designed to determine which short-term training program (VertiMax vs. depth jump) was more effective in increasing vertical jump. VertiMax is a new and uninvestigated training device consisting of a platform and elastic cords attached to the subject's waist. The results of the present study demonstrated that a twice-weekly 6-week program of depth jump training significantly increased vertical jump height by approximately 2 in. (∼11%), whereas the VertiMax and control groups demonstrated no significant change.
The use of plyometrics has been advocated for many years for improving athletic performance, especially in sports that require speed and jumping (2). During a plyometric exercise, the muscles undergo a very rapid stretch during the eccentric muscle action. This stretch-shortening cycle increases elastic energy, allowing for more power production. The stored elastic energy and stretch reflex response of muscles are essentially exploited in this manner, permitting more force to be produced by the muscle during the concentric muscle action (17). This only occurs when the time between the eccentric and concentric actions is very short (amortization time). Training programs that have used plyometric exercises have been shown to positively affect performance in power-related movements such as jumping and speed (2,8,10,20). In the present study, improvements were seen in vertical jump height, which support these studies.
Improved muscle performance due to a plyometric training program may be due in part to increased motor unit functioning. Previous studies have indicated that neuromuscular adaptations such as increased inhibition of antagonist muscles as well as activation and cocontraction of synergistic muscles may account for the improvements in increased vertical jump heights (5,7,12,22). This may partially explain the differences observed in the posttest measurements between the groups in our study. Only the depth jump training group demonstrated an improvement in vertical jumping height.
It is possible the VertiMax training group did not demonstrate improvements because the training apparatus increased amortization time due to the rubber band setup. Due to the rubber bands, subjects may have spent slightly longer time on the platform between jumps. This would be contradictory training to the actual plyometric jump training, which is trying to decrease the amortization time. Although subjects were encouraged to be quick off the ground, it was difficult at times, possibly due to the resistance created by the rubber band setup.
In previous studies involving vertical jump, static stretching was optional, mandatory, or not used at all. Stretching has traditionally been incorporated into warm-up routines to prepare muscles for activity, enhance performance, or prevent injuries (9). However, research points to the negative acute effects of stretching on force production, musculotendinous stiffness, and reflex sensitivity (1,16,21). The mechanism responsible for these effects have not been fully elucidated. However, some researchers have shown that these negative, acute effects may result in decreased vertical jump height.
In the present study, a dynamic warm-up was used before the pre- and post-tests as well as before each training session. No research studies investigating plyometrics could be found that used dynamic warm-up before jumping. Active warm-ups have been shown to improve coordination, flexibility, balance, and range of motion without the negative effects of static stretching prior to physical activity (4). Our results are unable to specify whether the active warm-up had a positive or negative effect on vertical jumping height.
The total volume for each program was equal by controlling the total number of jumps performed in each jump training session. Although the total volume was accounted for, equating the intensity levels proved more difficult. Both training programs required maximal effort with each jump. However, the perception of maximal effort is subjective. Verbal encouragement was used in an attempt to ensure maximal effort from every participant throughout the entire duration of the program, but objective measurements of maximal efforts were prohibitive.
It should be noted that although the VertiMax training group improvement was not significant, it approached statistical significance. It is possible that, if the present study had a larger sample size or trained for a longer duration, the VertiMax training group might have shown a significant improvement. However, the number of subjects and training duration were sufficient to demonstrate an improvement in the depth jump group. Even more significant is that this study used only recreationally trained subjects and therefore could have expected almost any training program to produce increased results. However, only the depth jump group demonstrated improvement. It should also be noted that subjects were instructed not to perform any plyometrics outside the study and to refrain from lower body strength training. We assume that subjects continued their normal physical activities of upper body strength training and aerobic activity following the given instructions.
This study was unique in that it is one of the first studies to identify which training program was more effective in improving vertical jump height. This study suggests that the depth jump training was the more effective training program by demonstrating a greater increase in vertical jump height from pre- to post-test.
This information may assist strength and conditioning professionals in program design for athletes or individuals who desire to improve their lower body explosive power in a very short time as in the power phase of their periodized program immediately preceding the season. A twice-weekly 6-week plyometric program involving depth jumps with varying volume and intensity appears to be more effective than the VertiMax training device with equal number of foot contacts.
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