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Quantification of Vertical Ground Reaction Forces of Popular Bilateral Plyometric Exercises

Wallace, Brian J1; Kernozek, Thomas W2; White, James M3; Kline, Dennis E3; Wright, Glenn A3; Peng, Hsien-Te4; Huang, Chen-Fu5

Journal of Strength and Conditioning Research: January 2010 - Volume 24 - Issue 1 - p 207-212
doi: 10.1519/JSC.0b013e3181c3b841
Original Research
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Wallace, BJ, Kernozek, TW, White, JM, Kline, DE, Wright, GA, Peng, H-T, and Huang, C-F. Quantification of vertical ground reaction forces of popular bilateral plyometric exercises. J Strength Cond Res 24(1): 207-212, 2010-The purpose of this study was to quantify the vertical ground reaction forces (VGRFs) developed during the performance of popular bilateral plyometric movements. Fourteen power-oriented track and field men of collegiate and national level randomly performed 3 trials of 9 different bilateral plyometric exercises in a single testing session. Three depth drop (DD) and 3 depth jump (DJ) conditions from 30, 60, and 90 cm heights (DD30, DD60, and DD90 and DJ30, DJ60, and DJ90) were tested, in addition to vertical jump (VJ), standing long jump (SLJ), and 2 consecutive jump (2CJ) conditions. Peak impact VGRFs were normalized to body weight. Additionally, all conditions were compared against the VJ in an intensity index. The SLJ condition resulted in a significantly higher peak VGRF than the 2CJ condition (p ≤ 0.05). 90DD, 90DJ, 60DD, and SLJ had a significantly greater peak VGRF (5.39, 4.93, 4.30, and 4.22 times body weight, respectively) than the VJ condition (3.34 times body weight). The 30DJ condition had an insignificantly smaller peak VGRF (2.78 times body weight) when compared with the VJ. Practitioners may use these findings to more effectively progress athletes in these movements based on their intensities.

1Biodynamics Laboratory, Department of Kinesiology and Health Promotion, University of Kentucky, Lexington, Kentucky; 2Department of Health Professions, Strzelczyk Clinical Biomechanics Laboratory, La Crosse Institute for Movement Science; 3Department of Exercise and Sport Science, Musculoskeletal Research Center, University of Wisconsin-La Crosse, La Crosse, Wisconsin; 4Department of Physical Education, Chinese Cultural University, Taipei, Taiwan; and 5Department of Physical Education, National Taiwan Normal University, Taipei, Taiwan

Address correspondence to Brian J. Wallace, brian.wallace@uky.edu.

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Introduction

Plyometric exercises are often prescribed in the training of athletes, including youth athletes (9,12). They are used as a training modality during various stages of athlete training programs, with aims of improving muscular power, enhancing athletic performance in activities involving the stretch-shortening cycle, or preventing injury (2). A plyometric exercise is generally considered to be any jumping-type exercise that aims to enable the involved muscles to reach maximal force in the shortest amount of time concentrically or eccentrically. These exercises typically involve the stretch-shortening cycle, consisting of eccentric, amortization, and concentric phases (14). Examples of lower extremity plyometric exercises include tuck jumps, bounds, countermovement vertical jumps (VJs), drop jumps, and depth jumps (DJ). However, landings without a subsequent jumping performance are also considered to be plyometrics (14).

Plyometric exercises are a form of resistance training that uses body weight as the load. Hence, they are subject to the same principles of mode, frequency, volume, progression, recovery, and intensity of other methods of training. Intensity is perhaps the most important of these because it often determines other parameters, such as volume and recovery. Intensity in plyometrics is defined as the amount of stress placed on the involved muscles, joints, and connective tissues involved in the movement (14). Often in weight training, intensity is quantified according to percent 1 repetition maximum. A similarly accepted way to quantify the intensity of plyometric exercises does not currently exist. Points of contact (i.e., unilateral or bilateral), speed, drop height, and the participant's weight are variables that have been used in practice to assess the intensity of plyometric exercises (14). However, while practical, these may not be a valid representation of the relative, approximate, or actual stresses involved during the performance of these types of exercises.

To date, the intensity of more than a few plyometric exercises has not been quantitatively studied (5,6,8,14). Additionally, we are unaware of any studies that have attempted to make practical recommendations regarding the relative impact intensities of different plyometric exercises vs. a sport-specific reference exercise. Thus, this study had 2 primary purposes: (a) to quantify the vertical ground reaction forces (VGRFs) generated during 9 different plyometric exercises and (b) to scale the landing forces of these exercises to those of a maximal VJ reference condition in an effort to create an intensity index useful to practitioners. We hypothesized that the VGRF would increase as drop height increased in both the depth drop (DD) and DJ conditions. We also hypothesized that the DD conditions would have greater peak VGRFs than the DJ conditions of the same height, with the standing long jump (SLJ) and 2 consecutive jump (2CJ) conditions being comparable in VGRF magnitude. Finally, we hypothesized that all conditions would result in higher VGRFs than the VJ condition, with the exception of the DD30 and DJ30 conditions, where 30 represents drop height in centimeters, due to the height of these conditions being relatively similar to the maximum VJ of the subjects.

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Methods

Experimental Approach to the Problem

In this study, a repeated measures design was used to investigate the differences in peak VGRF between 9 different plyometric exercises. While some kinetic variables for selected plyometric exercises have been published previously, those for several of the movements used in this investigation have not been described. The peak VGRF of the VJ was related to each of the other 8 exercises to create an intensity index. The VJ was deemed an appropriate movement for the basis of the index because it is both a common sport movement and an exercise often performed in training the programs used by many different types of athletes.

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Subjects

Fourteen power-oriented track and field men of collegiate and national level (mass = 87.2 ± 16.5 kg, height = 177.1 ± 5.6 cm, age = 22.5 ± 3.5 years) volunteered to serve as test subjects. All subjects were experienced in each exercise and were continuously participating in a sport-specific training program for at least 6 months prior to testing. No subjects were tested during their competitive season. The institution's Institutional Review Board (IRB) for Use of Human Subjects approved all procedures prior to any subjects being tested. All subjects read and signed the IRB-approved informed consent form, which outlined any foreseeable risks to them as a result of their participation, prior to their participation.

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Procedures

Each subject was tested for each of the different plyometric exercise conditions during a single laboratory visit. Subjects were instructed to abstain from rigorous physical activity for at least 24 hours prior to participating in the study to reduce possible performance altering neuromuscular fatigue. Before testing began, each subject's height, mass, and age were recorded. Subjects changed specific footwear (New Balance Running Shoe, Model 629; New Balance Athletic Shoe, Inc., Boston, MA, USA) to control for different shoe-sole absorption properties. A standardized dynamic warm-up consisting of various lower-body lunging and squatting movements was performed prior to the main testing protocol. Following the warm-up, subjects rested passively for 5 minutes prior to beginning the first testing condition.

The plyometric exercise conditions, performed in a random order, were DDs and DJs from heights of 30, 60, and 90 cm (DD30, DD60, and DD90 and DJ30, DJ60, and DJ90, respectively), as well as VJ, SLJ, and 2CJ conditions (Figure 1). Subjects jumped onto the force platform from the same starting position for the SLJ and 2CJ conditions. Three trials were completed at each exercise condition, and 90 seconds of passive rest was allotted between trials.

Figure 1

Figure 1

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Instrumentation

Kinetic data were recorded in EVaRT (Version 4.6; Motion Analysis Corporation, Santa Rosa, CA, USA) during the stance phase of each trial using 2 identical force platforms (Bertec 4060 NC; Bertec Corp., Columbus, OH, USA) collected at 1,200 Hz. One platform recorded right extremity data, and one recorded left extremity data. The dominant leg, defined as the one used to kick a ball, was analyzed for all participants. Six channels from each force platform (medial-lateral, anterior-posterior, and VGRFs and moments) were collected using a 12-bit analog to digital converter housed within a computer. Each platform's surface was flush with the surrounding floor and matched it in color so that the subjects could not intentionally target the force platform. Trials that did not result in each of the subject's feet making complete contact with the appropriate force platform were repeated. Raw ground reaction force data were exported, and VGRFs were processed in custom software programs using MATLAB (Mathworks, Inc., Natick, MA, USA). The onset of the impact phase was determined when the VGRF exceeded a 10 N threshold. Data were normalized to body weight (13).

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Statistical Analyses

Descriptive statistics (mean ± SD) were computed for the peak impact phase VGRF of each plyometric exercise relative to body weight. One-way repeated measures analyses of variance were performed (SigmaStat, Version 3.10; Systat Software, San Jose, CA, USA) to test for statistically significant differences within DD and DJ conditions and between all conditions relative to the VJ condition. Paired t-tests using Bonferroni corrections were used for post hoc comparisons within DD and DJ conditions. The Holm-Sidak procedure was used for post hoc comparisons relating the VJ condition to the other conditions. Paired t-tests using Bonferroni corrections were also used to compare height-matched DD and DJ conditions, as well as the SLJ vs. 2CJ. Intraclass correlation coefficients (ICCs) were calculated for each condition. The level of significance was set at p ≤ 0.05 for all analyses.

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Results

Data are reported as mean ± SD relative to times body weight (Table 1). The peak VGRFs for the SLJ and 2CJ conditions were significantly different, while the height-matched DD and DJ conditions were not. Thirty, 60, and 90 cm drop heights were all statistically different from each other within DD and DJ. The 30DD, VJ, 2CJ, and 60DJ all resulted in a mean peak VGRF between 3 and 4 times body weight (3.33, 3.34, 3.68, and 3.82, respectively). The only condition that resulted in VGRF of less than 3 times body weight was the 30DJ condition, which had an impact peak VGRF of 2.87 times body weight. The SLJ, 60DD, and 90DJ conditions resulted in VGRF between 4 and 5 times body weight, while the 90DD condition had the largest VGRF, equaling 5.39 times body weight. The ICCs, which ranged from 0.65 to 0.94, are shown in Table 2.

Table 1

Table 1

Table 2

Table 2

Figure 2 displays the peak VGRF of the VJ condition relative to each other condition. Only SLJ, 60DD, 90DJ, and 90DD were statistically different, with all having a higher peak VGRF. The 30DJ condition's impact peak was the only one that was less intense than the VJ condition with a peak VGRF equal to 91% of the VJ condition, although the difference was not significant. The conditions that resulted in the highest VGRFs were the 90DJ and 90DD conditions, which had a peak VGRF of 155% and 163% of the VJ condition, respectively.

Figure 2

Figure 2

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Discussion

Most authors have indicated that as drop height increases, peak ground and joint reaction forces also increase (1,10,11,17), but not all (5). Bobbert et al. (1) reported that peak VGRF increased significantly in a linear fashion between 20 and 40 cm and 40 and 60 cm in physically active men when landing from bounce jumps, as did McNitt-Gray (11) with DDs performed by gymnasts and recreationally trained men from heights ranging from 23 to 128 cm. What Bobbert et al. (1) reported as a bounce jump is similar to what was termed a DJ in this study. Significant differences between drop heights were also found in our study at each condition within both DD and DJ, as we hypothesized. The magnitude of the peak VGRF was greater as drop height increased, as would be expected, because contact velocity would increase proportional to drop height due to the uniform acceleration of gravity. Jensen and Ebben (5) did not find statistically different findings in peak VGRF between DJ from 46 and 61 cm. There were, however, rather large mean differences (3.25 vs. 3.91 N·kg1), so it is possible that the insignificant findings are a combination of the low subject number (n = 6) and 15 cm height difference vs. the 20 cm height difference used in other investigations.

The use of plyometrics in training is often aimed at accomplishing 1 of 2 training goals: (a) to accustom the athlete to absorbing high eccentric loads quickly or (b) to enhance an athlete's ability to perform stretch-shortening cycle movements in an effort to train lower extremity power output (2). Bobbert et al. (1) recommended that athletes should not perform plyometrics from heights above 40 cm because of the high-impact kinetics (i.e., peak VGRF, loading rates, and joint moments) they measured upon landings from the 60 cm height in their investigation. It should be noted that they used physically active men as subjects rather than athletes, who may be better able to tolerate high landing forces. Training for landing performances from heights equal to or greater than an athlete's maximum VJ height is often necessary, as these types of landing performances are common in sports such as volleyball, basketball, and gymnastics. It has been suggested that one of the characteristics that makes power and power-endurance athletes advanced is their ability to absorb large eccentric forces quickly, resulting in higher loading rates and peak ground reaction forces upon landing than recreationally trained athletes experience (15). Additionally, McNitt-Gray (11) reported that competitive gymnasts generated a higher peak VGRF than recreational athletes during DDs from heights of up to 128 cm, a relatively low drop height compared with those in some gymnastic events. The DD and SLJ conditions used in our study are likely to train eccentric loading more than the other conditions tested due to their higher VGRFs. Because they only load eccentrically and do not require an explosive concentric phase, they may be more appropriate to include in training programs for sports that involve landings without a subsequent jump, such as gymnastics. Training DD from heights of only 30 cm elicited an average peak VGRF of 3.33 times body weight, which is within the range of contact forces experienced by distance runners (18). The least intense DD condition (30DD) elicited comparable VGRFs to running, suggesting that it may more safely provide eccentric-only loading for less advanced athletes, especially because their peak VGRFs would be expected to be reduced in comparison to high-level athletes such as the ones in this investigation.

The DJ and 2CJ conditions performed in the present study are more likely to be beneficial in helping to elicit the stretch-shortening cycle than the DD or SLJ conditions because they also involve both amortization and concentric phases. The eccentric component of these exercises stretches the agonists involved in VJ (i.e., hip, knee, and ankle extensors), which may allow for the better utilization of the muscle spindles, resulting in more forceful concentric contractions of these muscle groups (14). Additionally, the DJ conditions resulted in lower peak VGRF than height-matched DDs, and the 2CJ condition had a lower peak impact than the SLJ, although the differences between DD and DJ conditions were insignificant. These results were the opposite of our hypothesis, although the SLJ and DJ exercises may not elicit the same impact-related eccentric benefits as DD exercises and the 2CJ condition because of the need for transition into a concentric phase. There may also be a ceiling as far as how high drop height can be and still elicit increased power output in DJ, which was previously mentioned as being one of the training goals of plyometrics. Walsh et al. (17) reported that maximum and mean power output decreased insignificantly as drop height increased from 40 to 60 cm, even while joint moments generally increased with drop height. Thus, in sporting activities where maximum power output is important (typically sports involving a single or repeated countermovement jump), it might benefit performance coaches to abstain from implementing landings from heights of 60 cm or greater.

The most novel part of our study was the development of an intensity index based on the impact forces of various bilateral plyometric exercises. In exercise prescription, there is no standardized index that practitioners can use as a basis to describe the intensity of a given plyometric exercise. As noted previously, the typical parameters of mode, frequency, volume, progression, recovery, and intensity need to be adhered to plyometric exercise training. Often, points of contact, speed, drop height, and the participant's body weight are used to quantify intensity (14). For example, unilateral drops and jumps are considered to be more intense than bilateral exercises, faster movements are considered to be more intense than slower movements, higher drop heights are considered to be more intense than lower ones, and heavier people are presumed to be exercising more intensely because they experience higher forces upon landing due to their relatively large mass. These methods of estimating intensity are used extensively in practice, but there is little objective data supporting their use. For example, while unilateral drops likely result in larger forces and moments on a single extremity than bilateral drops from the same height, we are unaware of any studies that have quantified this possible difference. It has been reported that unilateral VJs were 58% the height of bilateral VJs in male volleyball players, but the landing phase of the jumps was not studied (16). Additionally, using body weight as a measure of intensity may be difficult for practitioners who often develop a single plyometric workout or program for athletes of varying sizes on a team or in the same session at a performance center.

For the intensity index presented in this article, a maximum VJ was used as the reference condition. The VJ is a movement performed often in sport, and thus, it is useful to create an intensity index relative to it. The SLJ, 60DD, 90DJ, and 90DD conditions had statistically higher VGRFs than the VJ, while 30DJ was insignificantly lower. Perhaps, one of the reasons for the insignificant differences of some conditions compared with VJ is the highly trained status of these individuals. It would be expected that lesser trained individuals would have lower maximum VJ heights and thus lower peak VGRFs upon landing. Less advanced athletes also tend to use landing strategies that result in lower peak VGRFs, further reducing their peak VGRFs from all landing heights (10). Because of their reduced VJ height, we would hypothesize that the relative intensity of each condition would increase relative to the VJ.

Although the first attempt at developing a practical intensity index may prove useful for practitioners, there are some limitations to this study that should be considered. First, only VGRFs were studied, so the actual stresses experienced at the joint level can only be presumed. However, the relationship between VGRF and joint kinetics is strong in these types of activities (1,5,10). Another limitation is that only men of advanced athletic ability were used. It has been shown that women land differently than men do (3,4,7), and those differences may be maintained or may change with different plyometric exercises. Last, as previously mentioned, less advanced athletes could show different kinetic characteristics than the group used in this study, which could influence how practitioners would use this index with their athletes.

This study investigated the peak VGRFs for a variety of plyometric exercises that are commonly used in practice, some of which have not been quantified in the literature previously. Perhaps, more importantly, it established an index based on the VGRFs that will enable practitioners to know with more certainty the relative intensity of these popular plyometric exercises that they may already prescribe to their athletes. The subjects in this study were high-level athletes, so the results might not relate directly to lesser athletes, although general VGRF patterns would be expected to be qualitatively similar. The results of this study are not comprehensive for every plyometric exercise or every kinetic variable but provide a basis from which the relative intensities of common plyometric exercises can be gauged. Future studies should investigate if other kinetic variables, such as joint moments, show similar relative results as those presented in this article.

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Practical Applications

The use of plyometric exercises in the training of athletes has been popular for some time and is only increasing in popularity. The DD and SLJ conditions may be preferable for training force absorbing abilities, while DJ and the 2CJ condition may be preferable for training aims designed to enhance the performance of stretch-shortening cycle activities. Until now, quantitative kinetic recommendations on the relative impact-based intensity of these types of exercises were not available in a format useful to practitioners (14). However, the clinical uses of this investigation should be considered with respect to the practitioner's own clients, noting that the athletes in this study were relatively highly trained collegiate men. The discrepancy between conditions is likely to be less with lesser trained individuals, although the relational intensities would be expected to remain unchanged. The index can be used to more accurately monitor training intensities for one of these exercises at any given time based on if they desire an impact intensity approximately equal to or greater than (and to what degree) the commonly performed VJ. If practitioners are better able to monitor acute training intensity more effectively, it may lead to more preferable long-term performance gains for their athletes. It also better enables practitioners more certainly in safely progressing athletes, especially lesser trained ones who may be more susceptible to injury, through the appropriate prescription of various types of plyometric exercises.

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Acknowledgments

The authors report no conflicts of interest regarding this study. The principal investigator can be reached at brian.wallace@uky.edu. No sources of funding were sought or awarded for this study.

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Keywords:

biomechanics; kinetics; stretch-shortening cycle

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