Plyometrics are thought to improve performance in most competitive sports (27) and have been recommended for inclusion in a training program (1,32). Plyometrics also serve as an important training strategy for enhancing bone health (19) and potentially preventing and rehabilitating injuries (6). Meta-analyses demonstrate that plyometric training is effective, though considerable variation exists in the design of plyometric programs employed by researchers (7,21). The design of plyometric programs requires an understanding of a variety of variables such as exercise mode, frequency, volume, program length, recovery, progression, and intensity (28). Some of these variables, such as training frequency, are easily understood. On the other hand, plyometric intensity is difficult to quantify, though it has been suggested to be among the most important program design variables (10,14).
The intensity of resistance training is easily quantified because most forms of resistance have clearly labeled masses. Resistance training programs are progressed using some percentage of an athlete's repetition maximum (RM) or 1RM (1). Unlike resistance training intensity, plyometric intensity has been defined using anecdotal categories of low to high intensity or factors such as the number of points of contact during landing, the speed of the drill, the height of the jump, and the athlete's weight (28). Plyometric intensity also has been defined as the amount of stress placed on muscles, connective tissues, and joints and is dictated by the type of plyometric exercise that is performed (28). Based on this definition, it is possible to scientifically quantify the intensity of a variety of plyometric exercises based on the kinetic characteristics of the takeoff, airborne, and landing phases of each exercise.
In an attempt to quantify plyometric exercise intensity, previous research has examined vertical ground reaction forces (GRFs) and joint reaction forces of a limited number of plyometric exercises. For example, studies have compared kinetic and kinematic variables of drop jumps and pendulum jumps (12) and the GRF of unloaded and loaded drop jumps (34), of drop jumps of varying heights (4,24), and of 1-legged and 2-legged countermovement jumps (CMJs) (9,35). Research comparing the intensity of several different plyometric exercises is limited to studies quantifying exercise impulse and GRF (16), knee joint reaction force (14), rate of force development (15), and electromyography (EMG) (10). Of these studies, some did not assess kinetic variables (10), and those that did typically used a limited number of subjects, potentially restricting the finding of significant differences given the multiple pairwise comparisons performed (14-16).
Previous research examining plyometric exercises has both found (10,18,33) and failed to find (8,10) gender differences, depending on the outcome variables assessed. Thus, the relationship between gender on the specific intensity characteristics of plyometric exercises is not well understood. The purpose of this study was to quantify the intensity of plyometric exercises by evaluating kinetic variables associated with the takeoff, airborne, and landing phase of a variety of plyometric exercises believed to represent a continuum of exercise intensity based on previous research and anecdotal recommendation. This study also sought to assess gender differences in these variables.
Experimental Approach to the Problem
Kinetic characteristics of plyometric exercises serve as a valuable measure of the intensity of these exercises. A randomized repeated-measures experimental design was used to test the hypothesis that kinetic differences exist between plyometric exercises. The independent variables used in this study included the 8 different plyometric exercises assessed and gender. The dependent variables included the peak vertical GRF, minus body mass during the takeoff phase of the plyometric exercises (GRF-T), the time to takeoff (TTT), flight time (FT), jump height (JH), peak power (P), landing rate of force development (L-RFD), and peak vertical GRF, minus body mass during the landing phase of the plyometric exercises (GRF-L).
Twenty-six men and 23 women served as subjects. Subject characteristics including anthropometric data, previous athletic experience, current training experience, strength, and jumping ability are presented in Table 1. Inclusion criteria required subject involvement in National Collegiate Athletic Association (NCAA) Division-I, club, or recreational sports and participation in plyometrics as part of their annual training program. Subjects who were athletes were in the off-season of their sport season. Subjects were without orthopedic lower limb or known cardiovascular pathology and had no contraindications to resistance or plyometric training.
Exclusion criteria included any history of orthopedic lower limb or cardiovascular pathology that resulted in functional limitation in their sport or the performance of maximal effort plyometrics. The women subjects demonstrated 1RM back squat strength and vertical jump ability that was of 69.1 and 75.5%, respectively, of the values of the men. Thus, the women subjects demonstrated lower than average gender differences in lower body strength and power (8,23,25). Men and women possessed similar athletic backgrounds as evidenced by high school and college sports experience and weekly resistance and plyometric training experience (p > 0.05). The subjects were informed of the risks associated with the study and provided informed written consent. The investigation was approved by the Institutional Review Board for use of human subjects.
All subjects performed a habituation and testing session. Before each session, the subject warmed up with 3 minutes of low-intensity work on a cycle ergometer. Warm-up was followed by dynamic stretching exercises including 5 repetitions of each of the following: slow and fast body weight squats, 10-m forward, backward, and lateral lunges, 10-m walking quadriceps and hamstring stretches, 20-m skipping, and 5 CMJs of increasing intensity.
During the habituation session, subjects warmed up, rested for 4 minutes, and performed 2 repetitions of the CMJ, which was assessed using a Vertec (Sports Imports, Columbus, OH, USA). Subjects then rested for 4 minutes and performed their 5RM back squat test. The back squat test included 2 warm-up sets of 3 repetitions each at approximately 75 and 90% of the subject's estimated maximum ability. Subjects then performed the 5RM back squat test, similar to methods previously prescribed (2). After the back squat test, the subjects were given instruction and a demonstration on the correct performance of the plyometric exercises to be assessed during the test session. Subjects then performed each of these exercises until they mastered the correct technique. The plyometric exercises included line hops (LHs), 15.24-cm cone hops (CH), squat jumps (SJ), tuck jumps (TJ), CMJ, loaded CMJ with handheld dumbbells equal to 30% of the subjects previously assessed estimated 1 RM squat (dumbbell jump [DBJ]), depth jumps (DJs) from a box height that was normalized to the subjects previously assessed CMJ height (DJ), and single leg jumps (SLJs). These exercises were performed consistent with the methods previously described (28). These plyometric exercises were included in this study because they represent a variety of estimated (28) and previously research exercise intensities (10,14-16). Furthermore, these exercises represent a variety of plyometric conditions including those performed with submaximal JHs, added external mass, unilaterally and bilaterally, and with and without an eccentric phase to stimulate the stretch shortening cycle. Depth jump box height was normalized to vertical JH to control this variable because not doing so and using a standardized box height would result in a stimulus that is highly variable between subjects of differing jumping abilities. Additionally, if DJ height were not normalized to CMJ, the selection of a DJ box height would be merely arbitrary as no guideline exists to identify what height should be used, because a variety of box heights are often used in applied settings. After the habituation session, subjects recovered for at least 48 hours and returned for the testing session.
During the testing session, subjects warmed up with the same protocol they used before the habituation session followed by 5 minutes of rest. Subjects then performed 3 repetitions of each of the test plyometric exercises in a randomized order with 1 minute of rest between each exercise. This recovery duration between reps and sets was based on previously recommended work to rest ratios of at least 1:5 (28), and research demonstrating that maximum jump performance requires at least 15 seconds between repetitions (30). Thus, randomization and adequate rest between sets was used to control order and fatigue effects.
The test exercises were assessed with a 60 × 120-cm force platform (BP6001200, Advanced Mechanical Technologies Inc., Watertown, MA, USA), which was adhered to the laboratory floor according to the manufacturer's specifications. The force platform was calibrated with known loads to the voltage recorded before the testing session. Kinetic data were collected at 1,000 Hz, real time displayed and saved with the use of computer software (BioAnalysis 3.1, Advanced Mechanical Technologies, Inc., Watertown, MA, USA) for later analysis. All values were determined as the average of 3 trials for each plyometric exercise.
Dependent variables were selected to comprehensively evaluate each plyometric exercise, including variables associated with the takeoff (GRF-T, TTT), flight (FT, JH, P), and landing (L-RFD, GRF-L) phase. A number of these variables have been used previously in studies examining plyometric exercises or jumping and have been demonstrated to be reliable (13). The reliability of these variables was also assessed in this study. Peak vertical GRF-T, TTT, FT, JH, P, L-RFD, and GRF-L and were calculated from the force time records consistent with methods previously used. Peak vertical GRF-T was defined as the highest vertical GRF value attained from the force time record for the takeoff phase of each jump, minus body mass (15,17). Time to takeoff was defined as the time of onset of the flight phase minus the time of onset of the eccentric phase from the force time record. The time of onset of the eccentric phase was identified to be consistent based on methods previously demonstrated (13,18). Flight time was assessed as the period after the takeoff phase and before the landing phase where the GRF equaled zero. Jump height and P were analyzed because these variables are frequently used to assess jumping performance (5,20,22,26,29,36). Jump height was calculated from the force time records consistent with methods previously used (26). Peak power was calculated using the equation proffered by Canavan and Vescovi (5). The L-RFD was defined as the first peak of vertical GRF minus the initial vertical GRF upon landing, divided by the time to the first peak of GRF minus the time of initial GRF (3,4,15). Peak GRF-L was defined as the highest vertical GRF value attained during the landing phase of the plyometric exercise, minus body mass (14,16). Figure 1 shows a force time record from a CMJ and provides an example of the dependent variables obtained from it.
A 2-way mixed analysis of variance (ANOVA) with repeated measures for plyometric exercise type was used to evaluate the main effects for plyometric exercise type and the interaction between plyometric exercise type and gender, for GRF-T, TTT, FT, JH, P, L-RFD, and GRF-L. Significant main effects were further analyzed with Bonferroni adjusted pairwise comparison to identify the specific differences between the plyometric exercises. The trial-to-trial reliability of each dependent variable was assessed for each plyometric exercise using average measures intraclass correlation coefficient. In addition, a repeated-measures ANOVA was used to confirm that there was no significant difference (p > 0.05) between 3 trials of each plyometric exercise. A Pearson's correlation coefficient was used to assess the relationship between the JH and the L-RFD and GRF-L for each plyometric exercise. Independent samples t-tests were used to assess baseline gender differences in subject strength and jumping ability. Assumptions for linearity of statistics were tested and met. An a priori alpha level of p ≤ 0.05 was used with post hoc power and effect size represented by d and η 2 p, respectively.
The analysis of GRF-T revealed significant main effects for plyometric exercise type (p ≤ 0.001, η 2 p = 0.60, d = 1.00) and for the interaction between plyometric exercise type and gender (p ≤ 0.001, η 2 p = 0.17, d = 1.00). Analysis of TTT showed significant main effects for plyometric exercise type (p ≤ 0.001, η 2 p = 0.70, d = 1.00) but not for the interaction between plyometric exercise type and gender (p > 0.05). The analysis of FT revealed significant main effects for plyometric exercise type (p ≤ 0.001, η 2 p = 0.94, d = 1.00) but not for the interaction between plyometric exercise type and gender (p > 0.05). Analysis of JH showed significant main effects for plyometric exercise type (p ≤ 0.001, η 2 p = 0.94, d = 1.00) but not for the interaction between plyometric exercise type and gender (p > 0.05). Analysis of p showed significant main effects for plyometric exercise type (p ≤ 0.001, η 2 p = 0.95, d = 1.00) but not for interaction between plyometric exercise type and gender (p > 0.05). Analysis of L-RFD showed significant main effects for plyometric exercise type (p ≤ 0.001, η 2 p = 0.25, d = 1.00) but not for the interaction between plyometric exercise type and gender (p > 0.05). Analysis of GRF-L showed significant main effects for plyometric exercise type (p ≤ 0.001, η 2 p = 0.53, d = 1.00) but not for the interaction between plyometric exercise type and gender (p > 0.05). Results of Bonferroni adjusted pairwise comparisons for each dependent variable are presented in Tables 2–9. Intraclass correlation coefficients are shown in Table 10. Jump height was correlated with GRF-L for each plyometric exercise (p ≤ 0.05) and for the L-RFD for every plyometric exercise (p ≤ 0.05) other than the LH (p > 0.05).
This is the first comprehensive study of plyometric exercise intensity using kinetic data to quantify exercise variations and gender differences. A number of previous studies assessing plyometric exercises included only one (3,4,24,34) or 2 plyometric exercises (12,35) in the analysis. Results of this study demonstrate that men and women respond similarly to most of these exercises and the outcome variables assessed. A variety of differences in kinetic characteristics between the plyometric exercises were found. Thus, exercise intensity can be classified based on these differences.
Analysis of the takeoff GRF-T of each plyometric exercise has been performed less frequently (15,35) than the analysis of the GRF-L (3,4,12,14,17,34). The GRF-T provides practical information about the explosive muscular force production required of each plyometric exercise. Minor gender differences were present, indicating some variation in how men and women develop GRF-T for different plyometric exercises. For both men and women, the CH and TJ produce the highest GRF-T. This finding confirms the results of previous research regarding the plyometric exercises that produce the highest GRF-T (15). Thus, the intensity of the CH and TJ is high, based on the analysis of GRF-T. This finding is in contrast to reports that suggest that exercise JH dictates intensity and that hops and jumps in place are among the lowest intensity plyometric exercises (28). The CMJ and TJ also result in the similar GRF-T, which is not surprising because the knee extensors have been previously demonstrated to produce the highest level and second highest levels of EMG, respectively, during these exercises (10). This finding is also consistent with the previous finding that the TJ is a relatively high intensity plyometric exercise (14,15).
Line hops and CMJ are statistically similar for women and result in fairly similar values for men, demonstrating that jumps with dramatically different JHs and mechanics, produce similar GRF-T. The biggest difference between these exercises is the significantly longer TTT for the CMJ than for the LH. Thus, force is produced at a similar rate but for a longer period of time for a maximal jump such as the CMJ, compared to a submaximal jump such as the LH. In this study, the GRF-T is lowest in conditions where an external load is added such as the DBJ, when the stretch shortening cycle is not activated such as during the SJ, or when the body mass is overcome by the force produced by the muscles in only one leg as is the case during the SLJ. A bilateral deficit was found with men and women demonstrating single-leg GRF-T values that were approximately 73 and 70% or the bilateral CMJ, respectively. This finding confirms the presence, though exceeds the magnitude, of the bilateral deficit found for JH in this study and others (35).
Plyometric exercise TTT has not been previously assessed for multiple variations of plyometric exercises. This variable quantifies the duration of the stretch shortening cycle and the time it takes to generate the force needed to perform each plyometric exercise. Thus, TTT is similar to the contact time component of the reactive strength index (RSI), though the RSI is typically limited to the assessment of the DJ (11). In this study, the plyometric exercises with the lowest JH such as the LH and CH had the lowest TTT, most likely because of the lower time required to generate maximum force. Exercises such as the DJ also produced relatively low TTT, because subjects typically perform this exercise with relatively stiff landings and attempt to limit the knee and hip joint flexion, thus reducing the duration of the stretch shortening cycle (11) compared to other exercises. In this study, the SJ was performed without an eccentric phase so its TTT value is relatively low because the TTT includes only the duration of the concentric phase of the stretch shortening cycle for the SJ. Other plyometric exercise variations such as the SLJ and DBJ represent relative overload conditions where the entire mass of the body is managed by 1 leg or there is added mass of the dumbbells, respectively. Thus, it is not surprising that these plyometric exercise variations demonstrate the longest TTT. Plyometric exercise variations with a short TTT can be used to train activities that require the fast component of the stretch shortening cycle such as sprinting, whereas those that have a longer duration TTT may be more specific to slow component stretch shortening cycle activities such as maximal effort jumping (31). The absence of gender differences in TTT is consistent with previous work demonstrating no statistically significant difference in this variable between men and women during the performance of the CMJ (8).
Jump height is a practical measure of each plyometric exercise and has been previously assessed to compare unilateral and bilateral jumping performance (35). In the present study, JH was identical for the DJ and CMJ showing that when the DJ box height is normalized to the CMJ ability, the resultant jumps are similar. Presumably, this finding is because subjects are accustomed to landing from heights equivalent to their vertical jumping ability and jumping again, as sometimes occurs in sports events such as rebounding in basketball or blocking in volleyball. Although DJ and CMJ JHs are identical, previous research indicates that the CMJ produces appreciably more knee extensor muscle activation than the DJ, suggesting that the CMJ JH is achieved though neuromuscular mechanisms, whereas the DJ JH is produced by another phenomenon such as passive elastic force production. These findings suggest that the DJ is not inherently intense and is likely to only exceed the intensity of CMJ if the box height exceeds individual CMJ JH ability. This finding implies that this type of plyometric exercise needs to be individualized. Tuck jumps demonstrate a higher JH than a number of other plyometric exercises potentially because of the time in air method of calculating the jump (26), which may have been affected by the pronounced hip flexion produced and the delayed but rapid hip extension required to get the legs ready for landing, as previously reported (14). Dumbbell CMJ and SLJ JH values were similar and lower than most other plyometric exercises, because of the relative overload present in these conditions. Predictably, CH and LH resulted in comparatively lower JH, though each of these exercises includes a horizontal jumping component.
In this study, subjects demonstrated SLJs that were 52.6% of the bilateral condition, compared to 58.5% previously found (35), confirming the presence but demonstrating a smaller magnitude of bilateral deficit. In this study, JH was positively correlated with GRF-L for all plyometric exercises and with L-RFD for all exercises other than the LH. For some exercises, FTs for the CMJ and DJ were identical and because the FT or time in air calculation is used for P and JH calculations, it is predictable that the most of the plyometric exercise would parallel this value. One notable exception is the added mass condition associated with the DBJ, which produced relatively high P values despite low FT.
Only a limited number of studies have examined P of plyometric exercises (34) despite the fact it is believed to be important for athletic performance (32). In this study, the DBJ yielded the highest P, potentially because of the highest system mass compared to other plyometric exercises. This finding is in contrast to previous work that demonstrated that loaded jumps resulted in less P than unloaded jumps (34). However, this finding is difficult to interpret, because the magnitude of the loads was not reported (34). In this study, DJ and CMJ produced similar P potentially because of the normalization of the DJ to the CMJ height. Power values produced during the SJ and SLJ were comparatively lower because of the absence of stretch shortening cycle activation during the SJ and the unilateral nature of the SLJs. Cone hops and LH likely resulted in lower P values because they were submaximal exercise that resulted in lower JH and thus less subject effort compared to other plyometric exercises. The difference in P values between the CH and LH was predictable because the cone was higher than the line, requiring more subject effort to jump over and confirming that in many cases, JH determines plyometric exercise intensity as reported (28).
Studies assessing plyometric exercises often evaluate the GRF-L (4,12,14,16,34). However, L-RFD may be the best measure to approximate the rate of biomaterial loading, thought to be important for bone growth (19). The DJ and the CMJ had the highest mean and statistically similar L-RFD values demonstrating that when DJ box height is normalized to jumping ability, the intensity of these exercises is similar and calls into question the anecdotal classification of DJ as high-intensity exercises and the CMJ as a relatively low-intensity exercise (28). Ultimately, the DJ height must be considered in the context of CMJ ability to determine the DJ intensity. The DBJ, SJ, and TJ produced similar L-RFD. In some cases, the magnitude of the SLJ, CH, and LH L-RFD mirrors their JH suggesting that the acceleration of gravity may dictate the L-RFD, confirming the idea that JH is one of the important determinant of plyometric exercise intensity (14,28). In this study, the relationship between plyometric exercise JH and L-RFD was present for every plyometric exercise except for the LH. The order of plyometric exercise L-RFD from this study is similar to the order found for landing impulse in previous studies assessing a variety of plyometrics (16). Only one other study specifically assessed the L-RFD, with similar results, other than demonstrating that DBJ produced relatively low L-RFD (15), compared to the relatively high L-RFD found in this study.
Studies examining a limited number of variations of plyometric exercises have often evaluated GRF-L (4,12,14,16,34). This variable may be the best measure of plyometric exercises intensity for estimating the magnitude of the compressive strain, which is thought to be important for bone growth (19). In this study, the DJ, DBJ, and CMJ demonstrated similar GRF-L, indicating CMJs are as intense as DJs. Thus, DJ box height may have to exceed individual jumping ability for the DJ to be more intense than the CMJ. This finding shows that some jumps in place are as intense as the DJ and questions the validity of some anecdotal classifications of plyometrics. These findings are similar to those of previous research assessing the GRF-L of the DJ, CMJ, and DBJ (14,16). The present study demonstrated that the TJ and the SJ produced the next highest GRF-L, though the TJ was previously found to be among the highest in GRF-L (14). The TJ also has been shown to produce the highest mean knee joint reaction forces, believed to be due to the acceleration of the leg via hip extension to prepare the legs for landing, after substantial hip flexion associated with the exercise (14). It should be recognized that the SLJ may be the most intense plyometric exercise because the GRF-L are approximately 75% of those experienced in the bilateral condition, confirming previous reports that single-leg plyometrics are of the highest intensity (14). The relative intensity of the CH and LH, based on GRF-L, was similar to other variables assessed in this study. The CH was more intense than the LH, suggesting that JH is related to GRF-L as well. This finding is enhanced by the finding of a correlation between JH and GRF-L for each plyometric exercise assessed in this study. The relationship between JH and GRF-L has also been demonstrated with DJ of increasing height (4). However, some evidence indicates that DJ from a box as high as 0.52 m, do not cause greater tension, compression, or shear strain than running (24). In addition to potential application with athletes, previous research with nonathletic populations has recommended that intensity variables such as GRF-L should be considered for the rehabilitation of clinical populations (37).
The design of strength and conditioning programs requires the progressive increases of exercise intensity. Results of this study provide the practitioner with information that can be used to progress the intensity plyometric exercises in a program. A variety of kinetic variables are assessed. These results can be consulted and included in the design of a program based on the specific desired adaptation. For example, if improved athletic performance during the takeoff phase of jumping is desired, plyometric exercises with increasing GRF-T or TTT should be progressively implemented into a program. If the goals of a training program are to increase the subject's ability to land and attenuate GRFs, then plyometric exercises with increasing GRF-L or L-RFD should be progressively incorporated into the program. Similarly, data are presented that demonstrate a continuum of intensity for JH and the development of athletic power.
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