evaluation of plyometric intensity using.30

EVALUATION OF PLYOMETRIC INTENSITY

USING ELECTROMYOGRAPHY

WILLIAM P. EBBEN,1 CHRISTOPHER SIMENZ,1 AND RANDALL L. JENSEN2

1Department of Physical Therapy, Program in Exercise Science, Marquette University, Milwaukee, Wisconsin;2Department of Health, Physical Education, and Recreation, Northern Michigan University, Michigan

ABSTRACT

The purpose of this study was to investigate the motor unit

activation of the quadriceps (Q), hamstring (H), and gastrocne-

mius (G) muscle groups during a variety of plyometric exercises to

further understand the nature of these exercises. Twenty-four

athletes volunteered to perform randomly ordered plyometric

exercises, thought to cover a continuum of intensity levels,

including two-foot ankle hops; 15-cm cone hops; tuck, pike, and

box jumps; one- and two-leg vertical jump and reach; squat jumps

with approximately 30% of their 1RM squat load; and 30- and

61-cm depth jumps. Integrated electromyographic data were

analyzed for each exercise using a one-way repeated-measures

ANOVA. Results revealed significant main effects for the Q when

all subjects are analyzed, as well as for separate analysis of men,

women, subjects with vertical jumps greater than 50 cm, and

those with vertical jumps less than or equal to 50 cm (p # 0.05).

Significant main effects were also found for the Gmuscle group in

the analysis of all subjects, as well as for men and subjects with

vertical jumps greater than 50cm (p # 0.05). No significant main

effects were found for the H muscle group. Pairwise comparisons

revealed a variety of differences among plyometric exercises.

In some cases, plyometrics previously reported to be of high

intensity, such as the depth jump, yielded relatively little motor unit

recruitment comparedwith exercises typically thought to be of low

intensity. Results can assist the practitioner in creating plyometric

programs based on the nature of the motor unit recruitment.

KEY WORDS jumping, motor unit recruitment, program design

INTRODUCTION

Plyometric training has burgeoned in the last 3decades as an effective mode of training athletes.Predictably, evidence suggests that plyometrictraining results in superior outcomes when com-

bined with another method of training such as weight training

or when compared with other modes of training alone (1,6–9,16–18,20,23,30,32). Furthermore, plyometric training hasbeen shown to result in biomaterial adaptation that may beimportant for preventing injuries (2,4,12,13,31,33). Whereasthe effectiveness of this mode of training is unequivocal,information is limited regarding how to best design plyo-metric programs.

Little is known about program design, especially withrespect to quantifying the nature of the plyometric exerciseand how to best incorporate it into an optimal training pro-gram. Similar to other forms of training, plyometric programdesign requires an understanding of a variety of programdesign variables, such as exercise mode, frequency, volume,program length, recovery, progression, and intensity (19).Unlike resistance training, plyometric exercise intensity is notwell understood, although it may be the most importantprogram design variable. Typically, factors such as thenumber of points of contact during jump landings, speed ofthe exercise, height of the jump, and athlete’s body mass havebeen suggested as possible factors determining intensity (19).Additionally, anecdotal recommendations exist for categoriesof low- to high-intensity plyometric exercises (19).

Plyometric intensity has also been defined as the amount ofstress placed on involved muscles and connective tissue andjoints, and it is dictated by the type of exercise that isperformed (19). Given this definition, it is logical that inten-sity could be scientifically evaluated by examining a variety ofkinetic variables and by assessing the activation of musclethrough electromyography (EMG).

Previously, kinetic and kinematic variables associated witha limited number of plyometrics have been examined. Forexample, studies have compared kinetic and kinematicvariables of drop jumps and pendulum jumps (10), groundreaction forces (GRF) of unloaded and loaded drop jumping(28), the effect of feedback training on GRF, drop jumps ofvarying heights, and one-legged and two-legged counter-movement jumps (22,24,29). Research quantifying theintensity of a large number of plyometric exercises is limitedto studies by Jensen and Ebben (14,15), who examined 10different exercises and demonstrated that the impulse,eccentric rate of force development (RFD), GRF, and kneejoint reaction force (JRF) of plyometric exercises varydepending on the type of exercise performed and that some

Address correspondence to Dr. William P. Ebben, [email protected]

22(3)/861–868

Journal of Strength and Conditioning Research� 2008 National Strength and Conditioning Association

VOLUME 22 | NUMBER 3 | MAY 2008 | 861

of the previous anecdotal recommendations regardingintensity are not accurate. These studies provide a foundationfor understanding plyometric intensity, though many ques-tions remain.

Previous research examining motor unit activation duringplyometric exercises is limited to studies comparing variationsof a single exercise (21,24) or gender differences (5,11,34). Noprevious study has attempted to quantify differences in motorunit activity of more than two exercises. Therefore, thepurpose of this study was to evaluate integrated electromy-ography (IEMG) activity of the gastrocnemius (G), ham-strings (H), and quadriceps (Q) muscle groups during avariety of plyometric exercises to quantify differences amongthe exercises.

METHODS

Experimental Approach to the Problem

This study used a randomized repeated-measures design to testthe hypothesis that there are differences in the motor unitrecruitment of the G, H, and Q muscles groups dependent on the

type of plyometric exercise being performed. Independentvariables included the type of exercise performed, and dependentvariables included IEMG during each plyometric exercise.

Subjects

Twenty-four adult subjects (13 women and 11 men; age,22.65 6 3.42 years; weight, 76.15 6 18.61 kg) volunteered forthe study. All subjects participated in resistance training andeither recreational or intercollegiate sports and were familiarwith the plyometric exercises evaluated in the study. Meanvertical jumping ability and other subject characteristics aredescribed in Table 1. Subjects provided informed consentbefore participation in the study. Approval for use of humansubjects was obtained from the University’s internal reviewboard before beginning the study.

Testing Procedures

Warm-up before the plyometric exercises consisted of 5minutes of low-intensity work on a cycle ergometer, whichwas followed by static stretching, including one exercise foreach major muscle group with stretches held for 12 seconds,and activity-specific dynamic stretching. Subjects thenperformed two repetitions each of the 10 test plyometricexercises at 75% intensity. Subjects were then allowed at least5 minutes’ rest before beginning the test. Exercise selectionincluded a variety of common plyometric exercises that areperformed primarily in the vertical plane and are thought torepresent a continuum of intensities based on previousresearch (15) and anecdotal recommendations (19). Therandomly ordered exercises included depth jumps from 30.48cm and 61 cm, pike jump, tuck jump, single-leg vertical jumpand reach, double-leg vertical jump and reach, squat jumpholding dumbbells equal to 30% of 1RM squat, two-footankle hop, 15.24-cm cone hop, and a 61-cm box jump. A1-minute rest interval was maintained between each exerciseto allow recovery of the phosphagen system and to ensuremaximal effort for each exercise. Subjects performed only onerepetition of each exercise to minimize fatigue. Finally,randomization of the plyometric exercises reduced thelikelihood of an order effect.

TABLE 1. Subject characteristics.

Min. Max. Mean SD

All subjectsAge, years 19.00 31.00 22.61 3.40Weight, kg 51.55 118.60 76.60 18.56Vertical jump, cm 26.40 79.20 48.97 12.25

WomenAge, years 19.00 23.00 21.08 1.44Weight, kg 51.50 85.55 64.13 9.98Vertical jump, cm 26.40 52.80 42.35 9.13

MenAge, years 19.00 31.00 24.27 4.17Weight, kg 72.90 118.60 90.18 16.10Vertical jump, cm 39.60 79.20 56.21 11.30

TABLE 2. Main effects of analysis of variance for IEMG of the 10 plyometric exercises.

Total IEMG Quadriceps Total IEMG Hamstrings Total IEMG Gastrocnemius

Significance Significance Significance

All subjects 0.013* All subjects 0.115 All subjects 0.000*Men 0.000* Men 0.444 Men 0.019*Women 0.000* Women 0.407 Women 0.143VJ # 50 cm 0.003* VJ # 50 cm 0.452 VJ # 50cm 0.106VJ . 50 cm 0.009* VJ . 50 cm 0.407 VJ . 50cm 0.016*

IEMG = integrated electromyography; VJ = vertical jump.*Significantly different (p , 0.05).

862 Journal of Strength and Conditioning Researchthe TM

Evaluation of Plyometric Intensity Using Electromyography

Electromography

Electromyography was used to quantify muscle activity usinga four-channel shielded cable Biopac MP 100 EMG unit(Biopac Systems, Goleta, CA). The input impedance was 120kV, signal to noise ratio of 0.2mV and the common moderejection ratio was 100 dB. Electromyographic data wererecorded at 1,000 Hz using rectangular shaped (10 3 30 mm)Ag/Ag Cl bipolar surface electrodes (Noraxon USA, Inc.,Scottsdale, AZ) with an interelectrode distance of 10 mm.Electrodes were placed on the longitudinal axis of the muscleswith the H electrode placed over the biceps femoris halfwaybetween the gluteal fold and the popliteal fossa. The Qelectrode was placed over the rectus femoris halfway betweenthe greater trochanter and medial epicondyle of the femur.The G electrode was placed on the belly of the G, on thelongitudinal axis. A common reference electrode was placed10 mm anterior and between the medial condyle and medialmalleolus of the tibia. Skin preparation included shaving hair,

abrasion, and cleaning the surface with alcohol. Elastic tapewas applied to ensure electrode and cable placement and toprovide strain relief.

Surface electrodes were connected to an amplifier andstreamed continuously through an analog to digital converterto an IBM-compatible notebook computer. Electromyo-graphic data were managed with computer software (Acq-Knowledge 3.2; Biopac Systems, Inc.). Saved EMG data werefull wave-rectified and integrated (IEMG in mV�s21) for theeccentric and concentric phases of the plyometric exercise todetermine the number of active motor units and their firingrates. All data were filtered with a 10-Hz high-pass and a500-Hz low-pass filter.

Statistical Analyses

The data from the investigation are presented as mean 6

SD. The statistical analyses were undertaken with SPSS 13.0for Windows (SPSS, Inc., Chicago, IL) using a one-way,

TABLE 3. Integrated EMG for the quadriceps muscle group for all subjects.

CON BOX TUC VJ SJ30 ANK PIK SLJ DJ12 DJ24

5.65 62.35*

5.26 62.25*

5.09 62.41*

4.99 61.69†

4.55 61.77‡

4.48 62.12§

4.31 62.14§

3.48 61.77k

3.44 62.21{

2.96 61.37#

EMG= electromyography; DJ12 = depth jumps from 30.48 cm; DJ24 = depth jumps from 61 cm; PIK = pike jump (PIK); TUC = tuckjump; SLJ = single-leg vertical jump and reach; VJ = double-leg vertical jump and reach; SJ30 = squat jump holding dumbbells equal to30% of 1RM squat; ANK = two-foot ankle hop; CON = 15.24-cm cone hop; BOX = 61-cm box jump.

Values are mean 6 SD expressed in millivolts.*Significantly different (p , 0.05) from SLJ, DJ12, DJ24.†|Significantly different (p , 0.05) from DJ12, DJ24.‡Significantly different (p , 0.05) from SLJ, DJ12.§Significantly different (p , 0.05) from DJ24.kSignificantly different (p , 0.05) from CON, TUC, VJ, SJ30, BOX.{Significantly different (p , 0.05) from CON, VJ, BOX.#Significantly different (p , 0.05) from ANK, CON, TUC, PIK, VJ, SJ30, BOX.

TABLE 4. Integrated EMG for the quadriceps muscle group for men.

CON TUC SJ30 VJ BOX ANK PIK SLJ DJ24 DJ12

6.65 62.23*

5.81 62.68†

5.26 61.98†

5.09 61.42‡

4.61 62.28§

4.56 61.92§

3.80 62.14§

3.71 62.15k

3.52 61.52{

3.50 62.32**

EMG = electromyography; DJ12 = depth jumps from 30.48 cm; DJ24 = depth jumps from 61 cm; PIK = pike jump (PIK); TUC =tuck jump; SLJ = single-leg vertical jump and reach; VJ = double-leg vertical jump and reach; SJ30= squat jump holding dumbbells equalto 30% of 1RM squat; ANK = two-foot ankle hop; CON = 15.24-cm cone hop; BOX = 61-cm box jump.

Values are mean 6 SD expressed in millivolts.*Significantly different (p , 0.05) from ANK, PIK, SLJ, BOX, DJ12, DJ24.†Significantly different (p , 0.05) from SLJ, DJ12, DJ24.‡Significantly different (p , 0.05) from DJ24.§Significantly different (p , 0.05) from CON.kSignificantly different (p , 0.05) from CON, TUC, SJ30.{Significantly different (p , 0.05) from CON, TUC, VJ, SJ30.**Significantly different (p , 0.05) from CON, TUC, VJ, SJ30.


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TABLE 5. Integrated EMG for the quadriceps muscle group for woman.

BOX VJ PIK TUC CON ANK SJ30 DJ12 SLJ DJ24

5.86 62.25*

4.99 62.04†

4.82 62.22‡

4.54 62.08‡

4.42 61.97§

4.39 62.48‡

3.87 61.47§

3.42 62.31k

3.24 61.46{

2.38 61.05**

EMG= electromyography; DJ12 = depth jumps from 30.48 cm; DJ24 = depth jumps from 61 cm; PIK = pike jump (PIK); TUC= tuckjump; SLJ = single-leg vertical jump and reach; VJ = double-leg vertical jump and reach; SJ30 = squat jump holding dumbbells equal to30% of 1RM squat; ANK = two-foot ankle hop; CON = 15.24-cm cone hop; BOX = 61-cm box jump.

Values are mean 6 SD expressed in millivolts.*Significantly different (p , 0.05) from ANK, CON, SLJ, SJ30, DJ12, DJ24.†Significantly different (p , 0.05) from SLJ, DJ24.‡Significantly different (p , 0.05) from DJ24.§Significantly different (p , 0.05) from BOX, DJ24.kSignificantly different (p , 0.05) from BOX.{Significantly different (p , 0.05) from VJ, BOX.**Significantly different (p , 0.05) from ANK, CON, TUC, PIK, VJ, SJ30, BOX.

TABLE 6. Integrated EMG for the quadriceps muscle group for subjects with a VJ # 50 cm.

CON VJ ANK PIK TUC SJ30 BOX SLJ DJ12 DJ24

3.44 61.88*

3.40 61.68†

3.39 61.37‡

3.37 61.67†

3.17 61.69‡

2.71 61.09†

2.46 61.47§

2.30 61.00k

2.18 61.47{

1.65 60.97**


Values are mean 6 SD expressed in millivolts.*Significantly different (p , 0.05) from SLJ, DJ24.†Significantly different (p , 0.05) from DJ24.‡Significantly different (p , 0.05) from DJ12, DJ24.§Significantly different (p , 0.05) from BOX, DJ24.kSignificantly different (p , 0.05) from CON.{Significantly different (p , 0.05) from CON, TUC.**Significantly different (p , 0.05) from ANK, CON, PIK, VJ, SJ30.

TABLE 7. Integrated EMG for the quadriceps muscle group for subjects with a VJ . 50 cm.

CON TUC VJ SJ30 ANK BOX PIK SLJ DJ12 DJ24

6.29 62.70*

5.33 62.94†

4.82 61.18‡

4.79 62.16§

4.66 61.68k

4.61 62.17‡

4.52 61.82

3.44 62.42{

3.37 62.56k

3.28 61.75**


Values are mean 6 SD expressed in millivolts.*Significantly different (p , 0.05) from ANK, SLJ, DJ12, DJ24.†Significantly different (p , 0.05) from SLJ, DJ24.‡Significantly different (p , 0.05) from DJ24.§Significantly different (p , 0.05) from SLJ, DJ24.kSignificantly different (p , 0.05) from CON.{Significantly different (p , 0.05) from CON, TUC, SJ30.**Significantly different (p , 0.05) from CON, TUC, VJ, SJ30, BOX.



TABLE 8. Integrated EMG for the gastrocnemius muscle group for all subjects.

VJ CON TUC ANK PIK BOX SJ30 SLJ DJ12 DJ24

3.32 62.06*

3.25 62.15†

3.17 62.14‡

3.05 61.83§

3.07 61.97†

2.86 62.10*

2.61 61.53k

2.42 61.18{

2.33 61.60**

1.78 61.22††


Values are mean 6 SD expressed in millivolts.*Significantly different (p , 0.05) from SLJ, SJ30, DJ12, DJ24.†Significantly different (p , 0.05) from SLJ, DJ24.‡Significantly different (p , 0.05) from DJ12, DJ24.§Significantly different (p , 0.05) from DJ24.kSignificantly different (p , 0.05) from VJ, DJ24.{Significantly different (p , 0.05) from CON, PIK, VJ, DJ24.**Significantly different (p , 0.05) from TUC, VJ.††Significantly different (p , 0.05) from ANK, CON, TUC, PIK, VJ, SJ30, BOX.

TABLE 9. Integrated EMG for the quadriceps muscle group for men.

CON VJ TUC ANK PIK BOX SJ30 SLJ DJ12 DJ24

3.62 61.85*

3.47 61.69†

3.46 62.23

3.45 61.62†

3.16 61.46†

2.76 61.96

2.72 61.23

2.49 61.12‡

2.37 61.47

1.93 61.22§


Values are mean 6 SD expressed in millivolts.*Significantly different (p , 0.05) from SLJ, DJ24.†Significantly different (p , 0.05) from DJ24.‡Significantly different (p , 0.05) from CON.§Significantly different (p , 0.05) from ANK,CON, PIK, VJ.

TABLE 10. Integrated EMG for the quadriceps muscle group for subjects with a VJ . 50 cm.

CON VJ ANK PIK TUC SJ30 BOX SLJ DJ12 DJ24

3.44 61.88*

3.40 61.68†

3.39 61.37‡

3.37 61.67†

3.17 61.69‡

2.71 61.09†

2.46 61.46§

2.30 61.00k

2.18 61.47{

1.65 60.97**


Values are mean 6 SD expressed in millivolts.*Significantly different (p , 0.05) from SLJ, DJ24.†Significantly different (p , 0.05) from DJ24.‡Significantly different (p , 0.05) from DJ12.§Significantly different (p , 0.05) from DJ24.kSignificantly different (p , 0.05) from CON.{Significantly different (p , 0.05) from ANK, TUC.**Significantly different (p , 0.05) from ANK, CON, PIK, VJ, SJ30.



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repeated-measures ANOVA to test for main effects.Assumptions for linearity of statistics were tested and met.Significant main effects were further analyzed with Bonfer-roni adjusted pairwise comparison of within-subject differ-ences among the plyometric exercises. Data were analyzedfor the entire eccentric and concentric portions of theplyometric exercises. Interclass correlation coefficients (ICC)for reliability of the dependent measures ranged from 0.75 to0.93. For the sample size used, statistical power ranged from0.41 to 0.49 for the variables assessed in this investigation.The criterion for significance was set at an a level of p# 0.05.

RESULTS

Table 2 depicts the significant main effect for the Q when allsubjects were analyzed, as well as for separate analysis ofmen, women, subjects with vertical jumps greater than 50 cm,and those whose vertical jump was less than or equal to50 cm (p# 0.05). Significant main effects were also found forthe G muscle group when all subjects were analyzed, as wellas for men and those with vertical jumps greater than 50 cm(p # 0.05). No significant main effects were found for theH muscle group. Bonferonni adjusted pairwise comparison ofspecific plyometric exercise are presented for the Q musclegroups in Tables 3–7 and for the G muscle group in Tables8–10. Data are presented in each Table from high to lowmean values.

DISCUSSION

This is the first study to comprehensively evaluate motor unitrecruitment associated with plyometric exercises. Whereasnumerous studies demonstrate the effectiveness of plyomet-rics, little is known about how these exercises differ. Studiesattempting to quantify the differences among plyometricexercises are typically limited to evaluating kinetic orkinematic variables of one or two variations (10,21,22,28).Only two studies (14,15) have evaluated several plyometricexercises in an attempt to evaluate their qualitative differ-ences. These studies have focused on kinetic data such asground and knee joint reaction forces and impulse. Thesekinetic variables are an important for further understandingof plyometric intensity. However, studies examining EMGactivity, in particular, have previously only assessed variationsof a single exercise (3,5,11,24,29,34).

Results of the present study suggest that Q IEMG variesamong a number of plyometric exercises. When all subjectswere analyzed, exercises such as cone hops, box jumps, andtuck jumps resulted in more Q IEMG than exercises such asthe single-leg jump, and depth jumps from 30- and 61-cmboxes. Similarly, the vertical jump stimulates more Q IEMGthan either of the depth jumps, and ankle hops offer more QIEMG than depth jumps from 61-cm boxes. In fact, depthjumps resulted in the lowest mean Q IEMG of all the exercisesassessed. From the standpoint of motor unit recruitment,these finding contrast with previous anecdotal recommen-dations that depth jumps are the highest intensity form of

plyometrics and that exercise such as cone and ankle hops arelow intensity (19). The present findings, as a measure ofplyometric intensity, also contrast with previous researchindicating that exercises such as the single-leg jumps anddepth jumps result in the greatest impulse, peak GRF,eccentric rate of force development (14,15). With regard todepth jumps, the findings of the present study did not changewhen analyzed according to gender or jumping ability, asdepth jumps resulted in low IEMG in each analysis. Inaddition to the Q, analysis of the G indicated that exercisessuch as the vertical jump and cone hop resulted in greatermotor unit recruitment than single-leg jumps or depth jumpsand that exercises such as the tuck jump, ankle hop, and boxjump all resulted in greater motor unit recruitment than the61-cm depth jump. These findings also contrast witha number of anecdotal recommendations regarding exerciseintensity (19) and previous research examining kineticvariables of plyometric intensity (14,15).

Research has previously demonstrated that single-legjumps produce the greatest landing impulse (15), yet thepresent study demonstrated that this exercise produces lessIEMG than exercises such as the cone hop, tuck jump,vertical jump, squat jump with 30% of the squat load, and boxjumps (14). Similarly, previous research demonstrated thatthe single-leg jump, pike jump, and tuck jump produce thegreatest knee joint reaction, but in the present study, theseexercises produced less IEMG than a number of otherplyometrics (15).

Although not always meeting the level of statisticalsignificance, in all post hoc analyses of the Q and G musclegroups, the bilateral vertical jump resulted in higher averagelevels of motor unit recruitment than the single-leg jump, theunloadedvertical jumpresulted inmoremotorunit recruitmentthan squat jumps with 30% of subjects’ squat load, and in all butone post hoc evaluation, the 30-cm depth jump resulted inhigher levels of motor unit recruitment than the 61-cm depthjump. Thus, in conditions of greater loading because of single-leg jumping, added mass, or greater drop distance, motor unitrecruitment is surprisingly lower. These findings contrast withprevious research by Zazulak et al. (34), who demonstratedthat single-leg landing from depth jumps of greater heightproduced more EMG activity than lower depth jumps.Similarly, others have demonstrated greater EMG activityduring one-legged versus two-legged jumps (24,29).

Schmidtbleicher (25) previously reported that exerciseswith increasing stretching loads result in a reduction inmuscle activation and surface EMG. Thus, it is not surprisingthat jumps with added mass, depth jumps, and single-legjumps produced low EMG values compared with otherplyometric exercises in this study. Explanations of the resultsof the present study include the possibility that increasedloading selectively activates passive force-producing struc-tures of the strength shortening cycle more than the stretchreflex, which would increase motor unit recruitment.Previous research evaluating the squat demonstrated



increased power output during the concentric phase, despitesteady Q EMG, suggesting that force production may bea function of factors other than motor unit recruitment (27).Theoretically, conditions of higher loading could increase theeccentric and amortization phases, limiting the role of thestretch reflex but not the recovery of passive force. Incontrast, Bobbert et al. (3) ruled out stored elastic energy asan explanation for greater jump heights when comparingjumps with and without counter-movements. Anotherexplanation may be that plyometric exercises with thegreatest overload result in greater eccentric Q:H activationratios, resulting in Q-induced anterior tibial shear andlengthening of the anterior cruciate ligament, which hasbeen thought to trigger the anterior cruciate ligament musclereflex arc (26), potentially inhibiting the Q IEMG.

Hamstring EMG activity was highly variable amongsubjects, which most likely mitigated a finding of maineffects. This variation may be the result of biomechanicaldifferences in landing strategies and their effect on themoments of resistive force and external torque. Largeintersubject variability in H recruitment did not change whendata were analyzed based on gender and jumping ability.

Scientific evaluation of plyometric exercises is necessary todesign programs with progressive intensity for optimalperformance and/or rehabilitation of injuries. This study,along with previous research (14,15), demonstrates thatanecdotal observations regarding plyometric intensity havelimitations. Although not customary, it may be instructive tocompare the means of the pairwise comparisons from thisstudy (Tables 3–10) even when statistical significance wasnot attained, because it is likely that these averages aremore evidential than common anecdotal recommendationsregarding plyometric intensity. Although evaluating manyplyometric exercises is necessary, it is a challenge becausethe family-wise error rate of multiple comparisons and theBonferonni adjustment may resulted in an inflated type IIerror. Despite approximately threefold differences in meanIEMG of some of the exercises, no significance was foundin other cases.

Plyometric intensity may be evaluated according toa number of variables. Because the relative intensity ofplyometric exercises differs according to the kinetic andkinematic variables assessed (14,15), understanding intensityis not a simple matter. Thus, we propose a paradigm bywhich plyometrics are evaluated according to either variablesof performance enhancement or rehabilitation. For example,to understand the role of plyometrics in performanceenhancement, variables such as rate of force development,rate of force development during the time to takeoff,eccentric rate of force development, peak power, reactivestrength index, and motor unit recruitment may be useful.However, variables such as ground reaction force, jointreaction force, time to stabilization, and motor unit re-cruitment may be more useful for quantifying plyometricintensity for pre/rehabilitation. Future research should focus

on further quantifying these variables of intensity. Ultimately,because plyometric intensity varies according to the variablesassessed, practitioners could use the result of analysis that ismost applicable to the needs of those they serve.

PRACTICAL APPLICATIONS

Quantifying plyometric exercise intensity is important tooptimally progress this form of exercise for developingathletic ability, injury prevention, and rehabilitation. Un-derstanding plyometric intensity requires consideration ofa variety of kinetic variables and motor unit recruitment.Results of this study provide information about the degree ofmotor unit recruitment associated with various plyometricexercises. If motor unit recruitment is the primary goal of theplyometric program, the findings of this study presented inTables 3–10 should be considered. Selection of the exercisesoffering the greatest motor unit recruitment should beprioritized in the program design. Future research mayfurther evaluate plyometric intensity by examining a varietyof kinetic variables.

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