power training

Influence of Strength on Magnitude andMechanisms of Adaptation to Power Training

PRUE CORMIE1, MICHAEL R. MCGUIGAN2,3, and ROBERT U. NEWTON1

1School of Exercise, Biomedical and Health Sciences, Edith Cowan University, Perth, AUSTRALIA; 2New Zealand Academyof Sport North Island, Auckland, NEW ZEALAND; and 3Institute of Sport and Recreation Research New Zealand,Auckland University of Technology, Auckland, NEW ZEALAND

ABSTRACT

CORMIE, P., M. R. MCGUIGAN, and R. U. NEWTON. Influence of Strength on Magnitude and Mechanisms of Adaptation to

Power Training. Med. Sci. Sports Exerc., Vol. 42, No. 8, pp. 1566–1581, 2010. Purpose: To determine whether the magnitude of

performance improvements and the mechanisms driving adaptation to ballistic power training differ between strong and weak indi-

viduals. Methods: Twenty-four men were divided into three groups on the basis of their strength level: stronger (n = 8, one-repetition

maximum-to-body mass ratio (1RM/BM) = 1.97 T 0.08), weaker (n = 8, 1RM/BM = 1.32 T 0.14), or control (n = 8, 1RM/BM = 1.37 T

0.13). The stronger and weaker groups trained three times per week for 10 wk. During these sessions, subjects performed maximal-

effort jump squats with 0%–30% 1RM. The impact of training on athletic performance was assessed using a 2-d testing battery

that involved evaluation of jump and sprint performance as well as measures of the force–velocity relationship, jumping mechanics,

muscle architecture, and neural drive. Results: Both experimental groups showed significant (P e 0.05) improvements in jump

(stronger: peak power = 10.0 T 5.2 WIkgj1, jump height = 0.07 T 0.04 m; weaker: peak power = 9.1 T 2.3 WIkgj1, jump height =

0.06 T 0.04 m) and sprint performance after training (stronger: 40-m time = j2.2% T 2.0%; weaker: 40-m time = j3.6% T 2.3%).

Effect size analyses revealed a tendency toward practically relevant differences existing between stronger and weaker individuals in the

magnitude of improvements in jump performance (effect size: stronger: peak power = 1.55, jump height = 1.46; weaker: peak power =

1.03, jump height = 0.95) and especially after 5 wk of training (effect size: stronger: peak power = 1.60, jump height = 1.59; weaker:

peak power = 0.95, jump height = 0.61). The mechanisms driving these improvements included significant (P e 0.05) changes in the

force–velocity relationship, jump mechanics, and neural activation, with no changes to muscle architecture observed. Conclusions: The

magnitude of improvements after ballistic power training was not significantly influenced by strength level. However, the training had a

tendency toward eliciting a more pronounced effect on jump performance in the stronger group. The neuromuscular and biomechanical

mechanisms driving performance improvements were very similar for both strong and weak individuals. Key Words: BALLISTIC,

JUMP, SQUAT, SPRINT, NEUROMUSCULAR ADAPTATIONS

After strength training, the magnitude of improve-ments in strength and the mechanisms drivingthose adaptations differ as the strength level of

the athlete improves (14,17,33,37). Specifically, initial im-provements in strength are much greater and predomi-nately driven by neural adaptations (although early phasemuscular adaptations also occur), whereas further increasesin strength are progressively harder to achieve and morphol-ogical adaptations in the muscle become more important(14,17,33,37). Thus, training programs geared at signifi-cantly improving strength in individuals with an existing

high level of strength require a much more sophisticated de-sign (i.e., greater specificity and variation) (17,33). Althoughthe factors contributing to maximal strength and the appli-cation of strength training are well understood, much less isknown concerning the adaptations to and utilization of bal-listic power training. In particular, the influence of strengthlevel on both the magnitude of improvement and the mech-anisms driving adaptation after ballistic power training is notknown. Such knowledge is vital to the development of train-ing programs that most effectively improve maximal powerproduction and athletic performance in athletes with a widevariety of training backgrounds.

Cross-sectional comparisons have revealed that individ-uals with higher strength levels have markedly superiorpower production capabilities than those with a low level ofstrength (2,7,10,24,34,35). For example, significant differ-ences in power output and/or jump height between in-dividuals with significantly different strength levels havebeen reported in comparisons of well-trained athletes andrelatively untrained controls (10,24,35), athletes competi-tive in power-type sports (i.e., volleyball) and enduranceevents (7), rugby league players involved in national versus

Address for correspondence: Prue Cormie, Ph.D., School of Exercise,Biomedical and Health Sciences, Edith Cowan University, 270 JoondalupDr. Joondalup, WA 6027, Australia; E-mail: [email protected] for publication September 2009.Accepted for publication December 2009.

0195-9131/10/4208-1566/0MEDICINE & SCIENCE IN SPORTS & EXERCISE�

Copyright � 2010 by the American College of Sports Medicine

DOI: 10.1249/MSS.0b013e3181cf818d

1566

APP

LIED

SCIENCES

Copyright © 2010 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

state competitions (2), as well as the strongest and weakestin a pool of resistance-trained men with various trainingbackgrounds (34). These findings are supported by reportsof a significant and positive relationship existing betweenmaximal strength and maximal power production (1,30,34).While skeletal muscle mechanics that cause the force–velocity relationship dictate that maximal strength plays arole in muscular power, the development of muscular poweris influenced by a multitude of factors in addition to maximalstrength (27). For example, McBride et al. (24) observed thatdespite no differences in the maximal strength of national-level weightlifters and powerlifters (Smith machine squatone-repetition maximum-to-body mass ratio (1RM/BM) =2.86 T 0.15 and 2.88 T 0.14, respectively), weightliftersgenerated significantly greater power output during an un-loaded countermovement jump (CMJ; weightlifters = 63.0 T2.7 WIkgj1, powerlifters = 56.9 T 2.5 WIkgj1). Such obser-vations were attributed to the various training protocols com-monly used by these athletes (i.e., high force–high velocitytraining vs high force–low velocity training common toweightlifting and powerlifting, respectively) (24). Similarresults were observed in comparisons between well-trainedpower athletes and recreational bodybuilders with similarstrength levels (35). Therefore, although the developmentof maximal muscular power required for the successfulperformance of many athletic movements is influenced bya multitude of factors (27), stronger individuals have con-sistently been shown to display superior power outputthan individuals with a significantly lower strength level(2,7,10,24,34,35).

This raises the question of what mechanisms contribute tothe improved power production capabilities of stronger in-dividuals. Stronger individuals possess neuromuscular char-acteristics that form the basis for superior maximal powerproduction and ultimately contribute to enhanced athleticperformance. Specifically, an individual with a substantiallygreater level of strength (i.e., strong vs weak person) wouldhave larger whole-muscle cross sectional area (CSA), a re-sult of greater myofibrillar CSA of both Type I and Type IIfibers, with more pronounced hypertrophy of Type II fibersevident (18,22). Pennation angle may also be greater andpossibly even fascicle length (3,4,21). In addition, effective-ness of neural drive (i.e., recruitment, rate of onset, firingfrequency) as well as intermuscular coordination would be farsuperior in the significantly stronger individual (15,25,33).These neuromuscular characteristics would result in a shiftin the force–velocity relationship so that the force of musclecontraction would be greater for any given velocity of short-ening (19). As a consequence, the ability to generate maximalmuscular power, and therefore perform athletic movements,would be superior in a considerably stronger individual com-pared with a weaker person (19,23,38).

Despite the advantage of strength for maximal powerproduction and athletic performance, little is known re-garding whether neuromuscular characteristics of strongerindividuals allow for superior adaptation to ballistic power

training. More importantly, it is not known if the mech-anisms driving adaptations to ballistic power training areinfluenced by strength level. To the authors’ knowledge,only one intervention study examining the possible influenceof strength on improvements after ballistic power trainingexists (37). Wilson et al. (37) compared the improvementsbetween stronger (n = 4; squat 1RM/BM = 1.99 T 0.30) andweaker men (n = 5; squat 1RM/BM = 1.21 T 0.18) after 8 wkof drop jump training. Neither group significantly improvedeither jump and reach height or 20-m sprint time, and nocorrelation was found between strength level and the mag-nitude of the training-induced change in jump and reachheight (r = j0.13, n = 14) (37). Therefore, this previousstudy offers little insight into the influence of strength on themagnitude of improvements in athletic performance afterballistic power training. Furthermore, the investigation didnot involve measures to examine any mechanistic factors pos-sibly involved in adaptations to the training. Simulation datasuggest that although jump height is most sensitive to in-creases in strength-to-BM ratio (31), an increase in strengthdoes not translate into increased jump height unless control(i.e., intermuscular coordination—the appropriate magnitudeand timing of activation of agonist, synergist, and antagonistmuscles during a movement) is tuned to the strengthenedmuscle properties (6). These data are supported by the con-cept of a delayed training effect in which it takes consid-erable time and effort for an increase in strength to transmuteto improved performance in multijoint movements becauseintermuscular coordination needs to be adapted to the strongermotor units/muscles (39). Therefore, it may be speculatedthat a stronger individual exposed to ballistic power trainingwould improve performance by fine tuning the timing andpatterns of neural drive, thus improving intermuscular coor-dination and movement technique (although muscular adap-tations at the cellular level such as alterations to proteinexpression, calcium release, and uptake as well as relaxationtime may also contribute). It is unknown if the combinationof these potential changes with the underlying neuromuscularcharacteristics of the stronger individual would result in atraining response dissimilar to individuals with lower levelsof strength.

In comparisons of individuals not currently involved inballistic power training, previous research has demonstratedthat stronger individuals are able to generate superior levelsof maximal power than significantly weaker individuals(2,7,10,24,34,35). Although the enhanced maximal powerproduction of stronger individuals in the absence of ballisticpower training is theorized to be due to the underlying neu-romuscular characteristics of stronger individuals, it is un-clear if these characteristics improve the ability to adapt toballistic power training. Specifically, it is not known whetherballistic power training elicits performance improvementsin stronger individuals of a greater magnitude than thatwhich is achievable with weaker subjects. Furthermore, thereis a paucity of research investigating whether strength levelinfluences the mechanisms responsible for performance

INFLUENCE OF STRENGTH ON POWER Medicine & Science in Sports & Exercised 1567

APPLIED

SCIEN

CES


improvements after ballistic power training. Therefore, thepurpose of this experiment was to determine whether themagnitude of performance improvements and the mecha-nisms driving adaptation to ballistic power training differbetween strong and weak individuals.

METHODS

Experimental design. This study used a randomized,control design and was conducted during a period of 15 wk.Subjects were divided into two strata on the basis of their1RM/BM: stronger (1RM/BM 9 1.55) or weaker (1RM/BM G1.55). Subjects in the stronger stratum were allocated intothe stronger group. Subjects in the weaker stratum were ran-domized into one of two groups: weaker group or controlgroup. The stronger and weaker groups completed a total of10 wk of ballistic power training, while the control groupmaintained their normal level of activity throughout theduration of the study. Training involved three sessions perweek in which subjects performed maximal-effort jumpsquats with 0%–30% 1RM. Subjects completed a 2-d testingbattery before initiating training (baseline), after 5 wk oftraining (midtest—stronger and weaker groups only), andafter the completion of 10 wk of training (posttest). Subjectswere adequately familiarized to all testing procedures beforeactual assessment. Testing involved evaluation of jump andsprint performance as well as measures of the force–velocityrelationship, jumping mechanics, muscle architecture, andneural drive.

Subjects. Subjects were recruited on the basis of theirability to perform a back squat with proficient technique. Atotal of 35 men fulfilled all the testing and training re-quirements of this investigation. Data from 11 of these menwere removed on the basis of their 1RM/BM to establishtwo experimental groups with very distinct differences inmaximal strength (i.e., data from subjects with a 1RM/BMbetween 1.55 and 1.85 were not included). The remaining24 men were allocated into three groups: stronger group(n = 8, 1RM/BM = 1.97 T 0.08), weaker group (n = 8,1RM/BM = 1.32 T 0.14), or control group (n = 8, 1RM/BM= 1.37 T 0.13). Subjects’ characteristics throughout the du-

ration of the study are outlined in Table 1. The participantswere notified about the potential risks involved and gavetheir written informed consent. This study was approved bythe university’s human research ethics committee.

Training program. The training program consisted ofthree sessions per week separated by at least 24 h of rest.Each training session was initiated via a warm-up consistingof two sets of six submaximal jump squats with 0% 1RM(i.e., CMJ with no external load, just the resistance appliedby a carbon fiber pole (mass = 0.4 kg) held across theshoulders). During sessions 1 and 3 of each week, subjectsperformed seven sets of six maximal-effort jump squatsseparated by a 3-min recovery. Jump squats were performedat the load that maximized power output for each subject, asdetermined during the baseline testing session. Similar toprevious research, a load consistent with the subject’s BM(i.e., no external load or 0% 1RM) maximized power outputfor each of the participants in this study (11,12). The secondtraining session of each week included an additional warm-up set consisting of five submaximal jump squats with 30%1RM. Subjects then performed five sets of five maximal-effort jump squats with 30% 1RM separated by a 3-minrecovery. Subjects were encouraged to perform each jumpas rapidly as possible. Intensity was modified for eachsession so that an audible beep could be heard by subjectsduring jumps that reached 95% of the maximal poweroutput at that load from their previous training or testingsession. Previous literature has shown significant perfor-mance improvements after jump squat training with similarprogramming parameters at both 0% 1RM (11) and 30%1RM (25,38). Subjects refrained from any additional lowerbody resistance training, plyometrics, or sprint trainingthroughout the course of the study.

Testing protocol. Subjects rested for 3–5 d betweentheir previous training session and the midtest testing ses-sion and for 7–10 d between the final training session andthe posttest testing session to allow full recovery. Subjectscompleted two testing sessions separated by at least 3 d ofrecovery at the baseline, midtest, and posttest occasions.Testing session 1 was initiated with an examination ofthe vastus lateralis (VL) muscle architecture through

TABLE 1. Subject characteristics of the stronger, weaker, and control groups throughout the 10 wk of training.

BM (kg) Body Fat (%) 1RM/BM 40-m Sprint (s) Jump Height (m)

Stronger groupBaseline 79.1 T 12.8 12.2 T 3.9‡ 1.97 T 0.08† 5.41 T 0.25† 0.43 T 0.03‡Midtest 79.8 T 12.9 12.3 T 3.8‡ 1.93 T 0.11‡ 5.39 T 0.20‡ 0.50 T 0.03*Posttest 79.6 T 13.0 12.6 T 3.5‡ 1.88 T 0.11† 5.29 T 0.20† 0.50 T 0.03*,§

Weaker groupBaseline 79.9 T 14.5 17.3 T 3.8 1.32 T 0.14 5.90 T 0.27 0.38 T 0.04Midtest 79.2 T 14.2 17.5 T 4.0 1.38 T 0.16 5.79 T 0.20 0.42 T 0.07Posttest 79.1 T 13.8 17.7 T 3.6 1.39 T 0.17 5.69 T 0.22 0.44 T 0.06

Control groupBaseline 77.5 T 8.1 14.6 T 3.5 1.37 T 0.13 5.79 T 0.19 0.41 T 0.04Posttest 78.4 T 8.8 14.7 T 3.8 1.35 T 0.12 5.79 T 0.12 0.40 T 0.04

BM, body mass; 1RM:BM, squat one repetition maximum to body mass ratio.* Significantly (P e 0.05) different from baseline.† Significant (P e 0.05) difference between stronger group and all other groups.‡ Significant (P e 0.05) difference between stronger group and weaker group.§ Significantly (P e 0.05) different from control group.

http://www.acsm-msse.org1568 Official Journal of the American College of Sports Medicine

APP

LIED

SCIENCES


ultrasonography. Maximal dynamic strength was then as-sessed using a back squat 1RM to a depth consistent with aknee angle of at least 90- of flexion assessed using two-dimensional motion analysis. During a 30-min recovery,body composition was assessed using dual-energy x-rayabsorptiometry. Maximal isometric strength was then eval-uated using an isometric squat test performed at a knee angleof 140- to allow for the determination of maximum forceoutput at zero velocity (12,32). Adequate recovery waspermitted (10 min) before examination of jump squat per-formance across a series of intensities: 0% (i.e., no externalload or BM only), 20%, 40%, 60%, and 80% of squat 1RM.Subjects completed the jump squats in a randomized order,which was consistent across the three testing occasions foreach individual subject. Kinematic (both linear positiontransducer (LPT) and two-dimensional motion analysis),kinetic, and EMG data were obtained simultaneously through-out the testing session. After at least 3 d of recovery, sub-jects completed the second testing session involving a 40-msprint test.

Data acquisition and analysis procedures. Theback squat 1RM involved subjects completing a series ofwarm-up sets (four to six repetitions at 30% estimated1RM, three to four repetitions at 50% estimated 1RM, twoto three repetitions at 70% estimated 1RM, and one to tworepetitions at 90% estimated 1RM), each separated by 3 minof recovery. A series of maximal lift attempts was thenperformed until a 1RM was obtained. No more than fiveattempts were permitted with each attempt separated by5 min of recovery. This protocol has been frequently usedthroughout the previous literature for the assessment ofmaximal dynamic strength (11,12,30). Only trials in whichsubjects reached a relative knee angle (i.e., angle betweenthe midline of the lower leg and the midline of the thigh)G90- of flexion were considered successful. This depth wasvisually monitored during testing and was confirmed bytwo-dimensional motion analysis (stronger group: baseline= 85.7- T 4.2-, posttest = 82.5- T 6.8-; weaker group:baseline = 83.4- T 2.4-, posttest = 80.4- T 4.3-; controlgroup: baseline = 83.1- T 6.2-, posttest = 82.6- T 5.4-; raterreliability: r = 0.95). Although no significant differences inthe depth of the squat 1RM existed among the testing occa-sions, there is the potential that the slightly deeper depthobtained during posttesting could have influenced the 1RMvalues obtained.

The isometric squat test was performed with subjectsstanding on a force plate (9290AD; Kistler Instruments,Winterthur, Switzerland) in a back squat position pushingagainst an immovable rigid bar. The bar was positioned sothat subjects had a knee angle of 140- of flexion to allowfor the determination of maximal force output at zero ve-locity. Previous research has shown this knee angle to cor-respond with the highest isometric force output for the squatcompared with a range of other knee angles (32). Subjectswere instructed to perform a rapid, maximal effort to reachmaximal force output as soon as possible and maintain that

force for 3 s. The analog signal from the force plate wascollected for every trial at 1000 Hz using a data acquisitionsystem including an analog-to-digital card (cDAQ-9172;National Instruments, North Ryde, NSW, Australia). Cus-tom programs designed using LabVIEW software (Version8.2; National Instruments) were used for recording and an-alyzing the data. The signal was filtered using a fourth-order, low-pass Butterworth filter with a cutoff frequencyof 50 Hz. From laboratory calibrations, the voltage outputwas converted into vertical ground reaction force. Peakforce relative to BM was assessed as the maximal forceoutput during the 3-s period divided by the individual’sBM. The test–retest reliability for peak force relative to BMwas r = 0.98.

Performance of a jump squat involved subjects complet-ing a maximal-effort CMJ while holding a rigid bar acrosstheir shoulders. Subjects held a 0.4-kg carbon fiber pole forthe 0% 1RM jump squat, whereas for all other intensities,subjects held a 20-kg barbell loaded with the appropriateweight plates. Participants were instructed to keep constantdownward pressure on the bar throughout the jump andwere encouraged to move the resistance as fast as possibleto achieve maximal power output with each trial. The barwas not allowed to leave the shoulders of the subject, withthe trial being repeated if this requirement was not met. Aminimum of two trials at each load were completed, withadditional trials performed if both peak power and jumpheight were not within 5% of the previous jump squat.Adequate rest was enforced between all trials (3 min).

All jump squats were performed while the subject wasstanding on the force plate with a LPT (PT5A-150; CelescoTransducer Products, Chatsworth, CA) attached to the bar.The LPT was attached 10 cm to the left of the center of thebar to avoid any interference caused by movement of thehead during the jump. The LPT was mounted above the sub-ject, and the retraction tension of the LPT (equivalent to8 N) was accounted for in all calculations. Analog signalsfrom the force plate and LPT were collected for every trialat 1000 Hz and analyzed using custom programs designedusing LabVIEW software. The signal from the LPT wasfiltered using a fourth-order, low-pass Butterworth digitalfilter with a cutoff frequency of 10 Hz, and the voltage outputwas converted into displacement using laboratory calibra-tions. The vertical velocity of the movement was deter-mined using a first-order derivative of the displacement data.Power output was calculated as the product of the verticalvelocity and the vertical ground reaction force data. Ac-celeration of the movement was calculated using a second-order derivative of the displacement data and smoothedusing a fourth-order, low-pass Butterworth digital filter with acutoff frequency of 10 Hz. These data collection and analysismethodology have been validated previously (9), and the test–retest reliability for all jump variables examined was con-sistently r Q 0.90.

A series of performance variables was assessed duringthe jump squats. Peak force, velocity, power, displacement,


APPLIED

SCIEN

CES


and acceleration were determined as the respective maximalvalues achieved during the entire movement. Net impulsewas assessed as the integral of vertical ground reactionforce during the period of application in which force ex-ceeded that required during stationary standing (i.e., abovebody weight). Rate of force development (RFD) was de-termined between the minimum and maximum force thatoccurred throughout the movement. Similarly, rate of powerdevelopment (RPD) was also determined between the min-imum and maximum power that occurred throughout themovement. Average power output was calculated during theconcentric phase of the movement (i.e., time between min-imum displacement and takeoff). Instantaneous force andvelocity output at the time at which peak power occurredwas also examined and was termed force at peak power andvelocity at peak power, respectively. These values acrosseach of the loads examined were used to generate the force–velocity and force–power relationships for the jump squat.Velocity at takeoff was defined as the velocity of movementat the time at which the force output was first zero (i.e.,when the toes first left the force plate). Furthermore, timeto takeoff was determined as the time between the initiationof the countermovement (i.e., start of the eccentric phase)and the point that force was zero (i.e., end of the concentricphase—takeoff).

In addition to these instantaneous performance variables,analyses of parameters throughout the jump movement wereconducted. The power–time and force–time curves from eachindividual subject were selected from the beginning of theeccentric phase (i.e., initiation of countermovement as-sessed as the time at which a change in velocity first oc-curred) through to the end of the concentric phase (i.e., attakeoff when force and power reached zero). The velocity–time and displacement–time curves were selected from thebeginning of the eccentric phase to peak displacement (i.e.,zero velocity). Using a custom-designed LabVIEW program,the number of samples in each individual curve was thenmodified to equal 500 samples by changing the time delta(dt) between samples and resampling the signal (dt = numberof samples in the original signal/500). The sampling fre-quency of the normalized signals was calculated according tothe following equation:

normalized samplingfrequenzy ðHzÞ ¼ 1 second

½ no: samples in original signal

no: samples in normalized signal � � ½ secondsper sample

�Consequently, the sampling frequency of the modified

signals was then equivalent to 815 T 154 Hz for the power–time and force–time curves and 538 T 68 Hz for the velocity–time and displacement–time curves. This resampling allowedfor each individual’s power, force, velocity, and displace-ment curves to be expressed during equal periods of time(i.e., the 500 samples represented the relative time—from0% to 100%—taken to complete the jump). In other words,the various data sets were normalized to total movement

time so that data could be pooled. Each sample of the nor-malized power–, force–, velocity–, and displacement–timecurves was then averaged across subjects within the stronger,weaker, or control groups, resulting in averaged curves withhigh resolution (sampling frequency of 538–815 Hz). Thisallowed for power, force, velocity, and displacement through-out the jump to be compared across baseline, midtest, andposttest as well as between groups. Intraclass test–retestreliabilities for power–, force–, velocity–, and displacement–time curves during the CMJ have consistently been r Q 0.94,r Q 0.90, r Q 0.89, and r Q 0.92, respectively, using thismethodology (10).

Before assessment of sprint performance, subjects per-formed a warm-up consisting of 5 min of light jogging andthree submaximal 20-m sprints. The sprint test was initiatedfrom a standing start involving a staggered stance with thesame front–back leg orientation used for every trial through-out the baseline, midtest, and posttest. Subjects were in-structed to commence the sprint at will and accelerate asquickly as possible throughout the 40 m. Three trials wereperformed with each separated by a 3-min recovery. A seriesof six dual-beam timing gates (Speedlight; Swift Sports,Lismore, NSW, Australia) was used to record instanta-neous time at 5, 10, 20, 30, and 40 m (timing was com-menced when subject passed through a dual-beam timinggate positioned at their front foot). Flying 5 m was calculatedas the time between the 5- and 10-m gates, whereas flying15 m was calculated at the time between the 5- and 20-mgates. Intraclass test–retest reliability for all sprint perfor-mance variables examined was consistently r Q 0.90.

EMG of the VL, vastus medialis (VM), and biceps fe-moris (BF) was collected on the dominant leg during theisometric squat and all jump squats. Disposable surfaceelectrodes (self-adhesive Ag/AgCl snap electrode, 2-cm in-terelectrode distance, 1-cm circular conductive area; prod-uct 272; Noraxon USA, Inc., Scottsdale, AZ) were attachedto the skin over the belly of each measured muscle, distal tothe motor point, and parallel to the direction of musclefibers. A reference electrode was placed on the patella. Theexact location of the electrodes relative to the anatomicallandmarks was marked on a sheet of tracing paper after thefirst testing session to ensure consistent placement in sub-sequent tests. Each site was shaved, gently abraded, andcleansed with alcohol before electrode placement to mini-mize skin impedance. Raw EMG signals were collected at1000 Hz and amplified (gain = 1000, bandwidth frequency= 10–1000 Hz, input impedance G 5 k6; Model 12D-16-OSNeurodata Amplifier System; Grass Technologies, WestWarwick, RI). The amplified myoelectric signal was col-lected simultaneously with force plate and LPT data using adata acquisition system including an analog-to-digital card.Custom programs designed using LabVIEW software wereused for recording and analyzing the data. The signal wasfull-wave–rectified and filtered using a dual-pass, sixth-order, 10- to 250-Hz band-pass Butterworth filter as well asa notch filter at 50 Hz. A linear envelope was created using


APP

LIED

SCIENCES


a low-pass, fourth-order Butterworth digital filter with acutoff frequency of 6 Hz. Maximal voluntary contraction(MVC) for all muscles were determined by averaging theintegrated EMG signal during a 1-s period of sustainedmaximal force output after the initial peak in the force curveduring the isometric squat (intraclass test–retest reliabilityconsistently r Q 0.91). EMG activity during jumping wasanalyzed by averaging the integrated EMG signal from thebeginning of the eccentric phase to takeoff. To standardizefor the time taken to complete a jump squat, this value wasthen divided by the time to takeoff. The average integratedEMG (AvgIEMG) was then normalized by expressing itrelative to the MVC. This is similar to methods previouslyused when comparing EMG between movements with dif-ferent time components (29). The rate of rise in AvgIEMG(expressed as a percentage of MVC per second) was as-sessed during the 0% 1RM jump squat as the rate of changebetween minimum and maximum AvgIEMG throughoutthe movement (i.e., from the initiation of the countermove-ment to takeoff). Intraclass test–retest reliability for all EMGvariables examined was consistently r Q 0.80.

In vivo muscle architecture was assessed by B-mode ultra-sonography recorded using an ultrasound console (SSD-1000;Aloka Incorporated, Tokyo, Japan) with a 7.5-MHz, 9-cmlinear probe. The same experienced examiner completed allscans across the baseline, midtesting, and posttesting occa-sions. Scans were performed on the VL of the dominant leg,with subjects lying supine and their leg muscles completelyrelaxed. To assist with acoustic coupling, water-soluble trans-mission gel was applied to the transducer. Measurementswere taken at 50% of the thigh length calculated as half thedistance between the centers of the greater trochanter to thelateral condyle of the femur. Longitudinal images were ob-tained with the transducer oriented parallel to the musclefascicles and perpendicular to the skin. The impact of dis-crepancies in transducer location and orientation on architec-tural differences observed between baseline, midtest, andposttest was minimized using several techniques. The locationand two-dimensional orientation of the transducer relative toanatomical landmarks were mapped onto a sheet of tracingpaper to ensure that the same site was used across all testingoccasions. In addition, in an effort to minimize any differ-ences in the three-dimensional orientation of the transducerbetween testing occasions, the live, onscreen image duringmidtest and posttest was compared with those taken duringbaseline testing. This allowed the examiner to match theunique features of the ultrasound images (i.e., heterogeneitiesin the subcutaneous adipose tissue and echoes from inter-spaces among the fascicles). These procedures have been usedpreviously in literature, monitoring the impact of training onmuscle architecture (3). Images were digitally recorded, andthe superficial and deep aponeuroses were identified, withmuscle thickness measured as the distance between the apo-neuroses (3,20). Pennation angle was defined as the angle atwhich the fascicles arose from the deep aponeurosis (20).Intraclass test–retest reliabilities for muscle thickness and

pennation angle were r = 0.95 and r = 0.90, respectively. Inaddition, rater reliabilities for muscle thickness and pennationangle were r = 0.98 and r = 0.93, respectively.

Two-dimensional motion analysis was used to evaluate themovement mechanics during the squat 1RM and jump squats.A digital video camera (25 Hz; MV830i; Canon AustraliaPty Ltd., North Ryde, NSW, Australia) was positioned 3.1 mfrom the subject, perpendicular to the subjects’ sagittal plane.Dartfish software (Version 4.5 ProSuite; Dartfish, Sydney,NSW, Australia) was used to analyze the movements. Thevideo footage was deinterlaced into fields, yielding a 50-Hzsampling frequency. During the jump squats, the minimumand maximum knee, hip, and ankle joint angles were assessedat the transition between the eccentric and concentric phases(minimum) and the last field before takeoff where the foot isstill in contact with the force plate (maximum). The minimumknee, hip, and ankle angles were also assessed during thesquat 1RM, representing the joint angles at the transitionbetween the eccentric and concentric phases. Intraclass test–retest reliability and rater reliability for all joint angles as-sessed using motion analysis were consistently r Q 0.92 andr Q 0.93, respectively.

FIGURE 1—Change from baseline in peak power, average power, andmaximum jump height during the 0% 1RM jump squat at midtest (A)and posttest (B). *Significant (P e 0.05) change from baseline.


APPLIED

SCIEN

CES


Statistical analyses. A general linear model withrepeated-measures ANOVA followed by Bonferroni posthoc tests was used to examine the impact of training onperformance variables and to determine whether differencesexisted between the groups at baseline, at midtest, and atposttest. t-tests were used for comparisons between var-iables at baseline and posttest for the control group as wellas for comparison of variables between stronger and weakergroups at midtest. Statistical significance for all analyseswas defined by P e 0.05, and results were summarized asmeans T SD. Estimated effect sizes (ES) of G2 = 0.502 andG2 = 0.300 at observed power levels of 0.976 and 0.626 for

maximum peak power relative to BM after training existedfor stronger and weaker groups, respectively. In addition,estimated ES of G2 = 0.406 and G

2 = 0.319 at the observedpower levels of 0.821 and 0.644 existed for the compari-son of maximum peak power relative to BM between thestronger and weaker groups at baseline and posttest,respectively. Mean ES were also calculated to examineand compare the practical significance of the performanceimprovements among the experimental groups. Based on thestudy of Cohen (8), which suggests ES of 0.2, 0.5, and 0.8 torepresent small, moderate, and large effects, respectively,practical relevance was defined as an ES Q 0.8 for the

TABLE 2. Change in performance variables during the 0% 1RM jump squat (i.e., BM only) from baseline ($).

Stronger Group Weaker Group

0% 1RM Jump Squat $ at Midtest $ at Posttest $ at Midtest $ at Posttest

PowerPeak power (WIkgj1) 10.3 T 5.1* 10.0 T 5.2* 7.1 T 4.1* 9.1 T 2.3*Average power (WIkgj1) 8.6 T 4.8* 9.6 T 5.2* 7.0 T 2.3* 8.5 T 1.9*Rate of power development (WIkgj1Isj1) 343.0 T 164.4*,‡ 325.2 T 132.6* 169.3 T 79.4* 236.7 T 129.7*

ForcePeak force (NIkgj1) 6.3 T 5.4* 6.3 T 4.7* 3.5 T 3.7 4.7 T 4.2*RFD (NIkgj1Isj1) 76.8 T 46.1* 82.3 T 58.3* 55.3 T 28.8* 94.6 T 74.9*Force at peak power (NIkgj1) 2.8 T 2.1* 2.1 T 1.6* 1.5 T 1.8 1.4 T 1.9Net impulse (NIs) 38.9 T 22.6* 45.9 T 28.9* 48.8 T 26.5* 57.4 T 20.9*

VelocityPeak velocity (mIsj1) 0.43 T 0.29* 0.42 T 0.22* 0.34 T 0.34* 0.55 T 0.26*Velocity at peak power (mIsj1) 0.19 T 0.34 0.22 T 0.31 0.15 T 0.30 0.28 T 0.27Velocity at takeoff (mIsj1) 0.43 T 0.29* 0.41 T 0.24* 0.35 T 0.36* 0.60 T 0.30*

AccelerationPeak acceleration (mIsj1Isj1) 11.7 T 6.2*,‡ 14.6 T 6.5*,‡ 6.4 T 2.0* 8.6 T 4.7*

DisplacementPeak displacement (m) 0.07 T 0.03* 0.07 T 0.04* 0.04 T 0.05 0.06 T 0.04*

TimeTime to takeoff (s) j0.18 T 0.07* j0.19 T 0.6* j0.27 T 0.12* j0.31 T 0.13*,‡

* Significant (P e 0.05) change from baseline.‡ Significant (P e 0.05) difference between stronger group and weaker group.

TABLE 3. Comparison of 40-m sprint times among stronger, weaker, and control groups throughout the 10 wk of training.

Sprint Times (s)

5 m 10 m 20 m 30 m 40 m Flying 5 m Flying 15 m

Stronger groupBaseline 1.10 T 0.10 1.82 T 0.12 3.08 T 0.15‡ 4.25 T 0.19† 5.41 T 0.25† 0.72 T 0.03† 1.98 T 0.07†Midtest 1.08 T 0.02‡ 1.82 T 0.04‡ 3.07 T 0.09‡ 4.23 T 0.14‡ 5.39 T 0.20‡ 0.74 T 0.03 1.99 T 0.08‡Posttest 1.02 T 0.08§ 1.75 T 0.08§ 2.99 T 0.11‡ 4.14 T 0.15† 5.29 T 0.20† 0.73 T 0.01† 1.97 T 0.05†$ between baseline

and posttestj0.08 T 0.04*,§ j0.07 T 0.04* j0.09 T 0.08*,§ j0.11 T 0.11*,§ j0.12 T 0.11* 0.01 T 0.02 j0.01 T 0.07

ES 0.86 0.67 0.69 0.65 0.54 0.64 0.18Weaker group

Baseline 1.17 T 0.12 1.95 T 0.16 3.32 T 0.19 4.60 T 0.23 5.90 T 0.27 0.78 T 0.04 2.14 T 0.08Midtest 1.13 T 0.05 1.90 T 0.08 3.24 T 0.11 4.51 T 0.16 5.79 T 0.20 0.77 T 0.05 2.10 T 0.09Posttest 1.09 T 0.05 1.86 T 0.07 3.18 T 0.12 4.44 T 0.17 5.69 T 0.22 0.77 T 0.02 2.09 T 0.08$ between baseline

and posttestj0.08 T 0.09 j0.09 T 0.10 j0.13 T 0.09*,§ j0.16 T 0.10*,§ j0.22 T 0.14*,§ j0.01 T 0.03 j0.06 T 0.03*,§

ES 0.79 0.69 0.78 0.76 0.82 0.25 0.66Control group

Baseline 1.13 T 0.04 1.88 T 0.07 3.25 T 0.11 4.52 T 0.14 5.79 T 0.19 0.75 T 0.03 2.12 T 0.08Posttest 1.15 T 0.04 1.91 T 0.05 3.27 T 0.07 4.53 T 0.09 5.79 T 0.12 0.76 T 0.02 2.12 T 0.05$ between baseline

and posttest0.02 T 0.05 0.03 T 0.06 0.02 T 0.08 0.01 T 0.08 0.00 T 0.10 0.01 T 0.02 0.00 T 0.04

ES 0.41 0.46 0.24 0.10 0.01 0.40 0.07

Flying 5 m time is calculated as the time between 5 and 10 m.Flying 15 m time is calculated as the time between 5 and 20 m.The change in sprint times between baseline and posttests as well as ES of the change is also displayed.* Significant (P e 0.05) change from baseline to posttest.† Significant (P e 0.05) difference between stronger group and all other groups.‡ Significant (P e 0.05) difference between stronger group and weaker group.§ Significantly (P e 0.05) different from control group.


APP

LIED

SCIENCES


FIGURE 2—Training-induced changes to the force–velocity and force–power relationships for the jump squat. *Significantly (P e 0.05) differentpeak power relative to BM from baseline. +Approaching significantly (P e 0.10) different peak power relative to BM from baseline. xSignificantly(P e 0.05) different force relative to BM at peak power from baseline. #Significantly (P e 0.05) different velocity at peak power from baseline.


APPLIED

SCIEN

CES


purpose of this study. A statistical software package (SPSS,Version 13.0; SPSS, Inc., Chicago, IL) was used to performall statistical analyses.

RESULTS

Athletic performance. The stronger group had signif-icantly greater 1RM/BM than both the weaker and controlgroups at all testing occasions and displayed a practicallyrelevant decrease in 1RM/BM at posttest (ES = 0.93, equiv-alent to a 7 T 7-kg decrease in 1RM). Training resulted insignificant within-group changes in a multitude of jump per-formance variables between baseline and midtest or be-tween baseline and posttest sessions for both the strongerand weaker groups (Fig. 1 and Table 2). However, only thechanges in RPD at midtest and time to takeoff at posttestsignificantly differed between the groups (Table 2). Com-parison of the magnitude of changes in average power, peak

power, and peak displacement between the stronger andweaker groups is illustrated in Figure 1. Any practicallyrelevant differences between the groups were more pro-nounced after 5 wk of training, and they diminished some-what after 10 wk of training (Fig. 1). The stronger groupcompleted the 40-m sprint in significantly less time thanboth the weaker and control groups across all testing occa-sions (Table 3). Training resulted in a significant change intime between baseline and posttest at 5-, 10-, 20-, 30-, and40-m sprints for the stronger group and at 20-, 30-, and 40-msprints for the weaker group (Table 3). These changeswere significantly different from the change in time dis-played by the control group (which did not significantlyalter sprint time).

Force–velocity relationship. Training-inducedchanges to force–velocity and force–power relationshipsduring the jump squat are displayed in Figure 2. Significantdifferences between baseline and posttest were evident for

FIGURE 3—Between-group differences in the force–velocity (A–C) and force–power relationships (D–F) for the jump squat throughout the 10 wk oftraining. †Significant (P e 0.05) difference in force or power between stronger and all other groups. ‡Significant (P e 0.05) difference in force orpower between stronger and weaker groups. §Significantly (P e 0.05) different force or power from control group.

TABLE 4. Two-dimensional motion analysis variables assessed during the 0% 1RM jump squat.

Minimum Joint Angle Maximum Joint Angle

0% 1RM Jump Squat Knee Angle (-) Hip Angle (-) Ankle Angle (-) Knee Angle (-) Hip Angle (-) Ankle Angle (-)

Stronger groupBaseline 89.7 T 8.5 86.3 T 13.8 60.7 T 4.4 170.0 T 7.1 168.3 T 4.9 133.4 T 3.9Midtest 94.8 T 6.5 89.4 T 10.8 61.8 T 4.7 174.5 T 5.0 171.9 T 8.1 137.5 T 5.9Posttest 94.7 T 5.6 91.5 T 16.4 59.0 T 5.4 176.8 T 4.3 174.0 T 7.1 140.0 T 5.8

Weaker groupBaseline 85.9 T 7.1 79.3 T 8.8 60.1 T 3.0 170.2 T 6.4 168.1 T 8.1 136.0 T 8.2Midtest 90.1 T 9.5 84.8 T 18.7 59.9 T 3.0 174.8 T 4.9 172.8 T 7.1 137.4 T 6.6Posttest 89.3 T 11.1 88.4 T 20.4 57.7 T 3.7 171.7 T 6.3 172.7 T 9.5 138.1 T 5.2

Control groupBaseline 89.6 T 6.4 78.6 T 9.8 67.3 T 6.3 174.3 T 6.5 172.3 T 4.9 141.4 T 8.5Posttest 86.0 T 12.2 78.0 T 6.9 66.3 T 3.5 172.3 T 9.3 170.9 T 12.2 137.3 T 7.0


APP

LIED

SCIENCES


both stronger and weaker groups (Fig. 2). Between-groupcomparisons of the force–velocity and force–power rela-tionships during jump squats revealed several differences(Fig. 3).

Jump mechanics. No significant differences existedbetween the groups in either the minimum or the maximumjoint angles during the 0% 1RM jump squat at baseline,midtest, or posttest (Table 4). Furthermore, training did not

FIGURE 4—Comparison of the power–time curve during the 0% 1RM jump squat among the stronger, weaker, and control groups at baseline (A),midtest (B), and posttest (C ). Normalized time represents the time from initiation of countermovement to takeoff in A, B, and C. ‡Significant(P e 0.05) difference between stronger and weaker groups. 6Significant (P e 0.05) difference between stronger and control groups. §Significant(P e 0.05) difference between control and both stronger and weaker groups.

FIGURE 5—Training-induced changes to the power–time (A), force–time (B), velocity–time (C), and displacement–time (D) curves for the 0% 1RMjump squat in the stronger group. Normalized time represents the time from initiation of countermovement to takeoff for power and force (A and B)and from initiation of countermovement to peak displacement for velocity and displacement (C and D). *Significant (P e 0.05) difference betweenbaseline and both midtest and posttest. xSignificant (P e 0.05) difference between baseline and posttest.


APPLIED

SCIEN

CES


result in any changes to the joint angles at either the tran-sition between eccentric and concentric phases or the take-off during the 0% 1RM jump squat (Table 4). Investigationof the power–time curve throughout the jump revealed sig-nificant differences between the groups (Fig. 4). At base-line, significant differences in power between stronger andweaker groups existed from 76.0% to 92.8% of normalizedtime (Fig. 4A). Similar differences between the strongerand weaker groups were maintained throughout midtest(80.4%–91.0% normalized time; Fig. 4B) as well as posttest(73.8%–90.2% normalized time; Fig. 4C). After training,significant differences were evident between the controlgroup and both the stronger and weaker training groupsthroughout the power–time curve at posttest (4.4%–23.4%,40.8%–52.8%, and 69.4%–89.2% normalized time). In addi-tion, differences existed between the control and strongergroups from 53.0% to 60.8% normalized time (Fig. 4C).Several significant training-induced changes were also ob-served between baseline and both midtesting and posttestingsessions (Figs. 5 and 6). For the stronger group, significantdifferences between baseline and both midtest and posttest

existed during the following phases: (a) power: 6.6%–22.8%,42.2%–55.6%, and 74.2%–91.2% of normalized time; (b)force: 9.6%–28.4% and 48.8%–62.8% of normalized time;(c) velocity: 39.6%–62.6% and 69.6%–86.8% of normal-ized time; and (d) displacement: 57.4%–89.4% of normal-ized time. Significant differences between baseline andposttest also existed during the following phases: (a) power:70.6%–74.0% of normalized time; (b) force: 28.6%–32.4%,43.0%–48.6%, and 63.0%–87.0% of normalized time; (c)velocity: 36.0%–39.4% of normalized time; and (d) dis-placement: 46.2%–57.2% and 89.6%–100.0% of normalizedtime (Fig. 5). For the weaker group, significant differencesbetween baseline and both midtest and posttest existed dur-ing the following phases: (a) power: 3.6%–11.4%, 35.4%–52.6%, and 64.8%–89.6% of normalized time; (b) force:0.0%–23.2% and 38.0%–70.0% of normalized time; (c)velocity: 9.4%–32.0%, 39.2%–68.0%, and 75.0%–91.8% ofnormalized time; and (d) displacement: 58.0%–88.2% ornormalized time. Significant differences between baseline andposttest also existed during the following phases: (a) power:11.6%–18.2% of normalized time, (b) force: 70.2%–79.2%

FIGURE 6—Training-induced changes to the power–time (A), force–time (B), velocity–time (C ), and displacement–time (D) curves for the 0% 1RMjump squat in the weaker group. Normalized time represents the time from initiation of countermovement to takeoff for power and force (A and B)and from initiation of countermovement to peak displacement for velocity and displacement (C and D). *Significant (P e 0.05) difference betweenbaseline and both midtest and posttest. xSignificant (P e 0.05) difference between baseline and posttest.


APP

LIED

SCIENCES


of normalized time, and (c) displacement: 88.4%–92.4% ofnormalized time (Fig. 6).

Neuromuscular characteristics. No between- orwithin-group differences existed for muscle thickness, pen-nation angle, or lean mass of the leg across each of thetesting occasions (Table 5). However, percent change inpennation angle from baseline to posttest was significant(9.9%, P = 0.05) for the weaker group and approaching sig-nificance (6.5%, P = 0.07) for the stronger group. At base-line, no between-group differences were evident in any ofthe EMG variables assessed (Table 5). After training, nosignificant between- or within-group differences existed inAvgIEMG during the isometric squat (i.e., MVC) or the 0%1RM jump squat. However, the stronger power traininggroup significantly increased the rate of rise in AvgIEMGduring the 0% 1RM jump squat at posttest in both VM andVL (Table 5). Furthermore, the rate of rise in AvgIEMG ofthe VM and VL in the stronger power training group wassignificantly greater than that in the control group at post-test. The weaker power training group also displayed asignificant increase in the rate of rise in AvgIEMG of theVM during the 0% 1RM jump squat at both midtest andposttest (Table 5). No within-group changes in any EMGvariables were observed for the control group throughoutthe study.

DISCUSSION

This investigation revealed that the ability to adapt toballistic power training is quite similar for both strong andweak individuals. Despite trends toward superior improve-ments in maximal power production and athletic perfor-mance in stronger individuals (supported by much greatermean ES, especially after 5 wk of training), the magnitude of

improvements did not significantly differ between strongerand weaker groups (Fig. 1 and Tables 2 and 3). Further-more, the mechanisms driving these adaptations were sim-ilar for both groups (Figs. 2–4 and Tables 4 and 5).

Athletic performance. The current results indicatethat the experimental training effectively improved athleticperformance in both stronger and weaker individuals. Jumpheight and maximal power output during a CMJ weresignificantly enhanced after both 5 and 10 wk of training(Fig. 1 and Table 2). In addition, a range of jump perfor-mance measures (i.e., net impulse, movement velocity,RFD, and RPD) also showed significant improvement inboth experimental groups. These observations are similar toprevious research involving ballistic power training on ho-mogeneous groups of subjects with relatively low (11,38) ormoderate strength levels (25,28). Comparisons of the mag-nitude of these improvements in jump performance betweenthe experimental groups showed no statistically significantdifferences (Fig. 1 and Table 2). However, ES analysesrevealed that practically relevant differences existed betweenstronger and weaker individuals in the magnitude of im-provements in jump performance after ballistic power train-ing. Any practical differences between the groups were morepronounced after 5 wk of training and diminished somewhatafter 10 wk of training. For example, after 5 wk of training,the ES of improvements in CMJ peak power and jumpheight were 1.60 and 1.59, respectively, for the strongergroup compared with 0.95 and 0.61 for the weaker group.At the completion of 10 wk of training, ES were 1.55 and1.46 for the stronger group and 1.03 and 0.95 for the weakergroup. Thus, despite the lack of statistically significantdifferences between the experimental groups, ballistic powertraining had a tendency to elicit a more pronounced effect onthe magnitude of improvements of the stronger group, and

TABLE 5. Neuromuscular factors assessed throughout the 10-wk training program.

Stronger Group Weaker Group Control Group

Baseline Midtest Posttest Baseline Midtest Posttest Baseline Posttest

Muscle architectureMuscle thickness (cm) 3.29 T 0.38 3.30 T 0.38 3.26 T 0.37 3.02 T 0.45 3.07 T 0.46 3.08 T 0.48 3.06 T 0.30 3.06 T 0.29Pennation angle (-) 19.80 T 2.23 20.76 T 2.15 21.03 T 2.14 19.20 T 1.92 20.15 T 1.59 20.98 T 1.71 20.44 T 3.99 20.59 T 4.17Lean mass of the leg (kg) 11.35 T 1.62 11.56 T 1.69 11.50 T 1.62 10.82 T 1.54 10.76 T 1.50 10.66 T 1.55 11.28 T 1.61 11.30 T 1.49

Muscle activationAvgIEMG IS (MVC)VM (KV) 81.9 T 32.8 64.4 T 24.6 68.0 T 30.5 69.0 T 28.9 82.3 T 44.7 81.7 T 28.2 79.6 T 17.1 78.5 T 18.2VL (KV) 69.5 T 22.7 66.6 T 12.8 58.8 T 26.9 73.3 T 39.0 72.6 T 42.8 71.2 T 28.3 71.3 T 11.9 70.0 T 23.1BF (KV) 13.9 T 5.9 14.2 T 11.9 13.8 T 6.7 18.6 T 14.2 14.3 T 9.4 14.8 T 10.2 20.3 T 11.1 20.9 T 12.0

AvgIEMG 0% 1RM JSVM (% MVCIsj1) 114.5 T 24.2 145.2 T 25.8 148.0 T 45.3 108.8 T 29.0 121.9 T 52.3 115.3 T 19.0 81.3 T 13.6 84.6 T 23.2VL (% MVCIsj1) 115.2 T 28.3 122.3 T 20.0 135.0 T 28.5 118.8 T 46.8 130.1 T 59.3 106.0 T 25.7 107.1 T 30.0 109.7 T 28.5BF (% MVCIsj1) 125.6 T 21.8 195.5 T 102.0 200.9 T 112.5 100.8 T 58.8 181.7 T 73.8 174.4 T 90.3 103.0 T 53.2 104.9 T 76.8

Rate of EMG rise 0% 1RM JSVM (% MVCIsj1) 503.5 T 148.9 903.7 T 380.9 1057.3 T 492.0*,§ 352.3 T 174.4 725.1 T 305.9* 672.5 T 263.8* 373.0 T 209.7 386.7 T 227.4VL (% MVCIsj1) 479.2 T 210.2 634.1 T 188.6 971.4 T 338.6*,§ 389.6 T 187.4 866.0 T 672.0 654.1 T 283.0 403.6 T 209.5 392.9 T 146.8BF (% MVCIsj1) 922.6 T 413.9 1893.5 T 1170.1 1759.3 T 941.1 714.5 T 551.9 1180.0 T 619.0 1359.0 T 984.6 449.7 T 189.1 477.2 T 250.8

AvgIEMG, average integrated EMG; IS, isometric squat; JS, jump squat.Average integrated EMG (AvgIEMG) was assessed during an isometric squat MVC and during the 0% 1RM jump squat. Rate of rise in AvgIEMG was also assessed during the 0% 1RMjump squat. Muscle thickness and pennation angle were assessed through ultrasound images. Lean mass of the leg represents the average lean muscle mass of right and left legs asassessed through dual-energy x-ray absorptiometry scans.* Significant (P e 0.05) change from baseline.§ Significantly (P e 0.05) different from control group.


APPLIED

SCIEN

CES


this could hold great practical relevance. Furthermore, theseresults suggest that stronger subjects also display a tendencytoward more rapid improvements in performance after bal-listic power training than weaker individuals, which is alsoof considerable practical importance.

Ballistic power training also resulted in enhanced sprintperformance for both stronger and weaker groups (Table 3).The change in time between baseline and posttest was sig-nificant for both groups at 20, 30, and 40 m during a 40-msprint. In addition, the stronger group also displayed a sig-nificant reduction in time at the 5- and 10-m marks. Theimprovements in sprint performance for both groups wereof a practically relevant magnitude, representing a 7.3% and2.2% improvement in 5- and 40-m time (ES = 0.86 and0.54) in the stronger group and a 6.8% and 3.7% im-provement in 5- and 40-m time (ES = 0.79 and 0.82) in theweaker group. Although the additional significant improve-ments of the stronger group (i.e., at 5 and 10 m) may in-dicate a greater adaptability to the training stimulus, therewere no significant differences in the magnitude of per-formance enhancements between the stronger and weakergroups. Furthermore, the mean ES of the improvements insprint performance were quite similar between the experi-mental groups (Table 3). Previous research has reportedimprovements in sprint performance approaching signifi-cance after ballistic power training (38), but such trendshave not been consistently observed (25,37). Thus, the cur-rent findings are of great importance because this studyprovides the strongest evidence to date that sprint perfor-mance can be improved by ballistic power training in theform of vertical jumping. The successful transfer of jumpsquat training to sprint performance in the current study istheorized to be associated with the efficacy of the programdesign (i.e., highly sports-specific movements, frequency oftraining, effective load, repetition, set, and interset recoveryparameters).

The ability of the current study to elucidate whether themagnitude of performance improvements after ballisticpower training is influenced by strength level was limitedby several factors. First, the principle of diminished returnsdictates that initial improvements in muscular function areeasily invoked and further improvements are progressivelyharder to achieve (37). Thus, the training programs of in-dividuals with significantly greater strength levels (and moreexperienced training backgrounds) typically need to containadded variability, compared with weaker, inexperienced in-dividuals (27,37). However, the nature of the research ques-tions addressed by this study meant that the stronger andweaker groups completed the same training program duringa period of 10 wk. Although the training program was de-signed to contain a great deal of specificity to common athleticmovements, the experimental training lacked the level ofvariability required to maximize improvements in experiencedathletes. Consequently, the ability of this intervention tomaximize the adaptations of the stronger groupmay have beenaffected. Second, the current observations may have been

confounded by the stronger groups’ cessation of strengthtraining for the duration of the study (i.e., total of 15 wk). Toexamine the specific mechanisms driving adaptation to theballistic power training intervention, all subjects were in-structed to refrain from any lower body resistance training(or sprint training) outside the scope of the current study.Although this posed no impact on the strength level of theweaker group (i.e., not involved in any such training beforecommencing the experiment), the stronger group displayed astatistically nonsignificant but practically relevant decreasein maximal strength (4.6% decrease in 1RM/BM; ES = 0.91).The neuromuscular changes associated with detraining peri-ods of a similar time course (i.e., decreased neural drive andCSA [15]) are theorized to negatively impact the ability ofthe stronger group to adapt to the experimental training.Thus, the cessation of strength training is theorized to havenegatively affected the ability of the stronger group to adaptto the ballistic power training. It is imperative, therefore, thatfuture research is conducted that incorporates strengthmaintenance sessions during the training intervention to elu-cidate further the influence of strength on the magnitudeof improvements in athletic performance. Cognizant ofthese limitations, the fact that the stronger group showedsimilar performance improvements as the weaker group tothe ballistic power training intervention is of great practicalimportance.

Mechanismsresponsible for improvedperformance.Monitoring the training-induced changes to the force–velocityand force–power relationships during a sports-specific move-ment such as the jump squat offers some indication of themechanisms driving adaptations in the stronger and weakergroups. The ability of such an applied in vivo measure ofthe force–velocity relationship to delineate exact changes tomuscle mechanics after training is complicated by a range offactors including mixed fiber composition, muscle architec-tural characteristics, anatomical joint configuration, levels ofneural activation, as well as the complex nature of the jumpsquat movement (23). Despite these limitations, examina-tion of the force–velocity relationship in sports-specificmovements quantifies the ability of the intact neuromus-cular system to function under various loading conditions, in-formation essential to understanding muscular functionduring dynamic athletic movements. Data from this investi-gation revealed that strength level did somewhat influencethe training-induced changes to the jump squat force–velocityand force–power relationships (Figs. 2 and 3). Specifically,the stronger group showed a more velocity-specific responseto the training stimulus, displaying the greatest improvementsunder the lightest loading conditions (i.e., high-velocity, low-force portion of the force–velocity relationship). In contrast,ballistic power training with 0%–30% 1RM resulted in im-provements in velocity and power throughout a range ofloading conditions for the weaker subjects (i.e., similar im-provements at both high-velocity and high-force portions ofthe force–velocity relationship). These changes caused theslope of the force–velocity relationship to increase in the


APP

LIED

SCIENCES


stronger group (aided by a slight decrease in Fmax) but not inthe weaker group. Thus, the current data support the theoryof velocity specificity (13,19) and the notion that relativelyweak or inexperienced subjects display relatively nonspecificadaptations to training when compared with stronger, moreexperienced athletes. Although the stronger group seemedto display a more velocity-specific response, these apparentdissimilarities did not translate into major changes in the sig-nificant between-group differences that were evident beforetraining (Fig. 3). However, a significant between-groupdifference in force at peak power during the lightest loadexamined (i.e., 0% 1RM) that did not exist at baseline wasobserved at midtest and at posttest. In addition, between-group differences in power at 20% and 40% 1RM loads thatexisted before training were no longer present at posttest.These training-induced changes to the significant differ-ences between stronger and weaker groups also indicate thatstronger individuals displayed adaptations with greatervelocity specificity.

Another potential mechanism driving the observed im-provements in athletic performance was changes to move-ment mechanics during jumping. Although no significantchanges were observed in the minimum and maximum hip,knee, and ankle angles during the 0% 1RM jump squat(Table 4), significant training-induced changes in displace-ment, velocity, force, and power were evident throughoutthe 0% 1RM jump squat for both groups (Figs. 4–6). Thesechanges are theorized to have led to an optimization ofstretch-shorten cycle (SSC) function, which contributed tothe enhanced jump performance. Arguably, the primarymechanism driving the enhancement of performance duringSSC movements is the attainment of a greater level of forceat the beginning of the concentric phase in comparison toconcentric-only movements (5). Both stronger and weakergroups displayed significant increases in force during theeccentric phase and early in the concentric phase of the 0%1RM jump squat after training (Figs. 5 and 6). This increasewas generated by the improved acceleration of the BMduring the eccentric phase and, similar to comparisons be-tween SSC and concentric-only movements, resulted ingreater net impulse, velocity of movement, power output,and, ultimately, enhanced jump height after training. Theobserved changes are hypothesized to be due to the slightmodifications in jumping mechanics (i.e., a marginallyshorter but faster countermovement) and a significant de-crease in time to takeoff (Tables 3 and 4). Very little pre-vious research exists examining the impact of training onperformance variables throughout the entire movementhowever, similar results have been observed after ananalogous ballistic power training intervention involvingrelatively untrained men (10). Comparisons between thestronger and the weaker groups revealed no new significantbetween-group differences after training in joint angles ofthe hip, knee, and ankle (Table 4) or power outputthroughout the 0% 1RM jump squat (Fig. 4). Thus, changesto jump mechanics common to both stronger and weaker

individuals after ballistic power training involving jumpsquats are theorized to have contributed to improvements injump performance.

This investigation revealed that stronger and weakersubjects displayed similar adaptations in muscle architectureafter ballistic power training (Table 5). As expected, thetraining intervention did not elicit any significant changes tomuscle thickness (indicative of whole-muscle CSA [21]) orthe lean mass of the leg for either experimental group. Therelatively light loads used during ballistic power training(i.e., 0%–30% 1RM) were too small to elicit the necessarymechanical stimulus required to initiate a significant hyper-trophic response (16,17). In addition, ballistic power train-ing did not elicit any significant differences in the pennationangle of either the stronger or the weaker group. Changes inpennation angle have been reported previously, withincreases observed after heavy resistance training (4,21),although not consistently (3), and decreases in response tosprint training (4). These changes are believed to have apositive impact on the force- and velocity-generating ca-pacity of muscle, respectively. However, the potential im-pact of ballistic power training on changes to pennationangle has not been examined previously. On the basis ofthe current data, ballistic power training did not promptstructural changes to the muscle (i.e., muscle thickness orpennation angle). Furthermore, the initial strength level ofthe subject did not impact the type of adaptations in musclearchitecture. Muscular adaptations at an intracellular level(i.e., alterations to anaerobic and aerobic enzymes, musclesubstrates, and/or protein expression) and/or connective tis-sue remodeling may have contributed to the observed per-formance improvements (14). However, the potential forsuch adaptations cannot be established or rejected becausethese mechanisms were not assessed in the current study.

Neural adaptations in response to ballistic power train-ing were observed, with significant changes evident in theneural activation patterns of subjects regardless of their ini-tial strength level. Changes in EMG (indicative of alter-ations in motor unit recruitment, firing frequency, and/orsynchronization) have been previously reported with im-provements in performance after ballistic power training(16,25,36). However, this is one of the first experiments toshow training-induced changes in EMG during complex,multijoint, sports-specific movements. The current data in-dicate that ballistic power training resulted in significantincreases in the rate of EMG rise during dynamic athleticperformance (i.e., 0% 1RM jump squat) in both strongerand weaker groups. Thus, it is theorized that the ballisticpower training enhanced intermuscular coordination by op-timizing the magnitude and timing of muscle activation.Similarly, Hakkinen et al. (16) observed ballistic powertraining (jump squats with 0%–60% 1RM) to result in a38% increase in the rate of EMG rise during an isometricknee extension, which was reported to contribute to im-proved performance (a 24% improvement in isometricRFD). Although the current study cannot delineate whether


APPLIED

SCIEN

CES


the observed changes in EMG were brought about throughalterations in motor unit recruitment, firing frequency, and/orsynchronization, previous research involving intramus-cular EMG may offer some insight. During ballistic con-tractions, motor units have been reported to begin firing atvery high frequencies (even in excess of those required toachieve maximal force) followed by a rapid decline (40).The high initial firing frequency is believed to result in in-creased RFD, even if only maintained for a very shortperiod (26). van Cutsem et al. (36) reported the peak firingfrequency at the onset of ballistic contraction to increaseafter ballistic power training. Furthermore, these higherfiring frequencies were maintained for longer throughoutthe contraction after training. In addition, a training-inducedincrease in the percentage of doublet discharges (i.e., amotor unit firing two consecutive discharges in a e5-msinterval) at the onset of a ballistic contraction was alsoreported (5.2% of motor units displayed doublet dischargesbefore training, and this increased to 37.2% after 12 wk ofballistic power training). These training-induced changeswere reported to contribute to an 82.3% increase in the RFDand a 15.9% improvement in time to peak force during bal-listic contractions (36). Therefore, the ballistic power train-ing intervention of the current study may have inducedadaptations to the pattern of motor unit firing frequency thatsubsequently enhanced RFD capabilities and contributed toenhanced athletic performance. It is important to note thatthese mechanisms of adaptation as well as the performanceimprovements observed are specific to the movement patternand loading parameters used in the current study as powerproduced by muscle varies according to both the nature ofthe movement and the loading parameters used (12).

In conclusion, the ballistic power training program usedwas very effective at enhancing athletic performance, withboth groups showing significant improvements in maximalpower, jump height, movement velocity, and sprint perfor-mance. The magnitude of improvements in athletic perfor-mance after ballistic power training did not significantlydiffer between strong and weak subjects. However, ES anal-yses revealed that the training had a tendency toward pro-ducing a more pronounced effect on jump performance inthe stronger group (especially after only 5 wk of training)that is believed to hold great practical relevance. Further-

more, this conclusion is strengthened by the confoundinginfluences of the principle of diminished returns as well asthe stronger groups’ cessation of strength training. Therefore,because stronger individuals display superior performancebefore ballistic power training and have a tendency forgreater improvements after such training, it would be ad-vantageous for individuals to establish a solid foundation ofstrength before focusing on ballistic power training. Themechanisms driving the performance improvements werevery similar for both the stronger and the weaker groups.Ballistic power training involving sports-specific movementsincreased the rate of EMG rise during jumping, which,coupled with slight technique modifications believed to bespecific to this training stimulus, led to an improvement inSSC function. As a result, subjects were able to achievegreater force and more optimally timed force applicationresulting in higher acceleration and movement velocity inshorter periods. Thus, athletic performance was improvedthrough enhanced magnitude and RFD, translating to highervelocity and power production capabilities. Not only dothese findings provide a deeper understanding of the mech-anistic factors driving improvements in performance afterballistic power training but they also reveal that the neuro-muscular and biomechanical adaptations to such trainingare not influenced by strength level. These findings haveseveral implications for the design of ballistic power trainingprograms that effectively improve athletic performance. Theuse of ballistic jump squats with very light loads (i.e., 0%–30% 1RM) is sufficient to induce significant improvementsin jump and sprint performance of both strong and weakathletes. However, ballistic power training programs ofstronger athletes need to contain considerably more variabil-ity than programs of weaker athletes for continued perfor-mance improvements beyond 5 wk of training. Finally, theincorporation of strength maintenance sessions throughouta power training phase is vital because decrements in max-imal strength (and the ensuing neuromuscular alterations [15])are theorized to negatively affect the ability of stronger ath-letes to adapt to ballistic power training.

Funding from the National Strength and Conditioning Associationwas received for this work.

Results of the present study do not constitute endorsement bythe American College of Sports Medicine.

REFERENCES

1. Baker D, Nance S. The relation between strength and power inprofessional rugby league players. J Strength Cond Res. 1999;13(3):224–9.

2. Baker DG, Newton RU. Comparison of lower body strength,power, acceleration, speed, agility, and sprint momentum to de-scribe and compare playing rank among professional rugby leagueplayers. J Strength Cond Res. 2008;22(1):153–8.

3. Blazevich AJ, Cannavan D, Coleman DR, Horne S. Influence ofconcentric and eccentric resistance training on architectural adap-tation in human quadriceps muscles. J Appl Physiol. 2007;103(5):1565–75.

4. Blazevich AJ, Gill ND, Bronks R, Newton RU. Training-specificmuscle architecture adaptation after 5-wk training in athletes. MedSci Sports Exerc. 2003;35(12):2013–22.

5. Bobbert MF, Gerritsen KG, Litjens MC, Van Soest AJ. Why iscountermovement jump height greater than squat jump height?Med Sci Sports Exerc. 1996;28(11):1402–12.

6. Bobbert MF, Van Soest AJ. Effects of muscle strengthening onvertical jump height: a simulation study. Med Sci Sports Exerc.1994;26(8):1012–20.

7. Bourque PJ. Determinant of load at peak power during maximaleffort squat jumps in endurance and power trained athletes


APP

LIED

SCIENCES


[dissertation]. Fredericton (Canada): University of New Brunswick;2003. p. 44–60.

8. Cohen J. The concepts of power analysis. In: Cohen J, editor.Statistical Power Analysis for the Behavioral Sciences. Hillsdale(NJ): Lawrence Erlbaum Associates; 1988. p. 1–18.

9. Cormie P, McBride JM, McCaulley GO. Validation of powermeasurement techniques in dynamic lower body resistance exer-cises. J Appl Biomech. 2007;23(2):103–18.

10. Cormie P, McBride JM, McCaulley GO. Power–time, force–time,and velocity–time curve analysis of the countermovement jump:impact of training. J Strength Cond Res. 2009;23(1):177–86.

11. Cormie P, McCaulley GO, McBride JM. Power versus strength–power jump squat training: influence on the load–power relation-ship. Med Sci Sports Exerc. 2007;39(6):996–1003.

12. Cormie P, McCaulley GO, Triplett NT, McBride JM. Optimalloading for maximal power output during lower-body resistanceexercises. Med Sci Sports Exerc. 2007;39(2):340–9.

13. Coyle EF, Feiring DC, Rotkis TC, et al. Specificity of power im-provements through slow and fast isokinetic training. J ApplPhysiol. 1981;51(6):1437–42.

14. Folland JP, Williams AG. The adaptations to strength training:morphological and neurological contributions to increased strength.Sports Med. 2007;37(2):145–68.

15. Hakkinen K, Alen M, Komi PV. Changes in isometric force- andrelaxation-time, electromyographic and muscle fibre characteristicsof human skeletal muscle during strength training and detraining.Acta Physiol Scand. 1985;125(4):573–85.

16. Hakkinen K, Komi PV, Alen M. Effect of explosive type strengthtraining on isometric force- and relaxation-time, electromyographicand muscle fibre characteristics of leg extensor muscles. ActaPhysiol Scand. 1985;125(4):587–600.

17. Hakkinen K, Komi PV, Alen M, Kauhanen H. EMG, muscle fibreand force production characteristics during a 1 year training periodin elite weight-lifters. Eur J Appl Physiol. 1987;56:419–27.

18. Hakkinen K, Komi PV, Tesch PA. Effect of combined concentricand eccentric strength training and detraining on force-tme, mus-cle fibre and metabolic characteristics of leg extensor muscles.Scand J Sports Sci. 1981;3:50–8.

19. Kaneko M, Fuchimoto T, Toji H, Suei K. Training effect of dif-ferent loads on the force–velocity relationship and mechanical poweroutput in human muscle. Scand J Med Sci Sports. 1983;5(2):50–5.

20. Kawakami Y, Abe T, Kanehisa H, Fukunaga T. Human skeletalmuscle size and architecture: variability and interdependence.Am J Hum Biol. 2006;18(6):845–8.

21. Kawakami Y, Abe T, Kuno SY, Fukunaga T. Training-inducedchanges in muscle architecture and specific tension. Eur J ApplPhysiol. 1995;72:566–73.

22. MacDougall JD, Elder GCB, Sale DG, Moroz JR, Sutton JR.Effects of strength training and immobilization on human musclefibers. Eur J Appl Physiol. 1980;43:25–34.

23. MacIntosh BR, Holash RJ. Power output and force-velocityproperties of muscle. In: Nigg BM, MacIntosh BR, Mester J,editors. Biomechanics and Biology of Movement. Champaign (IL):Human Kinetics; 2000. p. 193–210.

24. McBride JM, Triplett-McBride NT, Davie A, Newton RU. Acomparison of strength and power characteristics between powerlifters, Olympic lifters, and sprinters. J Strength Cond Res. 1999;13(1):58–66.

25. McBride JM, Triplett-McBride T, Davie A, Newton RU. Theeffect of heavy- vs. light-load jump squats on the development ofstrength, power, and speed. J Strength Cond Res. 2002;16(1):75–82.

26. Miller RG, Mirka A, Maxfield M. Rate of tension developmentin isometric contractions of a human hand muscle. Exp Neurol.1981;72:267–85.

27. Newton RU, Kraemer WJ. Developing explosive muscular power:implications for a mixed method training strategy. Strength CondJ. 1994;16(5):20–31.

28. Newton RU, Kraemer WJ, Hakkinen K. Effects of ballistictraining on preseason preparation of elite volleyball players. MedSci Sports Exerc. 1999;31(2):323–30.

29. Newton RU, Kraemer WJ, Hakkinen K, Humphries B, MurphyAJ. Kinematics, kinetics, and muscle activation during explosiveupper body movements. J Appl Biomech. 1996;12:31–43.

30. Nuzzo JL, McBride JM, Cormie P, McCaulley GO. Relationshipbetween countermovement jump performance and multijoint iso-metric and dynamic tests of strength. J Strength Cond Res. 2008;22(3):699–707.

31. Pandy MG. An analytical framework for quantifying muscularaction during human movement. In: Winters JM, Woo SLY,editors. Multiple Muscle Systems Biomechanics and MovementOrganisation. New York (NY): Springer Verlag; 1990. p. 653–62.

32. Paulus DC, Reiser RF, Troxell WO. Pneumatic strength assessmentdevice: design and isometric measurement. Biomed Sci Instrum.2004;40:277–82.

33. Sale DG. Neural adaptations to strength training. In: Komi PV,editor. Strength and Power in Sport. Oxford (UK): BlackwellScience; 2003. p. 281–313.

34. Stone MH, O’Bryant HS, McCoy L, Coglianese R, Lehmkuhl M,Schilling B. Power and maximum strength relationships duringperformance of dynamic and static weighted jumps. J StrengthCond Res. 2003;17(1):140–7.

35. Ugrinowitsch C, Tricoli V, Rodacki AL, Batista M, Ricard MD.Influence of training background on jumping height. J StrengthCond Res. 2007;21(3):848–52.

36. van Cutsem M, Duchateau J, Hainaut K. Changes in single motorunit behaviour contribute to the increase in contraction speedafter dynamic training in humans. J Physiol. 1998;513(pt 1):295–305.

37. Wilson G, Murphy AJ, Walshe AD. Performance benefits fromweight and plyometric training: effects of initial strength level.Coach Sport Sci J. 1997;2(1):3–8.

38. Wilson GJ, Newton RU, Murphy AJ, Humphries BJ. The optimaltraining load for the development of dynamic athletic perfor-mance. Med Sci Sports Exerc. 1993;25(11):1279–86.

39. Zatsiorsky VM. Science and Practice of Strength Training.Champaign (IL): Human Kinetics; 1995. p. 47–65.

40. Zehr EP, Sale DG. Ballistic movement: motor control and muscleactivation. Can J Appl Physiol. 1994;19:363–78.


APPLIED

SCIEN

CES


power training

Documents

peak power

ballistic power training

evaluation of jump

jump performance effect

influence of strength

stronger n

weaker groups

weaker n