Influence of Strength on Magnitude and …...listic power training. In particular, the influence of...

Influence of Strength on Magnitude andMechanisms of Adaptation to Power Training

PRUE CORMIE', MICHAEL R. MCGUIGAN2'3 , and ROBERT U. NEWTON'

'School of Exercise, Biomedical and Health Sciences, Edith Cowan University, Perth, AUSTRALIA,; 2New Zealand Academy

of 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 Exere., 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 (IRM/BM) - 1.97 + 0.08), weaker (n = 8, IRM/BM - 1.32 + 0.14), or control (n - 8, 1RM/BM - 1.37 +

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 000 30% IRM. 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 < 0.05) improvements in jump

(stronger: peak power - 10.0 + 5.2 W-kg 1, jump height - 0.07 ± 0.04 m; weaker: peak power 9.1 + 2.3 W-kg _', jump height -

0.06 + 0.04 m) and sprint performance after training (stronger: 40-m time 2.2% + 2

.0%; weaker: 40-m time 3.6%, + 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 < 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

fter strength training, the magnitude of improve-ments in strength and the mechanisms driving

those 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 phase

muscular adaptations also occur), whereas further increases

in 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

Address for correspondence: Prue Connie, Ph.D., School of Exercise,

Biomedical and Health Sciences, Edith Cowan University, 270 Joondalup

Dr. 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

high level of strength require a much more sophisticated de-sign (i.e., greater specificity and variation) (17,33). Although

the factors contributing to maximal strength and the appli-

cation of strength training are well understood, much less is

known concerning the adaptations to and utilization of bal-

listic power training. In particular, the influence of strength

level on both the magnitude of improvement and the mech-

anisms driving adaptation after ballistic power training is not

known. Such knowledge is vital to the development of train-

ing programs that most effectively improve maximal power

production and athletic performance in athletes with a wide

variety of training backgrounds.

Cross-sectional comparisons have revealed that individ-

uals with higher strength levels have markedly superior

power production capabilities than those with a low level of

strength (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 have

been reported in comparisons of well-trained athletes and

relatively untrained controls (10,24,35), athletes competi-

tive in power-type sports (i.e., volleyball) and endurance

events (7), rugby league players involved in national versus

1566

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 forcevelocity 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 + 0.15 and 2.88 + 0.14, respectively), weightliftersgenerated significantly greater power output during an un-loaded countermovement jump (CMJ; weightlifters = 63.0 +

2.7 W-kg , powerlifters = 56.9 + 2.5 W'kg '). 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 I RM/BM - 1.99 ± 0.30) andweaker men (n - 5; squat I RM/BM = 1.21 + 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 = -0.13, n = 14) (37). Therefore, this previousstudy offers little insight into the influence of strength on themagnitude of improvements in athletic perfornance 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 (3 1), 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 internuscular 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 perforinance 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 POWERMedicine & Science in Sports & Exercise,,, 1567

improvements after ballistic power training. Therefore, the ration of the stn

purpose of this experiment was to determine whether the were notified

magnitude of performance improvements and the mecha- their written in.

nisms driving adaptation to ballistic power training differ the university's

between strong and weak individuals. Training pithree sessions

METHODS Each training sof two sets of

Experimental design. This study used a randomized, (i.e., CMJ with

control design and was conducted during a period of 15 wk. by a carbon fSubjects were divided into two strata on the basis of their shoulders). Du

1RM/BM: stronger (1RMiBM > 1.55) or weaker (IRM/BM < performed sev

1.55). Subjects in the stronger stratum were allocated into separated by a

the stronger group. Subjects in the weaker stratum were ran- at the load thatdomized into one of two groups: weaker group or control determined du

group. The stronger and weaker groups completed a total of previous resear

10 wk of ballistic power training, while the control group (i.e., no extem

maintained their normal level of activity throughout the for each of the

duration of the study. Training involved three sessions per training sessiolweek in which subjects performed maximal-effort jump up set consistin

squats with 0 -/o-30% IRM. Subjects completed a 2-d testing IRM. Subjectsbattery before initiating training (baseline), after 5 wk of effort jump sq

training (midtest-stronger and weaker groups only), and recovery. Subj

after the completion of 10 wk of training (posttest). Subjects as rapidly as

were adequately familiarized to all testing procedures before session so that

actual assessment. Testing involved evaluation of jump and during jumps

sprint performance as well as measures of the force-velocity output at that

relationship, jumping mechanics, muscle architecture, and session. Previ

neural drive. mance improv.

Subjects. Subjects were recruited on the basis of their programmingability to perform a back squat with proficient technique. A IRM (25,38).

total of 35 men fulfilled all the testing and training re- body resistanc

quirements of this investigation. Data from 11 of these men throughout the

were removed on the basis of their 1RM/BM to establish Testing pr

two experimental groups with very distinct differences in their previousmaximal strength (i.e., data from subjects with a 1RM/BM sion and for 7

between 1.55 and 1.85 were not included). The remaining the posttest te

24 men were allocated into three groups: stronger group completed tw(

(n = 8, 1RM/BM = 1.97 + 0.08), weaker group (n = 8, recovery at th

IRM/BM = 1.32 + 0.14), or control group (n = 8, IRM/BM Testing sessic

= 1.37 + 0.13). Subjects' characteristics throughout the du- the vastus la

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

BM (kg) Body Fat (%) 1RM/BM

Stronger groupBaseline 79.1 ± 12.8 12.2 ± 3.9t 1.97 + 0.08tMidtest 79.8 + 12.9 12.3 ± 3.8t 1.93 ± 0.11 t

Posttest 79.6 ± 13.0 12.6 -± 3.5t 1.88 + 0.11 tWeaker group

Baseline 79.9 + 14.5 17.3 ± 3.8 1.32 0.14

Midtest 79.2 + 14.2 17.5 ± 4.0 1.38+ 0.16Posttest 79.1 ± 13.8 17.7 ± 3.6 1.39 + 0.17

Control groupBaseline 77.5 ± 8.1 14.6 ± 3.5 1.37 + 0.13Posttest 78.4 ± 8.8 14.7+ 3.8 1.35 + 0.12

BM, body mass; 1RM:BM, squat one repetition maximum to body mass ratio.* Significantly (P < 0.05) different from baseline.

t Significant (P< 0.05) difference between stronger group and all other groups.t Significant (P_s 0.05) difference between stronger group and weaker group.§ Significantly (P_< 0.05) different from control group.

Tdy are outlined in Table 1. The participantsabout the potential risks involved and gaveformed consent. This study was approved by

human research ethics committee.rogram. The training program consisted ofper week separated by at least 24 h of rest.ession was initiated via a warm-up consistingsix submaximal jump squats with 0% 1 RMno external load, just the resistance applied

iber pole (mass - 0.4 kg) held across thering sessions 1 and 3 of each week, subjectsen sets of six maximal-effort jump squats3-min recovery. Jump squats were performedmaximized power output for each subject, asring the baseline testing session. Similar toch, a load consistent with the subject's BM

al load or 0% 1 RM) maximized power outputparticipants in this study (11,12). The secondn of each week included an additional warm-ig of five submaximal jump squats with 30%

then performed five sets of five maximal-[uats with 30% IRM separated by a 3-minects were encouraged to perform each jumppossible. Intensity was modified for eachan audible beep could be heard by subjectsthat reached 95% of the maximal powerload from their previous training or testingous literature has shown significant perfor-ements after jump squat training with similarparameters at both 0% 1iRM (11) and 30%

Subjects refrained from any additional lowerce training, plyometrics, or sprint training

course of the study.otocol. Subjects rested for 3-5 d betweentraining session and the midtest testing ses--10 d between the final training session and

sting session to allow full recovery. Subjectstesting sessions separated by at least 3 d of

te baseline, midtest, and posttest occasions.)n 1 was initiated with an examination ofateralis (VL) muscle architecture through

40-m Sprint (s)

5.41 ± 0.25t5.39 + 0.20f5.29 ± 0.20t

5.90 + 0.275.79 +0.205.69+ 0,22

Jump Height (m)

0.43 ± 0.03f0.50 ± 0.03*0.50 ± 0.03*§

0.38 ± 0.040.42 ± 0.070.44 ± 0.06

5.79 + 019 0.41 ± 0.045.79 ± 0.12 0.40 ± 0.04

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

ultrasonography. Maximal dynamic strength was then as-sessed using a back squat I RM 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 IRM.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 IRM involved subjects completing a series ofwarm-up sets (four to six repetitions at 30% estimatedI RM, three to four repetitions at 50% estimated IRM, twoto three repetitions at 70% estimated IRM, and one to tworepetitions at 90% estimated I RM), 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)<90' of flexion were considered successful. This depth wasvisually monitored during testing and was confirmed bytwo-dimensional motion analysis (stronger group: baseline= 85.70 + 4.2', posttest = 82.50 + 6.80; weaker group:baseline = 83.4' + 2.40, posttest = 80.4' + 4.30; controlgroup: baseline - 83.10 + 6.2', posttest = 82.6' + 5.40; raterreliability: r 0.95). Although no significant differences inthe depth of the squat 1 RM existed among the testing occa-sions, there is the potential that the slightly deeper depthobtained during posttesting could have influenced the IRMvalues 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 1400 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% 1 RM 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 > 0.90.

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

INFLUENCE OF STRENGTH ON POWER Medicine & Science in Sports & Exercise, 1569

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 (Hz)

1 second

no. samples in original signal 1 seconds

no. samples in normalized signal x per sample

Consequently, the sampling frequency of the modified

signals was then equivalent to 815 ±- 154 Hz for the power

time and force-time curves and 538 ±- 68 Hz for the velocity

time and displacement-time curves. This resampling allowed

for 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-from

0% to 100%0-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-time

curves 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 > 0.94,r > 0.90, r > 0.89, and r > 0.92, respectively, using this

methodology (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 were

performed 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-m

gates. Intraclass test-retest reliability for all sprint perfor-mance variables examined was consistently r > 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/AgCI 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 to

the 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, and

cleansed 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 < 5 kil; Model 12D-16-OSNeurodata Amplifier System; Grass Technologies, WestWarwick, RI). The amplified myoclectric 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

1570 Official Journal of the American College of Sports Medicine http://www.acsm-msse.org

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 I-s period of sustainedmaximal force output after the initial peak in the force curveduring the isometric squat (intraclass test-retest reliabilityconsistently r > 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 (AvglEMG) 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% 1 RM 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 > 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 I RM and jump squats.A digital video camera (25 Hz; MV830i; Canon AustraliaPty Ltd., North Ryde, NSW, Australia) was positioned 3.1 nifrom 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 I RM, representing the joint angles at the transitionbetween the eccentric and concentric phases. Intraclass testretest reliability and rater reliability for all joint angles as-sessed using motion analysis were consistently r > 0.92 andr > 0.93, respectively.

A MID-TEST

16

14

Zi 12

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0 4.

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B POST-TEST

16 1

14

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ff Te

Peak Power Average Power Jump Height

NMI

2-

E-,

OtM0,

3=

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0 F0.00Peak Power Average Power Jurmp Height

FIGURE I-Change from baseline in peak power, average power, andmaximum jump height during the 0% IRM jump squat at midtest (A)and posttest (B). *Significant (P < 0.05) change from baseline.

INFLUENCE OF STRENGTH ON POWER Medicine & Science in Sports & Exercise,, 1571

INFLUENCE OF STRENGTH ON POWERMedicine & Science in Sports & Exercise ... 1571

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

Stronger Group

0% IRM Jump Squat A at Midtest

PowerPeak power (W.kg-1) 10.3 + 5.1*

Average power (W.kg-') 8.6 ± 4.8*

Rate of power development (W.kg-l-s-1) 343.0 ± 164.4*4Force

Peak force (N-kg-') 6.3 + 5.4*

RFD (N.kg-'-s-') 76.8 + 46.1*

Force at peak power (N.kg ') 2.8 ± 2.1*

Net impulse (N-s) 38.9 ± 22.6*Velocity

Peak velocity (m.s-1) 0.43 ± 0.29*Velocity at peak power (m-s 1) 0.19 ± 0.34

Velocity at takeoff (m-s-1) 0.43 ± 0.29*Acceleration

Peak acceleration (ms- s 1) 11.7 + 6.2"$Displacement

Peak displacement (m) 0.07 + 0.03*Time

Time to takeoff (s) 0.18 ± 0.07*

Significant (P < 0.05) change from baseline.

4 Significant (P< 0.05) difference between stronger group and weaker group.

A• at Posttest

10.0 + 5.2*9.6 ± 5.2*

325.2 ± 132.6*

6.3 ± 4.7*82.3 ± 58.3*

2.1 ± 1.6*45.9 ± 28.9*

0.42 + 0.22*0.22 + 0.310.41 + 0.24*

14.6 + 6.5*4t

0.07 + 0.04*

-0.19 ± 0.6*

Statistical analyses. A general linear model withrepeated-measures ANOVA followed by Bonferroni post

hoc tests was used to examine the impact of training onperformance variables and to determine whether differencesexisted between the groups at baseline, at midtest, and at

posttest. t-tests were used for comparisons between var-

iables at baseline and posttest for the control group as well

as for comparison of variables between stronger and weaker

groups at midtest. Statistical significance for all analyseswas defined by P < 0.05, and results were summarized as

means + SD. Estimated effect sizes (ES) of 112 = 0.502 and-q2 = 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 _2 = 0.406 and q 2 = 0.319 at the observed

power levels of 0.821 and 0.644 existed for the compari-

son of maximum peak power relative to BM between the

stronger and weaker groups at baseline and posttest,

respectively. Mean ES were also calculated to examine

and compare the practical significance of the performance

improvements among the experimental groups. Based on the

study of Cohen (8), which suggests ES of 0.2, 0.5, and 0.8 to

represent small, moderate, and large effects, respectively,

practical relevance was defined as an ES > 0.8 for the

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

5m 1om 20 m

Sprint Times (s)30 m

Stronger groupBaseline 1.10 ± 0.10 1.82 ± 0.12 3.08 + 0.154 4.25 ± 0.1

Midtest 1.08 ± 0.024: 1.82 ± 0.044 3.07 ± 0.094: 4.23 ± 0.1

Posttest 1.02 ± 0.08§ 1.75 + 0.08§ 2.99 ± 0.114: 4.14 + 01

A between baseline -0.08 + 0.04*§ -0.07 ± 0.04* -0.09 + 0.08*"§ -0,11 ± 0.1and posffest

ES 0.86 0.67 0.69 0.65

Weaker groupBaseline 1.17 + 0.12 1.95 ± 0.16 3.32 ± 0.19 4.60 ± 0.2

Midtest 1.13 ± 0.05 1.90+ 0.08 3.24 ±- 0.11 4.51 +0.1

Posttest 1.09 ± 005 1.86 + 0.07 3.18 ± 0.12 4.44 + 0.1

A between baseline -0.08 ± 0.09 -0.09 +0.10 -0.13 ± 0.09*§ -0.16 ± 0.1and posttest

ES 0.79 0.69 0.78 0.76

Control groupBaseline 1.13 ± 0.04 1.88+ 0.07 3.25 ± 0.11 4.52 + 0.:

Posttest 1,15 ± 0.04 1.91 ± 0.05 3.27 + 0.07 4.53 ± 0.1

A between baseline 0.02 ± 0.05 0.03 ± 0.06 0.02 ± 0.08 0.01 +0.1and posttest

ES 0.41 0.46 0.24 0.10

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!< 0.05) change from baseline to posttest.

t Significant (P < 0.05) difference between stronger group and all other groups.

4: Significant (P < 0.05) difference between stronger group and weaker group.

§ Significantly (P!< 0.05) different from control group.

9t44:5t

231617lO*,§

140908

40 m

5.41 ± 0.25t5.39 ± 0.20t5.29 + 0.20t

-0,12 ± 0.11*

0.54

5.90 ± 0.275.79 ± 0.205H69 + 0,22

-0.22 ± 0.14*"§

0.82

5.79 ± 0.195.79 ± 0.120.00 + 0.10

0.01

Flying 5 m Flying 15 m

0.72 ± 0.03f0.74 + 0.030.73 0.01-t0.01 + 0.02

0.64

0.78 ± 0.040.77 + 0.050.77 ± 0.02

-0.01 + 0.03

0.25

1.98 + 0.07t1.99 + 0.084t1.97 + 005-t

-0.01 + 0,07

0.18

2.14 + 0.082.10 ± 0.092.09 ± 0.08

-0.06 ± 0.03*1§

0.66

0.75 ± 0.03 2.12 ± 0.080.76 ± 0.02 2.12 ± 0.050.01 + 0.02 0.00 ± 0.04

0.40 0.07


Weaker Group

A at Midtest

7.1 ± 4.1"7.0 ± 2.3*

169.3 + 79.4*

3.5 +3.755.3 ± 28.8*

1.5 ± 1.848.8 ± 26.5*

0.34 + 0.34*0.15 + 0.300.35 + 0.36*

6.4 ± 2.0*

0.04 ± 0.05

-0.27 + 0.12*

A at Posttest

9.1 ± 2.3*8.5 ± 1.9*236.7 + 129.7*

4.7 + 4.2*94.6 ± 74.9*

1.4 ± 1.957.4 + 20.9*

0.55 ± 0.26*0.28 + 0.270.60 + 0.30*

8.6 + 4.7*

0.06 + 0.04*

-0.31 + 0.13':t

a at Posttest

A Stronger Group: Force-Velocity Relationship3.5

" Baseline.. 0-. Mid-Test

3.0,-,_| - -v- Post-Test

72.5

0

Q. 2.0

1.5

Z 1.00_o

0.5

0.018 20 22 24 26 28 30 32 34 36 38 40 42

Force at Peak Power (Nlkg)

B80

60

40

20

0

Stronger Group: Force-Power Relationship

SBaselineT .. O.- Mid-Test*k O.u * -- Y- Post-Test

..N

t*

18 20 22 24 26 28 30 32 34 36 38 40 42

Force at Peak Power (N/kg)

Weaker Group: Force-Velocity RelationshipI-0- Baseline

0 ... Mid-Test- "y- Post-Test

D80

60

400Q.

a.

20

18 20 22 24 26 28 30 32 34 36 38 40 42

Weaker Group: Force-Power Relationship

- Baseline• .0O. Mid-Test

T. ~-'• Post-Test

18 20 22 24 26 28 30 32 34 36 38 40 42


Control Group: Force-Velocity Relationship-4le-- Baseline

-- •1'- Post-Test

F80

60

400a.

0(D(LJ

18 20 22 24 26 28 30 32 34 36 38 40 42



Control Group: Force-Power Relationship-4--Baseline]

- .y- Post-Test

18 20 22 24 26 28 30 32 34 36 38 40 42


FIGURE 2-Training-induced changes to the force-velocity and force-power relationships for the jump squat. *Significantly (P < 0.05) differentpeak power relative to BM from baseline. +Approaching significantly (P < 0.10) different peak power relative to BM from baseline. xSignificantly(P _< 0.05) different force relative to BM at peak power from baseline. #Significantly (P 5 0.05) different velocity at peak power from baseline.

INFLUENCE OF STRENGTH ON POWER

C3.5

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Medicine & Science in Sports & Exercise,. 1573

\

A Force-Velocity Relationship: Baseline

-. 0 Wo- Gr,p

C- 25

1. 25

o 1.0

0.5

0.010 20 22 24 26 28 30 32 34 36 38 40 42

Force at Peak Power (Nikg)

D Force-Power Relationship: Baseline

20

.0 S-ko.gGroui.Fo,. Weakera rPp

0 . Cw.aGr..

f..-..

10 23. 22 24 20 053 2 4 3 38 40 42

Force at Peak Power (NOkO)

I

B Force-Velocity Relationship: Mid-Test

W ,, ke,G-p

E2.5.

_ 2.

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0.5

0.0.

E

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o40

Force at Peak Power (N1kg)

Force-Power Relationship: Mid-Test

F• aror,Grpo-W-ke,r G-op

C Force-Velocity Relationship: Post-Test

3.0 o .. o Weaker Grip

05.0

10 20 22 24 26 20 30 32 3460 30 40 42

Force at Peak Powoer (Ntkg)

F Force-Power Relationship: Post-Test

ao

01810 20 22 24 26 28 30 32 34 36 38 40 42


. CW-1,r Gr,~p

8 20 22 24 26 28 30 32 30 36 38 40 42

Force at Peak Power (Nikg)

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 of

training. tSignificant (P < 0.05) difference in force or power between stronger and all other groups. JSignificant (P < 0.05) difference in force or

power between stronger and weaker groups. §Significantly (P < 0.05) different force or power from control group.

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 IRM/BM at posttest (ES = 0.93, equiv-alent to a 7 + 7-kg decrease in I RM). Training resulted insignificant within-group changes in a multitude ofjump 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 the

changes 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

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

Minimum Joint Angle

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

Stronger groupBaselineMidtestPosttest

Weaker groupBaselineMidtestPosttest

Control groupBaselinePosttest

89.7 ± 8.594.8 + 6.594.7 + 5.6

85.9 + 7.190.1 + 9.589.3 + 11.1

89.6 ± 6.486.0 + 12.2

86.3 + 13.889.4 + 10.891.5 + 16.4

79.3 + 8.884.8 + 18.788.4 + 20.4

78.6 + 9.878.0 + 6.9

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-, and

40-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

e (°)

60.7 + 4.461.8 +4.759.0 + 5.4

60.1 3.059.9 + 3.057.7 + 3.7

67.3 + 6.366.3 + 3.5

Knee Angle (o)

170.0 + 7,1174.5 + 5.0176.8 +4.3

170.2 + 6.4174.8 + 4.9171.7 + 6.3

174.3 + 6.5172.3 + 9.3

Maximum Joint AngleHip Angle (o)

168.3 + 4.9171.9 + 8.1174.0 + 7.1

168.1 + 8.1172.8 + 7.1172.7 f 9.5

172.3 + 4.9170.9 + 12.2

Ankle Angle (°)

133.4 i 3.9137.5 + 5.9140.0 5.8

136.0 8.2137.4 6.6138.1 5,2

141.4 8.5137.3 7.0


ao.

10

8N

so

80

60•0.

0

B

B

Normalised Time(% ofjump)

C

B

C.

Normalised Time(% of jump)

§ T oNormalised Time (% of jump)

FIGURE 4-Comparison of the power-time curve during the 0% IRM 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 < 0.05) difference between stronger and weaker groups. (DSignificant (P < 0.05) difference between stronger and control groups. §Significant(P < 0.05) difference between control and both stronger and weaker groups.

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% 1IRM jump squat at baseline,midtest, or posttest (Table 4). Furthen-nore, training did not

B- Baseline

†....Mid-Test- --- Post-Test

S! 0z80 100 LO

*

Normalised Time (% of jump)

x

100

Normalised Time(% of jump)

- Baseline.....Mid-Test

-- -Post-Test

* D 0.6

0.5

0.4

0.3

4.°

60 80 100 Q.W

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6Normalised Time (% of jump)

MBaseline.....Mid-Test

--- Post-Test

80 100


FIGURE 5-Training-induced changes to the power-time (A), force-time (B), velocity-time (C), and displacement-time (D) curves for the (. IR 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 < 0.05) difference betweenbaseline and both midtest and posttest. xSignificant (P < 0.05) difference between baseline and posttest.

C

A

0

a.

80

70

60

50

40

30

20

10

0

-10

-20

-30

-40

-50

-60

C

E

U 0

-1

-2

-3

INFLUENCE OF STRENGTH ON POWER Medicine & Science in Sports & Exercise,,, 1575

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%023 .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%0-22.8%,42.2%0-55.6%, and 74.20/6-91.2% of normalized time; (b)

force: 9.6%--28.4% and 48.80/o62.8% of normalized time;

(c) velocity: 39.6%-62.6% and 69.6%-86.8% of normal-

ized time; and (d) displacement: 57.40/6-89.4% of normal-

ized time. Significant differences between baseline and

posttest also existed during the following phases: (a) power:

70.6%--74.0% of normalized time; (b) force: 28.60/o-32.4%,

43.00/6-48.6%, and 63.0%487.0% of normalized time; (c)

velocity: 36.00%-39.4% of normalized time; and (d) dis-

placement: 46.2%-57.2% and 89.606-100.0% of normalized

time (Fig. 5). For the weaker group, significant differences

between baseline and both midtest and posttest existed dur-

ing the following phases: (a) power: 3.6%-o11.4%, 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: 9.40/6-32.0%, 39.2%-68.0%, and 75.0%-91.8% of

normalized time; and (d) displacement: 58.0%-88.2% or

normalized time. Significant differences between baseline and

posttest also existed during the following phases: (a) power:

11.6%-18.2% of normalized time, (b) force: 70.2%-79.2%

* B 35

30

25

5,

80 100 0

*


10040 60


*

D 0.6

0.5

0.4

0.3

•560 80 100 C

a

0.2

0.1

-0.2

-0.3

-0.4

-0.5

-0.6

- Baseline.....Mid-Test

----- Post-Test


FIGURE 6-Training-induced changes to the power-time (A), force-time (B), velocity-time (C), and displacement-time (D) curves for the 0% IRM

jump 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 < 0.05) difference between

baseline and both midtest and posttest. xSignificant (P < 0.05) difference between baseline and posttest.

1576 Official Journal of the American College of Sports Medicine

A 80

0

a.

50

40

30

20

10

-10

-20

-30

-40

-50

-60

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2

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- Baseline...... Mid-Test--- Post-Test

E

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80 100

http://www.acsm-msse-org

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

Stronger Group

Baseline Midtest Posttest

Muscle architectureMuscle thickness (cm)Pennation angle (o)Lean mass of the leg (kg)

Muscle activationAvgIEMG IS (MVC)

VM (MV)VL (;kV)BF (MtV)

AvgIEMG 0% 1 RM JSVM (% MVCOs 1)VL (% MVC.s -)BF (% MVCs -1)

Rate of EMG rise 0% 1 RM JS

3.29 ± 0.38 3 30 + 0.3819.80 + 2.23 20.76 + 2.1511.35 + 1.62 11.56 + 1.69

81.9 ± 32.8 64.4 ± 24,669.5 + 22.7 66.6 ± 12.813.9 ± 5.9 14.2 + 11.9

114.5 _ 24.2115.2 ± 28.3125.6 + 21.8

3.26 ± 0.3721.03 ± 2.1411.50 + 1.62

68.0 ± 30.558.8 ± 26.913.8 + 6.7

145.2 ± 25.8 148.0 ± 45.3122.3 + 20.0 135.0 + 28.5195.5 ±+ 102.0 200.9 + 112.5

Baseline

3.02 + 0.4519.20 ± 1.9210.82 + 1.54

69.0 + 28.973.3 ± 39.018.6 + 14.2

108.8 + 29.0118.8 + 46.8100.8 ± 58.8

Weaker Group

MidtestControl Group

Posttest Baseline Postlest

3.07 ± 0.46 3.08 ± 0.48 3.06 ± 0.3020.15 + 1.59 20.98 ± 1.71 20.44 + 3.9910.76 ± 1.50 10.66 + 1.55 11.28 + 1.61

82.3 ± 44.7 81.7 ± 28.2 79.6 + 17.172.6 + 42.8 71.2 + 28.3 71.3 ± 11.914.3 ± 9.4 14.8 + 10.2 20.3 f 11.1

121.9 ± 52.3130.1 ± 59.3181.7 + 73.8

115.3 + 19.0106.0 ± 25.7174.4 + 90.3

3.06 0.2920.59 4.1711.30 '1.49

78.5 '18.270.0 23.120.9 12.0

81.3 t 13.6 84.6 I 23.2107.1 + 30.0 109.7 28.5103.0 ± 53.2 104.9 76.8

VM (% MVC. s1) 503.5 + 148.9 903.7 ± 380.9 1057.3 ± 492.0*"§ 352.3 ± 174.4 725.1 ± 305.9* 672.5 + 263.8* 373.0 ± 209.7 386.7 F 227.4VL (% MVC.s 1) 479.2 + 210.2 634.1 ± 188.6 971.4 ± 338.6*,§ 389.6 + 187.4 866.0 + 672.0 654.1 ± 283.0 403.6 ± 209.5 392.9 '146.8BF (% MVCs 1) 922.6 +413.9 1893.5 1170.1 1759.3 + 941.1 714.5 ± 551.9 1180.0 ± 619.0 1359.0 +984.6 449.7 189.1 477.2 250.8

AvgIEMG, average integrated EMG; IS, isometric squat; JS, jump squat.Average integrated EMG (AvglEMG) 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 < 0.05) change from baseline.§ Significantly (P < 0.05) different from control group.

of normalized time, and (c) displacement: 88.40o-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% 1 RM 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% 1 RM 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

INFLUENCE OF STRENGTH ON POWER Medicine & Science in Sports & Exercise� 1577

INFLUENCE OF STRENGTH ON POWERMedicine & Science in Sports & Exercisec, 1577

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-m

sprint. 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 were

of a practically relevant magnitude, representing a 7.3% and

2.2% 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 in

sprint 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 the

form 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 the

magnitude 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 are

easily 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 group may have been

affected. Second, the current observations may have been

confounded by the stronger groups' cessation of strength

training for the duration of the study (i.e., total of 15 wk). To

examine the specific mechanisms driving adaptation to the

ballistic 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 the

weaker group (i.e., not involved in any such training before

commencing the experiment), the stronger group displayed a

statistically nonsignificant but practically relevant decrease

in maximal strength (4.6% decrease in I RM/BM; ES = 0.91).

The neuromuscular changes associated with detraining peri-

ods of a similar time course (i.e., decreased neural drive and

CSA [15]) are theorized to negatively impact the ability of

the stronger group to adapt to the experimental training.

Thus, the cessation of strength training is theorized to have

negatively affected the ability of the stronger group to adapt

to the ballistic power training. It is imperative, therefore, that

future research is conducted that incorporates strength

maintenance sessions during the training intervention to elu-

cidate further the influence of strength on the magnitude

of improvements in athletic performance. Cognizant of

these limitations, the fact that the stronger group showed

similar performance improvements as the weaker group to

the ballistic power training intervention is of great practical

importance.Mechanisms responsible for improved performance.

Monitoring the training-induced changes to the force-velocity

and force-power relationships during a sports-specific move-

ment such as the jump squat offers some indication of the

mechanisms driving adaptations in the stronger and weaker

groups. The ability of such an applied in vivo measure of

the force-velocity relationship to delineate exact changes to

muscle mechanics after training is complicated by a range of

factors including mixed fiber composition, muscle architec-

tural characteristics, anatomical joint configuration, levels of

neural activation, as well as the complex nature of the jump

squat movement (23). Despite these limitations, examina-

tion of the force velocity relationship in sports-specific

movements quantifies the ability of the intact neuromus-

cular system to function under various loading conditions, in-

formation essential to understanding muscular function

during dynamic athletic movements. Data from this investi-

gation revealed that strength level did somewhat influence

the training-induced changes to the jump squat force-velocity

and force-power relationships (Figs. 2 and 3). Specifically,

the stronger group showed a more velocity-specific response

to the training stimulus, displaying the greatest improvements

under the lightest loading conditions (i.e., high-velocity, low-

force portion of the force-velocity relationship). In contrast,

ballistic power training with 00/o-30% I RM resulted in im-

provements in velocity and power throughout a range of

loading conditions for the weaker subjects (i.e., similar im-

provements at both high-velocity and high-force portions of

the force-velocity relationship). These changes caused the

slope of the force-velocity relationship to increase in the


stronger group (aided by a slight decrease in F,,,) 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%0 IRM) that did not exist at baseline wasobserved at midtest and at posttest. In addition, between-group differences in power at 20% and 40% 1 RM 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% IRM jump squat(Table 4), significant training-induced changes in displace-ment, velocity, force, and power were evident throughoutthe 0%, 1 RM 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%I RM 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 countennovement) 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% 1 RM 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 perfonnance.

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% IRM) 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% IRM 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, Hdkkinen et al. (16) observed ballistic powertraining (jump squats with 0%o 60% IRM) 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

INFLUENCE OF STRENGTH ON POWER Medicine & Science in Sports & Exercise,,, 1579

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 <5-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 of

the movement and the loading parameters used (12).In conclusion, the ballistic power training program used

was 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-

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INFLUENCE OF STRENGTH ON POWER Medicine & Science in Sports & Exercise,. 1581

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TITLE: Influence of Strength on Magnitude and Mechanisms ofAdaptation to Power Training

SOURCE: Med Sci Sports Exercise 42 no8 Ag 2010

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