assessment of lower extremity muscle power in functionally-limited elders

194 Aging Clin Exp Res, Vol. 19, No. 3

Key words: aging, pneumatic, reliability, strength.Correspondence: Roger A. Fielding, Ph.D., Director, Nutrition, Exercise Physiology, and Sarcopenia Laboratory Jean Mayer USDA, Hu-man Nutrition Research Center on Aging, Tufts University, 711 Washington St., Boston, MA 02111, USA.E-mail: [email protected] March 20, 2006; accepted in revised form May 31, 2006.

ABSTRACT. Background and aims: The purpose ofthis study was to assess the reliability and concurrent va-lidity of a new methodology to evaluate lower extremitymuscle power in older, functionally limited men andwomen.Methods: A cross sectional evaluation was per-formed on 58 older men (n=27) and women (n=31)(74.2±0.9 years). Knee and hip (leg press) and knee ex-tensor power were evaluated on pneumatic and isokineticresistance equipment. Incremental single attempt power(IP) testing utilized a single attempt at attaining maximumpower at each of six external resistances and was com-pared to multiple attempt pneumatic power (MP) testingdetermined by the highest of 5 attempts at achievingmaximum power at two set resistances and also with pow-er determined by isokinetic dynamometry. Results: Legpress extension MP yielded significantly greater powerthan IP at both low (mean=225.3±11.85 and183.9±11.52 watts respectively, p<0.001) and high(mean=249.7±15.25 and 201.7±13.18 watts respec-tively, p<0.001) external resistances. Knee extensionMP also produced significantly greater power when com-pared to IP at low (mean=82.4±4.45 and 69.7±4.28watts respectively, p<0.001) and high (mean=93.7±6.3and 83.2±5.93 watts respectively, p<0.001) externalresistances. MP testing exhibited excellent reliability atboth low (leg press extension: Intra Class Correlation(ICC)=0.93, knee extension: ICC=0.87) and high (Legpress extension: ICC=0.85, Knee Extension: ICC=0.91)external resistances. MP knee extension at 70% 1 RM al-so showed good agreement with average isokinetic pow-er (R2=0.636). Conclusions: These findings supportthe reliability and concurrent validity of MP for theevaluation of muscle power in older individuals.(Aging Clin Exp Res 2007; 19: 194-199)©2007, Editrice Kurtis

Assessment of lower extremity muscle power infunctionally-limited eldersDamien Callahan2, Edward Phillips1,2,3, Robert Carabello1,Walter R. Frontera3, andRoger A. Fielding1,2,31Nutrition, Exercise Physiology, and Sarcopenia Laboratory, Jean Mayer USDA Human Nutrition Research Centeron Aging at Tufts University, Boston, MA, 2Human Physiology Laboratory, Department of Health Sciences, SargentCollege of Health and Rehabilitation Sciences, Boston University, Boston, MA, 3Department of PhysicalMedicine and Rehabilitation, Harvard Medical School, Spaulding Rehabilitation Hospital, Boston, MA, USA

INTRODUCTIONSkeletal muscle strength, defined as the maximum

capacity to generate force, declines with advancing age (1,2). Decrements in strength have been associated with lossof function and increased disability (3). However withage, muscle power, the product of force and velocity, hasbeen shown to decline earlier and more precipitouslythan muscle strength (4).

Isokinetic power of the ankle flexors has been report-ed to be 7.5 times lower in institutionalized recurrent “fall-ers” compared to “non-fallers” (5) and leg extensor pow-er has been shown to explain 85% of the variance in gaitspeed in nursing home residents (6). In communitydwelling, older individuals, lower extremity muscle pow-er has also been demonstrated to be a strong independentpredictor of performance-based measures of physicalfunction (7, 8) and self-reported disability (9). In fact, re-cent studies have found measures of lower extremitypower to be more predictive of physical functioning thanstrength (10, 11).

Although lower extremity power has emerged as a crit-ical determinant of impairment in older individuals, no sin-gle measure of lower extremity power has been adoptedand universally applied. Approaches to the measure-ment of muscle power in older individuals have included:modified cycle ergometer anaerobic power testing (12),isokinetic dynamometry (5, 8), unloaded leg extensorpower evaluation (13), and assessments using dynamicpneumatic resistance training equipment (9). However,limitations exist in all of these methods including theavailability of equipment, participant safety, lack of taskspecificity, and financial cost.

The use of dynamic, pneumatic resistance equip-ment for the evaluation of muscle power has been re-ported most frequently (14-16). One protocol using

Aging Clinical and Experimental Research

pneumatic resistance exercise employing a single trial ofleg power at each of six external resistances (IP) at vary-ing percentages of the 1 repetition maximum (1 RM) hasshown excellent reliability in young as well as older in-dividuals (7, 9, 17-19). Although reliable, the IP protocolmay not be optimal for all applications particularly infunctionally-limited elders. In particular, one limitation inthis protocol is the performance of a single repetition ateach external resistance which prevents any assessmentof the variability of this measure between trial attempts.Nor is it known whether IP yields the highest absolutepower scores compared to evaluation protocols that al-low more familiarization and repetition at each externalresistance. Furthermore, no systematic evaluation ofthis method, which has demonstrated reliability, hasbeen conducted with alternative protocols using pneu-matic resistance exercise equipment or with the wellestablished clinical assessment of leg power using isoki-netic dynamometry.

The purpose of the present study was to evaluate theconcurrent validity and reliability of a new protocol for as-sessing lower extremity muscle power with pneumatic re-sistance equipment in mobility-limited, older, communitydwelling men and women. We propose that the newtesting protocol which allows for multiple attempts atachieving maximum velocity and therefore maximumpower will yield significantly higher performance andbetter reliability than measurement protocols involving asingle attempt at varying external resistances that arecurrently widely applied. A secondary objective was tocompare the new protocol with a well established andwidely used clinical measurement for power assessment us-ing isokinetic dynamometry.

METHODSStudy PopulationRecruitment was centered in the Boston area and ac-

complished through advertisements in local publica-tions and through the Harvard Research CooperativeProgram on Aging volunteer database. Participantswere enrolled in a large randomized exercise interventiontrial, but all strength and power testing were adminis-tered prior to the start of the training. All volunteerssigned an informed consent form and were made awareof all potential risks and benefits associated with pro-cedures of the study prior to enrollment. This studywas approved by the Boston University InstitutionalReview Board.

Participants were eligible for the study if they werecommunity dwelling and aged 65 years or older. Theywere excluded if they had acute or terminal illness, cog-nitive impairment as defined by a score of 23 or below onthe Folstein Mini Mental State Examination (20), suf-fered from symptomatic coronary artery disease, unstablecongestive heart failure, or had a myocardial infarction or

fracture in the previous 6 months. Other exclusion criteriaincluded uncontrolled hypertension (>150/90 mmHg) andthe presence of neuromuscular disease or drugs affectingneuromuscular function. Participants had to demonstratefunctional limitation as defined by a score of 10 or belowon the Short Physical Performance Battery (SPPB) (21).The SPPB is a twelve-point summary scale characterizingperformance for three tests including balance, habitual gaitand repeated chair-rise that is predictive of morbidityand mortality in older individuals (21). Participants meet-ing these preliminary qualifications were examined by aphysician and underwent a supervised graded exercise teston a treadmill prior to enrollment.

Testing was performed twice, at the same time ofday separated by one week. The entire testing procedurewas performed in approximately 60 minutes. All partic-ipants were tested by the same evaluator and used thesame equipment and procedures. Before initiating testing,each participant was familiarized with the equipment viavisual demonstration of its use, and practice at very low re-sistance.

Pneumatic Resistance TestingKnee extension (KE) strength and power were tested

using knee extension pneumatic strength training equip-ment (K400, Keiser Sports Health Equipment Inc., Fres-no, CA). Participants were seated with knees in 90 de-grees of flexion and an adjustable seat back was posi-tioned so that the participant’s femoral lateral epicondylewas aligned with the axis of rotation of the machineslever arm. Participants extended their knee against a padpositioned one inch proximal to the medial maleolus. Thepneumatic strength training equipment in these testsutilizes cylinders pressurized with air to provide variableresistance. Actuation of the lever arm compresses air inthe cylinder, while metered compression of the gaswithin the cylinder provides resistance. An ultrasonicsystem mounted on the cylinder monitors relative move-ment over time allowing for the calculation of distance,velocity, and consequently, work and power. These val-ues are viewed on a configurable digital display. Softwareengineered for this equipment calculated work and pow-er during the concentric phase of each repetition bysampling system pressure (equivalent to force) and po-sition 400 times per second. Average power was calcu-lated from data collected between 5% and 95% of theconcentric phase. The first and last five percent of themeasured range of motion are not analyzed on thisequipment in order to minimize the effect of signalnoise at the beginning and end of each movement.There were no significant differences for measurementsof strength or power between right and left legs and da-ta are presented for the left leg only.

Leg press extension (LP) strength and power weretested using pneumatic bilateral seated leg press equip-

Leg power assessment in the elderly

Aging Clin Exp Res, Vol. 19, No. 3 195

Aging Clin Exp Res 19: 194-199, 2007©2007, Editrice Kurtis

ment (K400, Keiser Sports Health Equipment Inc,Fresno, CA). Participants were seated so that theirfeet were flat on the foot plates, with their toes andknees slightly angled outward, and their knees in 90 de-grees of flexion. The participants were instructed topush through their heels and toes with a smooth con-trolled motion until their legs were almost fully extend-ed. The bilateral leg press machine utilized the sametechnology to produce the pneumatic resistance and tocalculate distance, velocity, work, and power as theknee extension equipment.

Strength as defined as the 1RM was measured forboth tests as reported previously (19). Each participantwas instructed to perform a unilateral knee extension(knee extension testing) and a bilateral knee and hip ex-tension (leg press testing) through his or her full range ofmotion. Following a few attempts at very low resis-tance to establish an individual’s range of motion, the ex-ternal resistance was progressively increased until theparticipant could no longer complete a repetitionthrough their full range of motion. A lighted bar on thedigital display that corresponded to excursion of the leverarm was used to confirm achievement of full range ofmotion for each attempt. Throughout the testing pro-cedure a rest period of approximately 2 minutes wasprovided between repetitions.

The incremental single attempt power (IP) protocolwas applied unilaterally to each leg at six relative in-tensities equal to 40%, 50%, 60%, 70%, 80%, and90% of the measured 1RM. Participants were instruct-ed to complete a single repetition as quickly as possiblethrough their full range of motion at each external re-sistance. At each resistance setting, participants per-formed one maximal effort with a 30 second rest be-tween repetitions.

Multiple Attempt Power (MP) was then measured af-ter a 5 minute rest for both exercises. Each participantwas instructed to again complete each repetition asquickly as possible through their full range of motion ata resistance equal to 40% of 1RM for both tests. Thiswas repeated four more times for a total of five repeti-tions, each separated by 30 seconds. The highest mea-sured power was recorded as the MP 40%. Externalresistance was then raised to 70% 1RM and the partic-ipant was again instructed to extend their leg as quicklyas possible through five repetitions. The highest of thesevalues was then recorded as the MP 70%. We specificallychose these two relative intensities because we havepreviously shown that participants generate maximumpower (MP 70%) and maximum velocity (MP 40%) atthese specific relative intensities (11).

Isokinetic dynamometryA Cybex II isokinetic dynamometer (Computer

Sports Medicine, Inc., Stoughton, MA) was used to

measure isokinetic knee extensor peak torque (N-m) andpower (watts) at 90° sec-1. Velocity was set and heldconstant at 90° sec-1 for the peak torque measures us-ing a PC attached to the dynamometer. As with thepneumatic testing, participants were seated with kneesin 90 degrees of flexion and an adjustable lever arm waspositioned such that its axis of rotation was aligned withthe participant’s femoral lateral epicondyle. A shinpad at the end of the lever arm was positioned one inchproximal to the medial maleolus. During isokinetictesting, participants were instructed to extend andthen flex their leg as quickly and forcefully as possiblesix times in rapid succession. Peak torque was record-ed as the best of these six repetitions. Average isokineticpower was evaluated concurrently with measures ofisokinetic torque at 90° sec-1. A PC interfaced with theequipment calculated average power per repetitionfrom torque produced through the complete concentricrange of motion. The highest value from 6 attemptswas recorded.

Statistical analysisData were analyzed using SPSS and StatView software

packages. All values are reported as means ± standard er-ror. Two way, fixed model intraclass correlation coeffi-cients were used to determine test/retest reliability usingSPSS data editors. StatView software was used for theanalysis of simple regression between measures of strengthand power. Paired t tests were used to assess mean dif-ferences between MP and ISP testing at 40% and 70%1RM. Statistical significance was accepted at p<0.05.Comparisons between MP, IP, and isokinetic tests wereperformed on results obtained from the second evaluationin all participants.

RESULTSParticipant characteristicsFifty-eight volunteers (27 male, 31 female) complet-

ed all aspects of the study protocol. Participants wereaged 74.2±0.9 and had an SPPB score of 7.7±0.2.Body mass was 78.8±2.16 kg and BMI was 28.9±0.8kg⋅m-2. No adverse effects nor untoward outcomeswere reported by any of the participants from the test-ing procedures.

MP testing vs IP testingLeg press extension (LP) and knee extension (KE) test-

ing produced excellent agreement between IP and MP atboth 40% and 70% 1 RM (LP: r2=0.85 and 0.84, re-spectively, and KE: r2=0.88 and 0.95, respectively,p<0.001). However MP testing demonstrated signifi-cantly greater leg power at both resistances for each ofthe individual exercises (Fig. 1). With leg press extension,at an external resistance equal to 40% 1 RM, MP test-ing generated 18% higher power than IP testing

D. Callahan, E. Phillips, R. Carabello, et al.



(225.3±11.8 vs 183.9±11.5 watts, respectively,p<0.001) (Table 1a) and MP was also associated with a22% higher velocity (87.1±2.7 vs 71.0±3.0 cm⋅s-1,respectively, p<0.00) (Table 2a). At 70% 1 RM, MP forleg press extension generated 19% greater power thanIP testing (249.7±15.2 vs 201.7±13.2 watts, respec-tively, p<0.001) (Table 1a) and MP was also associatedwith a 24% higher velocity (56.8±2.6 vs 45.8±2.3cm⋅s-1, respectively p<0.00) (Table 2a). At an externalresistance of 40% 1 RM, on the knee extension, MPtesting generated 15% higher power than ISP testing(82.4±4.4 vs 69.7±4.3 watts, respectively, p<0.001)(Table 1b) and was also associated with a 16% higher ve-locity (2.4±0.1 vs 2.1±0.1 rad⋅s-1, respectively, p<0.00)(Table 2b). At 70% 1 RM, MP generated 11% greaterpower than IP testing (93.7±6.3 vs 83.2±5.9 watts, re-spectively, p<0.001) (Table 1b) and was also associatedwith a 15% higher velocity (1.6±0.1 vs 1.9±0.1 rad⋅s-1,respectively, p<0.00) (Table 2b).

MP Reliability and ValidityAll measures of power showed excellent reliability

(Tables 1a and 1b). In all cases, MP demonstrated greaterreliability than the IP procedure. Knee extension MPtesting at 70% 1 RM also showed good agreement withassessment of isokinetic knee extensor power at 90°sec-1 (r2=0.64, p<0.001) (Fig. 2).

DISCUSSIONThe major finding from the present study is that the ab-

solute power output generated during knee extensionexercise is significantly greater with the MP testing pro-tocol than with the IP testing protocol. Although the re-liability of both testing protocols was excellent, reliabilityof the MP test protocol was slightly better than the IP testprotocol. Finally, the MP test protocol exhibited goodagreement with knee extension average power mea-sured by isokinetic dynamometry, an established test ofmusculoskeletal performance (22).




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er(W

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40

20

0KE 40 KE 70

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Pow

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)

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Fig. 1 - Left knee extensor power (1a) and Leg press (1b) powerproduction from single attempt pneumatic power testing proto-col (ISP) (filled bars) and multiple attempt pneumatic powertesting protocol (MP) (open bars) at 40% and 70% of 1RM. MPtesting resulted in significantly greater power at both 40% and70% 1RM when compared to ISP (asterisks indicate p<0.001).

Table 1 - Leg press (a), and (b) knee extension power and reliability(1 repetition maximum=1 RM; single trial of leg power at each ofsix external resistances=IP; multiple attempt pneumatic pow-er=MP). All power units are in Watts.

Power Intra Class(mean±SE) Correlation

a. Leg Press Extension1 RM 559.1 (25.71) 0.97IP 40% 183.9 (11.52) 0.75MP 40% 225.3 (11.85) 0.93IP 70% 201.7 (13.18) 0.76MP 70% 249.7 (15.25) 0.85

b. Knee Extension1 RM 64.7 (3.5) 0.92IP 40% 69.7 (4.28) 0.80MP 40% 82.4 (4.45) 0.87IP 70% 83.2 (5.93) 0.78MP 70% 93.7 (6.30) 0.91Isokinetic (90°) 60.6 (3.03) 0.84

Table 2 - Leg press (a) and (b) knee extension velocity (single tri-al of leg power at each of six external resistances=IP; multiple at-tempt pneumatic power=MP). Leg press extension velocity unitsare in cm/sec. Leg extension velocity units are in radians/sec.

Velocity(mean±SE)

a. Leg Press ExtensionIP 40% 71.04 (2.96)MP 40% 87.09 (2.71)IP 70% 45.84 (2.34)MP 70% 56.79 (2.63)

b. Knee ExtensionIP 40% 2.09 (0.07)MP 40% 2.43 (0.07)IP 70% 1.62 (0.07)MP 70% 1.87 (0.07)

Lower extremity muscle power has emerged as astrong independent predictor of performance-based mea-sures of physical function (7, 8, 10-12) and self-reporteddisability (9, 23). Several randomized trials of exercise inolder individuals have evaluated the effects of variousresistance training interventions on lower extremity mus-cle power (18, 19, 24-28). In addition, studies have ex-amined the role of other anabolic interventions such astestosterone and growth hormone on lower extremitymuscle power (29-31). However, all of these studieshave employed different methods to assess lower ex-tremity power and none, with exception of the study bySkelton et al. (24), has used methodology and equip-ment that has been evaluated for the reliability and validityof lower extremity power in older individuals. In the pre-sent study, we present a methodology (MP testing pro-tocol) that is feasible to perform in functionally limited old-er individuals, has concurrent validity with isokinetic dy-namometry, a widely accepted measure of power as-sessment, yields the greatest absolute power and demon-strates excellent reliability in our sample.

Several cross sectional and intervention studies haveused the incremental single attempt power (IP) test to eval-uate leg power in older adults (18, 19, 25, 26). Al-though there was excellent agreement between the IP andMP protocol, results from the present study indicate MPtesting produces greater measures of power in function-ally-limited, older men and women when compared to IPtesting. The present data suggest that previous meth-ods of measuring leg power may have underestimated theabsolute leg power of the leg press extensors and the kneeextensors and that the MP protocol is optimal for the as-sessment of absolute leg press extension and knee ex-tensor power. Although this systematic underestimationof leg power by the IP may represent an adequate mea-sure in cross sectional studies, in intervention studieswhere repeated measures of leg power are performed, theresultant power by IP may be influenced by learning and

repeated attempts. Because of single attempts at differentpercentages of the 1 RM, the power determined by IPmay be negatively influenced by practice effects, a con-founder which may be reduced with repeated trials in-herent with MP. We contend that this learning effectmay be minimized with the MP testing protocol.

In addition to the higher power output recorded duringthe MP testing, this methodology has proven suitableand as safe as other methods in this population of olderindividuals with functional limitations. In the presentstudy, MP 70% testing correlated well with average isoki-netic power production (R2=0.636), indicating its con-current validity with a recognized measure of muscu-loskeletal performance but also, unlike isokinetic test-ing, is performed during dynamic movement and not at apredetermined velocity. The relative lack of agreement andthe lower absolute power generated with isokinetic pow-er testing compared to MP 70% may be due to the in-ability of the participant to maximally accelerate during thefixed velocity isokinetic test and possible mechanical dif-ferences between both testing procedures (eg: knee andhip positioning). Therefore the MP methodology may bemore functionally relevant than isokinetic testing.

CONCLUSIONSIn summary, the MP test of leg power evaluated here

was highly reliable, demonstrated greater absolute pow-er values than IP, had concurrent validity and was well tol-erated in older individuals with functional limitations. In-vestigators are encouraged to utilize this newly validatedand reliable measure of leg power in future cross sectionaland intervention studies in older individuals.

ACKNOWLEDGEMENTSThe authors wish to acknowledge the support of Dennis Keiser and

Keiser Sports Health Equipment (Fresno, CA). Subjects were recruit-ed from a Volunteer Registry supported by the Hebrew SeniorLife In-stitute for Aging Research, and grants from the National Institute on Ag-ing 5 P01 AG04390, Older Americans Independence Center SubGrant2 P60 AG08812 and Massachusetts Alzheimer’s Disease ResearchCenter Subgrant 2 P50 AG05134. This work was supported by theNational Institute on Aging (NIA) grant number AG18844 and this workis based upon work supported by the U.S. Department of Agriculture,under agreement No. 58-1950-4-401. Any opinions, findings, con-clusion, or recommendations expressed in this publication are those ofthe author(s) and do not necessarily reflect the view of the U.S. De-partment of Agriculture.

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00 20 40 60 80 100 120 140 160

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Isokinetic Power (W)

R2 = 0.6361

Fig. 2 - Simple regression analysis comparing left knee extensorpower measured via MP 70% and average isokinetic power at 90°sec-1. There was a significant positive correlation (R2=0.636) be-tween measures (p<0.001).

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