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RIGINAL ARTICLE

ssociation of Activity Limitations and Lower-Limb Explosivextensor Power in Ambulatory People With Stroke

avid H. Saunders, MPhil, Carolyn A. Greig, PhD, Archie Young, MD, Gillian E. Mead, MD

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ABSTRACT. Saunders DH, Greig CA, Young A, Mead GE.ssociation of activity limitations and lower-limb explosive

xtensor power in ambulatory people with stroke. Arch Physed Rehabil 2008;89:677-83.

Objective: To determine whether the explosive lower-limbxtensor power of the affected and unaffected sides, and anysymmetry, are associated with activity limitations after stroke.

Design: Cross-sectional observational study of baseline datarom a randomized controlled trial.

Setting: Measurements made in a hospital clinical researchacility.

Participants: Community-dwelling (N�66) subjects withtroke who were independently ambulatory. Subjects’ meange was 72�10 years.

Interventions: Not applicable.Main Outcome Measures: The lower-limb extensor power

f each lower limb (in W/kg), performance of specific func-ional activities (comfortable walking velocity, Functionaleach Test, chair-rise time, Timed Up & Go test), and global

ndices of activity limitation (FIM instrument, Rivermead Mo-ility Index, Nottingham Extended Activities of Daily Living).

Results: Low lower-limb extensor power in either lowerimb was the principal factor from among the confounders weecorded that significantly (R2 range, .21–.46) predicted theimitation of specific functional activities, and low lower-limbxtensor power in either lower limb was the principal predic-ive factor for global indices of activity limitation (R2 range,13–.38). The degree of asymmetry of lower-limb extensorower between legs was low and had little or no predictivealue.Conclusions: In ambulatory persons with stroke, activity

imitations are associated with deficits in lower-limb extensorower of both lower limbs, and not the severity of any residualsymmetry. These findings suggest that interventions to in-rease lower-limb extensor power in both lower limbs mighteduce activity limitations after stroke.

Key Words: Activities of daily living; Cerebrovascular ac-ident; Physical fitness; Rehabilitation.

© 2008 by the American Congress of Rehabilitation Medi-ine and the American Academy of Physical Medicine andehabilitation

From the Department of Physical Education, Sport and Leisure Studies (Saunders),nd Geriatric Medicine, School of Clinical Sciences and Community Health, Univer-ity of Edinburgh, Edinburgh, UK (Greig, Young, Mead).

Supported in part by the Chief Scientist Office of the Scottish Executive (grant no.ZB/4/46) and the Research into Ageing (fellowship no. 236).No commercial party having a direct financial interest in the results of the research

upporting this article has or will confer a benefit upon the authors or upon anyrganization with which the authors are associated.Reprint requests to David H. Saunders, MPhil, Scottish Centre for Physical Edu-

ation Sport and Leisure Studies, University of Edinburgh, St Leonards Land,olyrood Rd, Edinburgh, EH8 8AQ, Scotland, e-mail: Dave.Saunders@ed.ac.uk.

i0003-9993/08/8904-00294$34.00/0doi:10.1016/j.apmr.2007.09.034

HE ABILITY OF MUSCLE to generate force can bedescribed in terms of muscle strength and explosive power.

trength is the magnitude of maximal force generation whereasxplosive power output is a velocity-dependent characteristicefined as the greatest rate of work achieved during a single,allistic, resisted contraction.1 Explosive power deterioratesaster than strength (3%–4% vs 1%–2% a year) during healthyging.2

Although strength and power are both important for execu-ion of functional activities, lower-limb extensor power is moremportant than knee extensor strength for stair climbing, chairising, and walking,3-5 and when impairment is asymmetrical,ower-limb extensor power is a better predictor of the fre-uency of falling than strength alone.6

Although people who have survived a stroke are often el-erly, may be less active than prior to their stroke, and mayave unilateral limb weakness, surprisingly little is knownbout the extent to which explosive power might be impairednd whether this might have adverse functional consequences. pilot study of 11 ambulatory subjects 1 year after stroke withirtually no residual neurologic deficit7 found that both lower-imb extensor power and knee extensor muscle strength of bothower limbs were substantially lower than that of age- andex-matched healthy subjects.8,9 Further unpublished data in-icated that impairment in lower-limb extensor power waspproximately double that of muscle strength.

Bilateral impairment in explosive power,7 or muscletrength,7,10,11 observed after stroke could arise for severaleasons both directly and indirectly associated with stroke.irst, bilateral motor deficits can arise directly from a unilateral

esion.12 Second, reduced habitual physical activity, either be-ore and/or after stroke may cause muscle atrophy.13 Third, theresence of comorbid disease (including poor nutrition) beforend/or after stroke could impair motor function.

Low muscle strength after stroke is associated with poorerformance of walking and stair climbing,14 chair rising,15

nd impaired motor function.16 Only 1 small study (N�14) hasxplored the functional associations of explosive power aftertroke.17 It showed that asymmetry in lower-limb extensorower was associated with reduced walking performance. Thearticipants were unusually young (mean, 46.4�8.4y), and thiselationship should be examined in people with stroke of moreypical age (ie, �70y).18 Moreover, the relationship of powerith other aspects of activity limitation should be examined to

xplore the potential benefits that might result from attempts tomprove explosive power after stroke. This is important be-ause fitness training can be presented in such a way as topecifically improve explosive power19 and this might reducectivity limitations, and so reduce participation restriction aftertroke.

The aim of this study was to determine in older, ambulatoryeople with stroke, whether the lower-limb extensor power ofhe affected and unaffected sides, and any asymmetry, weressociated with (1) performance of specific functional activitiesreaching, walking, and rising from a chair), and (2) global

ndices of activity limitation (FIM instrument, Nottingham

Arch Phys Med Rehabil Vol 89, April 2008

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xtended Activities of Daily Living [NEADL], Rivermeadobility Index [RMI]).

METHODS

articipantsAll participants in this study (N�66) were recruited to a ran-

omized trial of exercise or relaxation after stroke (table 1).20 We

Table 2: Untransformed Data for Lower-Limb Extensor Power, MIndices of Ac

Variable n

Lower-limb extensor powerAffected side LLEP (W/kg) 64Unaffected side LLEP (W/kg) 61Asymmetry ratio (aff LLEP/unaff LLEP) 60

Specific functional activitiesFRT (cm) 63Comfortable walking velocity (m/s) 64TUG test (s) 61Chair-rise time (s) 60

Global indices of activity limitationFIM instrument 66RMI 66NEADL 65

bbreviations: aff, affected; LLEP, lower-limb extensor power; unaffAffected LLEP lower than unaffected LLEP (t�3.77, P�.001).The non-normal data included in this table could be transformed t

Table 1: Partici

Characteristics

Age (y) NSex (male/female) 36Stature (m) NTime from stroke (d) NSmoking history

Smoker/Ex-/Non-/UC 25/1Walking aids

Stick/orthosis/zimmer/none 28/4Body mass (kg) NStroke type (TAC/LAC/PAC/POC/UC) 2/19/Lesion type (ischemic/hemorrhagic/UC) 60Lesion side (left/right/both/UC) 37/2Hospital care (inpatient/outpatient) 56Inpatient length of stay (d) NBlood pressure

Systolic (mmHg) NDiastolic (mmHg) N

ComorbiditiesPrior strokePrior transient ischemic attackIschemic heart diseaseLeft ventricular failureHypertensionPrior malignancyDiabetesMiscellaneousNoneTotal no. per participant N

bbreviations: IQR, interquartile range; LAC, lacunar; NA, not apptandard deviation; TAC, total anterior circulation; UC, unclear.

The non-normal data included in this table could be transformed to a nThe non-normal data included in this table could be transformed to a n

rch Phys Med Rehabil Vol 89, April 2008

elected these 66 patients after screening 301 patients for trialligibility (all 301 had required either inpatient or outpatientare after an acute stroke in 1 of 4 Edinburgh hospitals). Trialnclusion criteria were (1) independently ambulatory (with orithout walking aids), (2) living within the recruitment catch-ent area, (3) completion of inpatient and outpatient stroke

ehabilitation, and (4) absence of dysphasia or confusionudged severe enough to prevent safe participation in exercise

res of Performance of Specific Functional Activities, and GlobalLimitation

Mean � SD Median (IQR)

NA 0.92 (0.53–1.49)*†

NA 1.05 (0.73–1.56)†

0.89�0.24 NA

26.53�6.65 NA0.67�0.24 NA

NA 11.68 (8.17–16.09)‡

NA 1.28 (0.83–1.70)‡

NA 117.5 (114–122)§

NA 13 (11–14)§

NA 17 (12–19)§

ffected.

ormal distribution using square root.

Characteristics

Mean � SD Median (IQR)

71.85�9.91 NANA NA

1.67�0.09 NANA 152 (83–278)

1 NA NA

NA NA72.64�15.29 NA

/1 NA NANA NANA NANA NANA 19 (9–44)

140.03�18.10 NA73.16�9.50 NA

NA NANA NANA NANA NANA NANA NANA NANA NANA NA

1.97�1.35 NA

e; PAC, partial anterior circulation; POC, posterior circulation; SD,

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679LOWER-LIMB POWER AND POSTSTROKE ACTIVITY LIMITATIONS, Saunders

r relaxation classes or to preclude informed consent. Absoluteontraindications to exercise in elderly people21 and walkingimited by pain were applied as exclusion criteria. Approvalas obtained from the local research ethics committee.For participants who presented with no lateralizing signs but

ad relevant stroke lesions evident on brain imaging, we con-idered the affected side to be ipsilateral to the side of posteriorirculation lesions, and contralateral for all others.

easurementsPrior to randomization, we measured lower-limb extensor

ower during hip and knee extension while the subject waseated on a Nottingham Power Rig.22,a Ten maximal pushesere encouraged using each lower limb with a rest (minimum0s) between each push. Mitchell et al23 reported that 10epetitions were sufficient to obtain peak lower-limb extensorower values in elderly people rehabilitating after proximalemoral fracture. Power to body mass ratio (W/kg of bodyass) was recorded for each push and the highest value

chieved was recorded for the affected and unaffected lowerimbs. Asymmetry in lower-limb extensor power was ex-ressed as a ratio (affected lower-limb extensor power/unaf-ected lower-limb extensor power) and used to indicate hemi-aresis. The lower-limb extensor power technique is valid andeliable in healthy elderly people22 and reliable in persons with

ig 1. The relationships between the explosive lower-limb extensorower limbs and performance of specific functional activities assesselocity. When arms were used for assistance during chair rising aor clarity the lines of best fit with 95% CIs are included only for th

troke.17 f

Functional Reach Test (FRT),24 Timed Up & Go (TUG)est,25 and chair-rising time9 were recorded in triplicate. Par-icipants were asked not to use walking aids or arms duringhair rising. The average velocity of comfortable walking wasetermined during three 3-minute bouts of self-paced walkinground a 17-m circuit with a 5-minute break between eachalk. Participants were instructed to walk at their “comfortableace.”26 The following global indices of activity limitationere recorded during face-to-face interview: FIM instrument,27

MI,28 and NEADL.29 The above measures have been found toe reliable in persons with stroke30-33 or elderly people.34

We recorded age, sex, time since stroke, stature, smoking,se of walking aids, the incidence of key comorbid diseases,nd the total number of comorbid diseases (see table 1) asotential confounding factors.8,10,35

ata AnalysisNormally distributed data were reported as mean and stan-

ard deviation (SD). Non-normal data were expressed as me-ian and interquartile range (IQR) and transformed to a nor-ative distribution prior to any statistical analysis. Affected

nd unaffected legs were compared using a paired t test.tepwise multiple linear regression was used to identify (1)hether any of the confounders predicted lower-limb extensorower, and (2) whether lower-limb extensor power and con-

er (LLEP), measured in W/kg, of the affected (�) and unaffected (□)sing the FRT, TUG test, chair-rising time, and comfortable walking

G test the data are marked differently (� affected, � unaffected).affected lower-limb extensor power data.

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ounders predicted activity limitation measures. When lower-

Arch Phys Med Rehabil Vol 89, April 2008

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imb extensor power was the only significant predictor ofctivity limitation, the regression coefficients were used toenerate nonlinear models (and 95% confidence interval [CI])f the untransformed graphed data. Analyses were performedith SPSSb and Graphpad Prizm.c A P value of less than .05as considered statistically significant.

RESULTSA successful measure of lower-limb extensor power was

chieved in both legs of 60 (91%) of 66 participants and at leastleg of 65 (98%) of 66 participants. The reasons preventing

ata collection were leg pain (n�4) and equipment failuren�2). The average lower-limb extensor power increased by6% (affected) and 55% (unaffected) throughout the 10 repe-itions, but approached asymptotic values between repetitions 8nd 10 during which further increase was trivial (2.3% affectedeg, 0.3% unaffected leg). The proportion of participants witheiling values in the global indices was low (FIM instrument,/66; RMI, 11/66; NEADL, 3/66). The data for lower-limbxtensor power, specific functional activities, and global indi-es of activity limitation are summarized in table 2.

Median affected lower-limb extensor power was 42% (IQR,7–66) and unaffected lower-limb extensor power was 54%IQR, 37–71) of that expected in age- and sex-matched sub-ects.8,9 Affected lower-limb extensor power was significantlyower than unaffected lower-limb extensor power (t�3.77,�.001), but the difference was small (�10%; median, .14W/g) and the extensor power of each lower limb were highlyorrelated (R2�.68, P�.001). When the influence of age, sex,ime since stroke, smoking, and incidence of comorbid dis-ase(s) on lower-limb extensor power was examined, unaf-

Table 3: The Results of Stepwise Multiple Linear Regression Analyratio) and Potential Confounding Predictor Variables (age, sex,

smoking history) on Performance of Specific Functi

Dependent Variable

M

Affected Side for LLEP

Specific functional activitiesFRT* LLEP .32 P�.01

Stature .28 P�.023

R2 .21 P�.001Comfortable walking speed LLEP .54

R2 .28 P�.001TUG test LLEP .68

R2 .46 P�.001Chair-rise time* LLEP .63

R2 .38 P�.001Global indices of activity

limitationFIM instrument LLEP �.64 P�.001

Stature .23 P�.039R2 .35 P�.001

RMI LLEP �.58R2 .33 P�.001

NEADL LLEP �.64 P�.001Smoking .21 P�.045R2 .38 P�.001

OTE. Standardized � coefficients are reported for each individual inalues for each overall model where this could be fitted.bbreviation: NS, no significant regression model solution.Use of walking aids omitted from models.

ected lower-limb extensor power was predicted (weakly) only F

rch Phys Med Rehabil Vol 89, April 2008

y sex and age (R2�.18, P�.001), and affected lower-limbxtensor power by sex only (R2�.14, P�.002). None of theactors predicted the asymmetry ratio.

Low values of affected or unaffected lower-limb extensorower appear associated with limitation in each specific func-ional activity (fig 1). Lower-limb extensor power showedronounced curvilinear associations with chair-rising time, andUG test. When walking speed (in m/s) was expressed as a

unction of time (in s/m), the same curvilinear association wasbserved. All 3 dynamic physical functions showed reducederformance when lower-limb extensor power was below ap-roximately 1W/kg with no increase in performance above thisalue.Both affected and unaffected lower-limb extensor power

ere significant predictors of performance in each functionalctivity (table 3). Comfortable walking velocity, chair-riseime, and TUG test performance were predicted exclusively byffected and unaffected lower-limb extensor power, with eacheg having similar influence. Five participants with low valuesf lower-limb extensor power (�1W/kg) found chair risingmpossible without using their arms; their data were excludedrom the regression analysis of the TUG test and chair rising.unctional reach was predicted by lower-limb extensor power,ut not exclusively or as strongly as were other activities. Theatio of affected/unaffected lower-limb extensor power had noredictive importance for performance of specific functionalctivities.

Lower-limb extensor power was nearly exclusive as a pre-ictor of global indices of activity limitation from among theariables included in the regression models (fig 2, see table 3),he only exceptions being marginal contributions of stature to

f the Lower-Limb Extensor Power (affected side, unaffected side,ure, time since stroke, use of walking aids, comorbid disease,Activities and Global Indices of Activity Limitation

e of LLEP Included in Regression Model

Unaffected Side for LLEPAsymmetry Ratio (affected

LLEP/unaffected LLEP)

LLEP .48 P�.001 Stature .38Stature .54 P�.001Sex .39 P�.019R2 .33 P�.001 R2 .13 P�.003LLEP .65 Age �.31R2 .41 P�.001 R2 .08 P�.020LLEP .60R2 .35 P�.001 R2 NA NSLLEP .56 Age �.30R2 .30 P�.001 R2 .07 P�.026

LLEP �.38

R2 .13 P�.003 R2 NA NSLLEP �.53R2 .27 P�.001 R2 NA NSLLEP �.41 LLEP �.29

R2 .16 P�.001 R2 .07 P�.027

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681LOWER-LIMB POWER AND POSTSTROKE ACTIVITY LIMITATIONS, Saunders

ended to be stronger for affected lower-limb extensor powerhan unaffected lower-limb extensor power, but asymmetry inower-limb extensor power did not predict FIM instrument orMI scores and had only marginal predictive value forEADL scores.When statistical analyses were repeated after excluding sub-

ects with prior stroke (11/66) the multivariate R2 values in-reased slightly and marginal variables were dropped from theodels, leaving affected and unaffected lower-limb extensor

ower as the exclusive predictors of performance or limitationf activities.

DISCUSSIONThis study shows that among a sample of ambulatory sub-

ects with stroke (mean age, 72y), the unaffected lower-limbxtensor power was lower than expected and that low lower-imb extensor power in either leg was associated with (1)educed performance in some everyday dynamic functionalctivities that involve the legs, and (2) activity limitation asssessed using more global scale indices. Asymmetry in lower-imb extensor power was small and not predictive of limita-ions.

Our data suggest lower-limb extensor power is important forhe performance of dynamic day-to-day lower-limb activitieshat require rapid rates of muscle contraction. Associationsere strongest with comfortable walking velocity, TUG test,

nd chair-rising time. When lower-limb extensor power is very o

ow, performance of chair rising may be impossible for somenless modified (eg, use of arms). This is compatible withimilar observations in healthy elderly people.2 As expected,he weakest association between lower-limb extensor power (ofither leg) and physical function was with functional reach,robably because this is not limited by speed of movement.Our data show a convincing association between low lower-

imb extensor power and increased global indices of activityimitation even though not all questions within each scaleirectly addressed performance of activities involving the low-r-limb extensors.

In elderly persons with functional impairments, power out-ut during leg-press exercise, a procedure similar to lower-limbxtensor power determination, was found to be associated withtair climbing ability, chair-rise time and habitual gait veloci-y,36 and with self-reported functional status.5 These observa-ions resemble the types of association found in our study.

In a small study of unusually young (46y) ambulatory sub-ects with stroke17 substantial asymmetry in lower-limb exten-or power was observed (mean, 43%) and this was inverselyssociated with walking speed (Spearman ���.76, P�.01).hese younger participants had RMI values with a mean of 13nd walking speeds with a mean of .70m/s, which were similaro our data (see table 2). It is plausible that the greater lower-imb extensor power of their stronger side, with a mean of.99W/kg, allowed functional compensation. Asymmetry in

Fig 2. The relationship between the explosive lower-limb ex-tensor power, measured in W/kg, of the affected (�) andunaffected (□) lower limbs and global indices of activity limi-tation assessed using FIM instrument, RMI, and NEADL. Forclarity the lines of best fit with 95% CIs are included only forthe unaffected explosive lower-limb extensor power data.

ur typically older participants (mean age, 72y) was not pre-

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682 LOWER-LIMB POWER AND POSTSTROKE ACTIVITY LIMITATIONS, Saunders

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ictive of activity limitation to any important extent, probablyecause little asymmetry (10%) existed. This lack of asymme-ry may have occurred because our participants had made aood neurologic recovery. Second, substantial asymmetry maye unusual in the older ambulatory person with stroke simplyecause lower-limb extensor power is already low prior totroke, and a threshold effect limits the reduction in lower-limbxtensor power that can occur without rendering the participantonambulatory. The lack of asymmetry in our data suggestshat the low values of lower-limb extensor power could haverisen due to the influence of factors that act bilaterally (ie,ilateral motor effects, comorbid disease, habitual physicalnactivity).

Longitudinal poststroke deterioration could cause bilateraloss of lower-limb extensor power. Although no longitudinalata of lower-limb extensor power are available, 1 small studyas reported an approximate 30% loss of strength of the ipsi-ateral leg during the week after stroke.10 Another, however,ound no poststroke deterioration.37 In our study, neither timefter stroke nor comorbid disease(s) were predictive of lower-imb extensor power or activity limitations, perhaps becauseur sample was homogeneous due to restrictive eligibilityriteria. Although it is not possible to identify the underlyingause for low lower-limb extensor power and activity limita-ions, habitual physical inactivity before and/or after strokeemains a possible cause.

High-velocity resistance training in 25 healthy elderly per-ons (age, 60–80y) increased explosive power of the kneextensors and this is associated with significant improvementsn chair rising, walking, and reaching ability.38 Extrapolatingndings from studies of elderly people suggest that increasingffected and unaffected lower-limb extensor power might im-rove activity and independence after stroke. We are unawaref any studies to date that have examined this type of trainingfter stroke.

We successfully measured peak lower-limb extensor powern more than 90% of our participants; this compares favorablyith our experience of this measurement in healthy elderlyeople (78%) using the same equipment.8 This suggests thatmbulatory persons who have had stroke can perform theepeated, high-velocity, resisted muscle contractions needed tomprove explosive power. In addition, if lower-limb extensorower is impaired due to reduced habitual physical activity,here is no reason why reversal through suitable training shouldot occur. Therefore training lower-limb extensor power aftertroke may be feasible.

tudy LimitationsThe main limitation of this study was that we recruited a

omogeneous sample of high functioning independently am-ulatory subjects with stroke. Homogeneity may have limitedhe strength of the observed associations. The potential self-election of fitter participants would also tend give rise to aigher functioning cohort. Participants had minimal hemipare-is so it is difficult to speculate on the functional importance ofower-limb extensor power for those with more severe impair-ent. Future work should therefore include more impaired

articipants and examine the role of other confounding factors,uch as stage of motor recovery and pre- and poststroke habit-al physical activity levels. Outcome measures could be ex-ended to include indices of participation restriction.

CONCLUSIONSIn a sample of ambulatory subjects with stroke of mean age

2 years activity limitations were associated with bilateral

rch Phys Med Rehabil Vol 89, April 2008

eficits in lower-limb extensor power and not with the severityf any residual asymmetry. These data suggest that the feasi-ility and effectiveness of training interventions to improveuscle explosive power after stroke should be explored.

Acknowledgments: Gillian E. Mead, MD, was the principalnvestigator for a trial of exercise or relaxation after stroke from whichhese data are derived.

We thank the staff on the stroke units at the Royal Infirmary ofdinburgh, Astley, Ainslie, and Liberton Hospital, who assisted in

dentifying and recruiting patients. We thank Claire Fitzsimons, PhD,lasdair MacLullich, PhD, Susan Shenkin, MD, and Gail Carin-Levy,Sc, who assisted with some patient assessments. Susan Lewis PhDnalyzed the data from the STARTER (Stroke: A Randomized Trial ofxercise of Relaxation) trial, from which these data are derived. Simonoleman, PhD, advised on the regression analyses.

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Suppliers. Medical Engineering Unit, University of Nottingham, Queens Med-

ical Centre, Nottingham, NG7 2UH, UK.. Version 12; SPSS, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.. Version 4.0; Graphpad Prizm, 11452 El Camino Real, #215, San

Diego, CA 92130.

Arch Phys Med Rehabil Vol 89, April 2008