Archives of Physical Medicine and Rehabilitation
Volume 89, Issue 4 , Pages 677-683, April 2008

Association of Activity Limitations and Lower-Limb Explosive Extensor Power in Ambulatory People With Stroke

  • David H. Saunders, MPhil

      Affiliations

    • Department of Physical Education, Sport and Leisure Studies, University of Edinburgh, Edinburgh, UK
    • Corresponding Author InformationReprint requests to David H. Saunders, MPhil, Scottish Centre for Physical Education Sport and Leisure Studies, University of Edinburgh, St Leonards Land, Holyrood Rd, Edinburgh, EH8 8AQ, Scotland
    • ,
  • Carolyn A. Greig, PhD

      Affiliations

    • Geriatric Medicine, School of Clinical Sciences and Community Health, University of Edinburgh, Edinburgh, UK.
    • ,
  • Archie Young, MD

      Affiliations

    • Geriatric Medicine, School of Clinical Sciences and Community Health, University of Edinburgh, Edinburgh, UK.
    • ,
  • Gillian E. Mead, MD

      Affiliations

    • Geriatric Medicine, School of Clinical Sciences and Community Health, University of Edinburgh, Edinburgh, UK.

Article Outline

Abstract 

Saunders DH, Greig CA, Young A, Mead GE. Association of activity limitations and lower-limb explosive extensor power in ambulatory people with stroke.

Objective

To determine whether the explosive lower-limb extensor power of the affected and unaffected sides, and any asymmetry, are associated with activity limitations after stroke.

Design

Cross-sectional observational study of baseline data from a randomized controlled trial.

Setting

Measurements made in a hospital clinical research facility.

Participants

Community-dwelling (N=66) subjects with stroke who were independently ambulatory. Subjects’ mean age was 72±10 years.

Interventions

Not applicable.

Main Outcome Measures

The lower-limb extensor power of each lower limb (in W/kg), performance of specific functional activities (comfortable walking velocity, Functional Reach Test, chair-rise time, Timed Up & Go test), and global indices of activity limitation (FIM instrument, Rivermead Mobility Index, Nottingham Extended Activities of Daily Living).

Results

Low lower-limb extensor power in either lower limb was the principal factor from among the confounders we recorded that significantly (R2 range, .21–.46) predicted the limitation of specific functional activities, and low lower-limb extensor power in either lower limb was the principal predictive factor for global indices of activity limitation (R2 range, .13–.38). The degree of asymmetry of lower-limb extensor power between legs was low and had little or no predictive value.

Conclusions

In ambulatory persons with stroke, activity limitations are associated with deficits in lower-limb extensor power of both lower limbs, and not the severity of any residual asymmetry. These findings suggest that interventions to increase lower-limb extensor power in both lower limbs might reduce activity limitations after stroke.

Key Words: Activities of daily living, Cerebrovascular accident, Physical fitness, Rehabilitation

 

THE ABILITY OF MUSCLE to generate force can be described in terms of muscle strength and explosive power. Strength is the magnitude of maximal force generation whereas explosive power output is a velocity-dependent characteristic defined as the greatest rate of work achieved during a single, ballistic, resisted contraction.1 Explosive power deteriorates faster than strength (3%–4% vs 1%–2% a year) during healthy aging.2

Although strength and power are both important for execution of functional activities, lower-limb extensor power is more important than knee extensor strength for stair climbing, chair rising, and walking,3, 4, 5 and when impairment is asymmetrical, lower-limb extensor power is a better predictor of the frequency of falling than strength alone.6

Although people who have survived a stroke are often elderly, may be less active than prior to their stroke, and may have unilateral limb weakness, surprisingly little is known about the extent to which explosive power might be impaired and whether this might have adverse functional consequences. A pilot study of 11 ambulatory subjects 1 year after stroke with virtually no residual neurologic deficit7 found that both lower-limb extensor power and knee extensor muscle strength of both lower limbs were substantially lower than that of age- and sex-matched healthy subjects.8, 9 Further unpublished data indicated that impairment in lower-limb extensor power was approximately double that of muscle strength.

Bilateral impairment in explosive power,7 or muscle strength,7, 10, 11 observed after stroke could arise for several reasons both directly and indirectly associated with stroke. First, bilateral motor deficits can arise directly from a unilateral lesion.12 Second, reduced habitual physical activity, either before and/or after stroke may cause muscle atrophy.13 Third, the presence of comorbid disease (including poor nutrition) before and/or after stroke could impair motor function.

Low muscle strength after stroke is associated with poor performance of walking and stair climbing,14 chair rising,15 and impaired motor function.16 Only 1 small study (N=14) has explored the functional associations of explosive power after stroke.17 It showed that asymmetry in lower-limb extensor power was associated with reduced walking performance. The participants were unusually young (mean, 46.4±8.4y), and this relationship should be examined in people with stroke of more typical age (ie, >70y).18 Moreover, the relationship of power with other aspects of activity limitation should be examined to explore the potential benefits that might result from attempts to improve explosive power after stroke. This is important because fitness training can be presented in such a way as to specifically improve explosive power19 and this might reduce activity limitations, and so reduce participation restriction after stroke.

The aim of this study was to determine in older, ambulatory people with stroke, whether the lower-limb extensor power of the affected and unaffected sides, and any asymmetry, were associated with (1) performance of specific functional activities (reaching, walking, and rising from a chair), and (2) global indices of activity limitation (FIM instrument, Nottingham Extended Activities of Daily Living [NEADL], Rivermead Mobility Index [RMI]).

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Methods 

Participants 

All participants in this study (N=66) were recruited to a randomized trial of exercise or relaxation after stroke (table 1).20 We selected these 66 patients after screening 301 patients for trial eligibility (all 301 had required either inpatient or outpatient care after an acute stroke in 1 of 4 Edinburgh hospitals). Trial inclusion criteria were (1) independently ambulatory (with or without walking aids), (2) living within the recruitment catchment area, (3) completion of inpatient and outpatient stroke rehabilitation, and (4) absence of dysphasia or confusion judged severe enough to prevent safe participation in exercise or relaxation classes or to preclude informed consent. Absolute contraindications to exercise in elderly people21 and walking limited by pain were applied as exclusion criteria. Approval was obtained from the local research ethics committee.

Table 1. Participant Characteristics
CharacteristicsnMean ± SDMedian (IQR)
Age (y)NA71.85±9.91NA
Sex (male/female)36/30NANA
Stature (m)NA1.67±0.09NA
Time from stroke (d)NANA152 (83–278)
Smoking history
Smoker/Ex-/Non-/UC25/12/28/1NANA
Walking aids
Stick/orthosis/zimmer/none28/4/2/32NANA
Body mass (kg)NA72.64±15.29NA
Stroke type (TAC/LAC/PAC/POC/UC)2/19/32/12/1NANA
Lesion type (ischemic/hemorrhagic/UC)60/5/1NANA
Lesion side (left/right/both/UC)37/27/1/1NANA
Hospital care (inpatient/outpatient)56/10NANA
Inpatient length of stay (d)NANA19 (9–44)
Blood pressure
Systolic (mmHg)NA140.03±18.10NA
Diastolic (mmHg)NA73.16±9.50NA
Comorbidities
Prior stroke11NANA
Prior transient ischemic attack4NANA
Ischemic heart disease22NANA
Left ventricular failure2NANA
Hypertension31NANA
Prior malignancy7NANA
Diabetes3NANA
Miscellaneous50NANA
None5NANA
Total no. per participantNA1.97±1.35NA

Abbreviations: IQR, interquartile range; LAC, lacunar; NA, not applicable; PAC, partial anterior circulation; POC, posterior circulation; SD, standard deviation; TAC, total anterior circulation; UC, unclear.

For participants who presented with no lateralizing signs but had relevant stroke lesions evident on brain imaging, we considered the affected side to be ipsilateral to the side of posterior circulation lesions, and contralateral for all others.

Measurements 

Prior to randomization, we measured lower-limb extensor power during hip and knee extension while the subject was seated on a Nottingham Power Rig.22,a Ten maximal pushes were encouraged using each lower limb with a rest (minimum 30s) between each push. Mitchell et al23 reported that 10 repetitions were sufficient to obtain peak lower-limb extensor power values in elderly people rehabilitating after proximal femoral fracture. Power to body mass ratio (W/kg of body mass) was recorded for each push and the highest value achieved was recorded for the affected and unaffected lower limbs. Asymmetry in lower-limb extensor power was expressed as a ratio (affected lower-limb extensor power/unaffected lower-limb extensor power) and used to indicate hemiparesis. The lower-limb extensor power technique is valid and reliable in healthy elderly people22 and reliable in persons with stroke.17

Functional Reach Test (FRT),24 Timed Up & Go (TUG) test,25 and chair-rising time9 were recorded in triplicate. Participants were asked not to use walking aids or arms during chair rising. The average velocity of comfortable walking was determined during three 3-minute bouts of self-paced walking around a 17-m circuit with a 5-minute break between each walk. Participants were instructed to walk at their “comfortable pace.”26 The following global indices of activity limitation were recorded during face-to-face interview: FIM instrument,27 RMI,28 and NEADL.29 The above measures have been found to be reliable in persons with stroke30, 31, 32, 33 or elderly people.34

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

Data Analysis 

Normally distributed data were reported as mean and standard deviation (SD). Non-normal data were expressed as median and interquartile range (IQR) and transformed to a normative distribution prior to any statistical analysis. Affected and unaffected legs were compared using a paired t test. Stepwise multiple linear regression was used to identify (1) whether any of the confounders predicted lower-limb extensor power, and (2) whether lower-limb extensor power and confounders predicted activity limitation measures. When lower-limb extensor power was the only significant predictor of activity limitation, the regression coefficients were used to generate nonlinear models (and 95% confidence interval [CI]) of the untransformed graphed data. Analyses were performed with SPSSb and Graphpad Prizm.c A P value of less than .05 was considered statistically significant.

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Results 

A successful measure of lower-limb extensor power was achieved in both legs of 60 (91%) of 66 participants and at least 1 leg of 65 (98%) of 66 participants. The reasons preventing data collection were leg pain (n=4) and equipment failure (n=2). The average lower-limb extensor power increased by 76% (affected) and 55% (unaffected) throughout the 10 repetitions, but approached asymptotic values between repetitions 8 and 10 during which further increase was trivial (2.3% affected leg, 0.3% unaffected leg). The proportion of participants with ceiling values in the global indices was low (FIM instrument, 2/66; RMI, 11/66; NEADL, 3/66). The data for lower-limb extensor power, specific functional activities, and global indices of activity limitation are summarized in table 2.

Table 2. Untransformed Data for Lower-Limb Extensor Power, Measures of Performance of Specific Functional Activities, and Global Indices of Activity Limitation
VariablenMean ± SDMedian (IQR)
Lower-limb extensor power
Affected side LLEP (W/kg)64NA0.92(0.53–1.49)⁎, †
Unaffected side LLEP (W/kg)61NA1.05(0.73–1.56)†
Asymmetry ratio (aff LLEP/unaff LLEP)600.89±0.24NA
Specific functional activities
FRT (cm)6326.53±6.65NA
Comfortable walking velocity (m/s)640.67±0.24NA
TUG test (s)61NA11.68(8.17–16.09)‡
Chair-rise time (s)60NA1.28(0.83–1.70)‡
Global indices of activity limitation
FIM instrument66NA117.5(114–122)§
RMI66NA13(11–14)§
NEADL65NA17(12–19)§

Abbreviations: aff, affected; LLEP, lower-limb extensor power; unaff, unaffected.

⁎Affected LLEP lower than unaffected LLEP (t=3.77, P<.001).

†The non-normal data included in this table could be transformed to a normal distribution using square root.

‡The non-normal data included in this table could be transformed to a normal distribution using reciprocal.

§The non-normal data included in this table could be transformed to a normal distribution using square root of reflected data.

Median affected lower-limb extensor power was 42% (IQR, 27–66) and unaffected lower-limb extensor power was 54% (IQR, 37–71) of that expected in age- and sex-matched subjects.8, 9 Affected lower-limb extensor power was significantly lower than unaffected lower-limb extensor power (t=3.77, P<.001), but the difference was small (≈10%; median, .14W/kg) and the extensor power of each lower limb were highly correlated (R2=.68, P<.001). When the influence of age, sex, time since stroke, smoking, and incidence of comorbid disease(s) on lower-limb extensor power was examined, unaffected lower-limb extensor power was predicted (weakly) only by sex and age (R2=.18, P=.001), and affected lower-limb extensor power by sex only (R2=.14, P=.002). None of the factors predicted the asymmetry ratio.

Low values of affected or unaffected lower-limb extensor power appear associated with limitation in each specific functional activity (fig 1). Lower-limb extensor power showed pronounced curvilinear associations with chair-rising time, and TUG test. When walking speed (in m/s) was expressed as a function of time (in s/m), the same curvilinear association was observed. All 3 dynamic physical functions showed reduced performance when lower-limb extensor power was below approximately 1W/kg with no increase in performance above this value.

  • View full-size image.
  • Fig 1. 

    The relationships between the explosive lower-limb extensor power (LLEP), measured in W/kg, of the affected (■) and unaffected (□) lower limbs and performance of specific functional activities assessed using the FRT, TUG test, chair-rising time, and comfortable walking velocity. When arms were used for assistance during chair rising and TUG test the data are marked differently (× affected, + unaffected). For clarity the lines of best fit with 95% CIs are included only for the unaffected lower-limb extensor power data.

Both affected and unaffected lower-limb extensor power were significant predictors of performance in each functional activity (table 3). Comfortable walking velocity, chair-rise time, and TUG test performance were predicted exclusively by affected and unaffected lower-limb extensor power, with each leg having similar influence. Five participants with low values of lower-limb extensor power (<1W/kg) found chair rising impossible without using their arms; their data were excluded from the regression analysis of the TUG test and chair rising. Functional reach was predicted by lower-limb extensor power, but not exclusively or as strongly as were other activities. The ratio of affected/unaffected lower-limb extensor power had no predictive importance for performance of specific functional activities.

Table 3. The Results of Stepwise Multiple Linear Regression Analysis of the Lower-Limb Extensor Power (affected side, unaffected side, ratio) and Potential Confounding Predictor Variables (age, sex, stature, time since stroke, use of walking aids, comorbid disease, smoking history) on Performance of Specific Functional Activities and Global Indices of Activity Limitation
Dependent VariableMeasure of LLEP Included in Regression Model
Affected Side for LLEPUnaffected Side for LLEPAsymmetry Ratio (affected LLEP/unaffected LLEP)
Specific functional activities
FRT⁎LLEP.32P<.01LLEP.48P<.001Stature.38
Stature.28P<.023Stature.54P<.001
Sex.39P<.019
R2.21P<.001R2.33P<.001R2.13P<.003
Comfortable walking speedLLEP.54 LLEP.65 Age−.31
R2.28P<.001R2.41P<.001R2.08P<.020
TUG testLLEP.68 LLEP.60
R2.46P<.001R2.35P<.001R2NANS
Chair-rise time⁎LLEP.63 LLEP.56 Age−.30
R2.38P<.001R2.30P<.001R2.07P<.026
Global indices of activity limitation
FIM instrumentLLEP−.64P<.001LLEP−.38
Stature.23P<.039
R2.35P<.001R2.13P<.003R2NANS
RMILLEP−.58 LLEP−.53
R2.33P<.001R2.27P<.001R2NANS
NEADLLLEP−.64P<.001LLEP−.41 LLEP−.29
Smoking.21P<.045
R2.38P<.001R2.16P<.001R2.07P<.027

NOTE. Standardized β coefficients are reported for each individual independent variable having significant predictive value, and adjusted R2 values for each overall model where this could be fitted.

Abbreviation: NS, no significant regression model solution.

⁎Use of walking aids omitted from models.

Lower-limb extensor power was nearly exclusive as a predictor of global indices of activity limitation from among the variables included in the regression models (fig 2, see table 3), the only exceptions being marginal contributions of stature to FIM instrument, and smoking to NEADL scores. Associations tended to be stronger for affected lower-limb extensor power than unaffected lower-limb extensor power, but asymmetry in lower-limb extensor power did not predict FIM instrument or RMI scores and had only marginal predictive value for NEADL scores.

  • View full-size image.
  • Fig 2. 

    The relationship between the explosive lower-limb extensor power, measured in W/kg, of the affected (■) and unaffected (□) lower limbs and global indices of activity limitation assessed using FIM instrument, RMI, and NEADL. For clarity the lines of best fit with 95% CIs are included only for the unaffected explosive lower-limb extensor power data.

When statistical analyses were repeated after excluding subjects with prior stroke (11/66) the multivariate R2 values increased slightly and marginal variables were dropped from the models, leaving affected and unaffected lower-limb extensor power as the exclusive predictors of performance or limitation of activities.

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Discussion 

This study shows that among a sample of ambulatory subjects with stroke (mean age, 72y), the unaffected lower-limb extensor power was lower than expected and that low lower-limb extensor power in either leg was associated with (1) reduced performance in some everyday dynamic functional activities that involve the legs, and (2) activity limitation as assessed using more global scale indices. Asymmetry in lower-limb extensor power was small and not predictive of limitations.

Our data suggest lower-limb extensor power is important for the performance of dynamic day-to-day lower-limb activities that require rapid rates of muscle contraction. Associations were strongest with comfortable walking velocity, TUG test, and chair-rising time. When lower-limb extensor power is very low, performance of chair rising may be impossible for some unless modified (eg, use of arms). This is compatible with similar observations in healthy elderly people.2 As expected, the weakest association between lower-limb extensor power (of either leg) and physical function was with functional reach, probably because this is not limited by speed of movement.

Our data show a convincing association between low lower-limb extensor power and increased global indices of activity limitation even though not all questions within each scale directly addressed performance of activities involving the lower-limb extensors.

In elderly persons with functional impairments, power output during leg-press exercise, a procedure similar to lower-limb extensor power determination, was found to be associated with stair climbing ability, chair-rise time and habitual gait velocity,36 and with self-reported functional status.5 These observations resemble the types of association found in our study.

In a small study of unusually young (46y) ambulatory subjects with stroke17 substantial asymmetry in lower-limb extensor power was observed (mean, 43%) and this was inversely associated with walking speed (Spearman ρ=−.76, P<.01). These younger participants had RMI values with a mean of 13 and walking speeds with a mean of .70m/s, which were similar to our data (see table 2). It is plausible that the greater lower-limb extensor power of their stronger side, with a mean of 1.99W/kg, allowed functional compensation. Asymmetry in our typically older participants (mean age, 72y) was not predictive of activity limitation to any important extent, probably because little asymmetry (10%) existed. This lack of asymmetry may have occurred because our participants had made a good neurologic recovery. Second, substantial asymmetry may be unusual in the older ambulatory person with stroke simply because lower-limb extensor power is already low prior to stroke, and a threshold effect limits the reduction in lower-limb extensor power that can occur without rendering the participant nonambulatory. The lack of asymmetry in our data suggests that the low values of lower-limb extensor power could have arisen due to the influence of factors that act bilaterally (ie, bilateral motor effects, comorbid disease, habitual physical inactivity).

Longitudinal poststroke deterioration could cause bilateral loss of lower-limb extensor power. Although no longitudinal data of lower-limb extensor power are available, 1 small study has reported an approximate 30% loss of strength of the ipsilateral leg during the week after stroke.10 Another, however, found no poststroke deterioration.37 In our study, neither time after stroke nor comorbid disease(s) were predictive of lower-limb extensor power or activity limitations, perhaps because our sample was homogeneous due to restrictive eligibility criteria. Although it is not possible to identify the underlying cause for low lower-limb extensor power and activity limitations, habitual physical inactivity before and/or after stroke remains a possible cause.

High-velocity resistance training in 25 healthy elderly persons (age, 60–80y) increased explosive power of the knee extensors and this is associated with significant improvements in chair rising, walking, and reaching ability.38 Extrapolating findings from studies of elderly people suggest that increasing affected and unaffected lower-limb extensor power might improve activity and independence after stroke. We are unaware of any studies to date that have examined this type of training after stroke.

We successfully measured peak lower-limb extensor power in more than 90% of our participants; this compares favorably with our experience of this measurement in healthy elderly people (78%) using the same equipment.8 This suggests that ambulatory persons who have had stroke can perform the repeated, high-velocity, resisted muscle contractions needed to improve explosive power. In addition, if lower-limb extensor power is impaired due to reduced habitual physical activity, there is no reason why reversal through suitable training should not occur. Therefore training lower-limb extensor power after stroke may be feasible.

Study Limitations 

The main limitation of this study was that we recruited a homogeneous sample of high functioning independently ambulatory subjects with stroke. Homogeneity may have limited the strength of the observed associations. The potential self-selection of fitter participants would also tend give rise to a higher functioning cohort. Participants had minimal hemiparesis so it is difficult to speculate on the functional importance of lower-limb extensor power for those with more severe impairment. Future work should therefore include more impaired participants and examine the role of other confounding factors, such as stage of motor recovery and pre- and poststroke habitual physical activity levels. Outcome measures could be extended to include indices of participation restriction.

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Conclusions 

In a sample of ambulatory subjects with stroke of mean age 72 years activity limitations were associated with bilateral deficits in lower-limb extensor power and not with the severity of any residual asymmetry. These data suggest that the feasibility and effectiveness of training interventions to improve muscle explosive power after stroke should be explored.

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Acknowledgments 

Gillian E. Mead, MD, was the principal investigator for a trial of exercise or relaxation after stroke from which these data are derived.

We thank the staff on the stroke units at the Royal Infirmary of Edinburgh, Astley, Ainslie, and Liberton Hospital, who assisted in identifying and recruiting patients. We thank Claire Fitzsimons, PhD, Alasdair MacLullich, PhD, Susan Shenkin, MD, and Gail Carin-Levy, BSc, who assisted with some patient assessments. Susan Lewis PhD analyzed the data from the STARTER (Stroke: A Randomized Trial of Exercise of Relaxation) trial, from which these data are derived. Simon Coleman, PhD, advised on the regression analyses.

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  • a Medical Engineering Unit, University of Nottingham, Queens Medical Centre, Nottingham, NG7 2UH, UK.
  • b Version 12; SPSS, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.
  • c Version 4.0; Graphpad Prizm, 11452 El Camino Real, #215, San Diego, CA 92130.

 Supported in part by the Chief Scientist Office of the Scottish Executive (grant no. CZB/4/46) and the Research into Ageing (fellowship no. 236).

 No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated.

PII: S0003-9993(07)01840-0

doi:10.1016/j.apmr.2007.09.034

Archives of Physical Medicine and Rehabilitation
Volume 89, Issue 4 , Pages 677-683, April 2008

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