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Relationship Between Muscle Strength and the Time Taken to Complete a Standardized Walk-Turn-Walk Test

^a National Institute on Aging, Gerontology Research Center, Baltimore, Maryland
^b Johns Hopkins Bayview Medical Center, Baltimore, Maryland
^c Department of Kinesiology, University of Maryland, College Park
^d The Johns Hopkins University, School of Nursing, Baltimore, Maryland
^e Florida Gerontological Research and Training Services, Palm Harbor

E. Jeffrey Metter, National Institute on Aging, Gerontology Research Center, 5600 Nathan Shock Drive, Baltimore, MD 21224 E-mail: metterj{at}grc.nia.nih.gov.

Decision Editor: John Faulkner, PhD

	Abstract

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We examined the effects of age and gender on the relationship between knee strength and walking time during a walk-turn-walk test in 176 male and 168 female generally healthy participants of the Baltimore Longitudinal Study of Aging who were aged 21–89 years. Subjects were timed as they walked 50 ft (15.24 m), turned around, and walked back to the starting point, both at a comfortable pace and as fast as possible. Isokinetic concentric knee extensor strength was measured at 30°/s by using a Kin-Com dynamometer. Both comfortable and fast gait times increased with increasing age for both women and men, starting in middle age. An interaction was found between gender and age showing that older women are slower than older men at both paces. Gait time decreased linearly with increasing knee extensor strength, plateauing at higher strength levels (>130 N m for comfortable gait, and 190 N m for fast gait). Most women occupied the linear part of the curve below the plateau. Adjustment for body size, age, physical activity, and particularly number of steps to complete the task removed the relationship between strength and gait time for the comfortable gait. Women took longer to complete the walk-turn-walk test than men at older ages, were on the linear part of the strength–gait time relationship, and used more steps to complete the task, all of which may contribute to their greater likelihood of frailty in later years.

INCREASING age is associated with greater susceptibility to physical disability, which can lead to frailty in the elderly population ⁽¹⁾. Women live longer and tend to have more years of functional dependence than men, and they have a greater likelihood of becoming frail ⁽²⁾⁽³⁾. In addition, women are smaller, with less muscle mass and muscle strength, and thus have less functional reserve.

Functional reserve has been defined as a level of physical fitness (frequently considered in relation to muscle strength) beyond which further increases in fitness do not lead to further improvements in physical function ⁽⁴⁾. Because muscle mass and strength decline by as much as 50% to 60% with increasing age ^{(5)(6)(7)(8)(9)}, functional reserve should also decline, contributing to increased frailty ^{(4)(10)(11)(12)(13)(14)(15)}. Little attention has been given to this relationship in young healthy adults.

Gait is an important component of personal mobility and functional independence. Gait speed declines, as does stride length, with increasing age ^{(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)}. Guralnik and colleagues ⁽²⁶⁾ have found that simple measures such as timed gait are correlated to functional performance and disability. Furthermore, timed gait has been used to study the nonlinear relationship between physical function and muscle strength, with no difference between older women and men in their strength-to-gait speed relationship ⁽¹⁵⁾.

In a cross-sectional analysis, we examined the age-associated changes in timed gait by using a walk-turn-walk test in healthy female and male participants of the Baltimore Longitudinal Study of Aging (BLSA); the participants were aged from 21 to 89 years. This test adds the element of a turn to the linear walk that is typically used to evaluate gait. Specifically, our objectives were as follows: ⁽¹⁾ to evaluate the relationship between age and timed gait for this test, ⁽²⁾ to determine the relationship between knee extensor muscle strength and gait time by sex, and ⁽³⁾ to ascertain the contribution of body size, physical activity, and number of steps to the gait–strength relationship.

	Methods

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Subjects
Subjects consisted of 344 (176 men and 168 women) generally healthy participants in the BLSA ⁽²⁷⁾ who had timed gait and muscle strength testing. BLSA participants tend to be well educated, with over two thirds having some college education. All subjects receive a health status evaluation, including a thorough physical examination. Subjects were excluded who had clinical cardiovascular, neurological, or musculoskeletal diseases, and who had any recent major surgery within 6 months or potential aggravating conditions. Height and weight were checked at each visit, with weight measured to the nearest 0.1 kg and height to the nearest 0.1 cm. Written informed consent was obtained from the subjects prior to entry into the study. This study was approved by the Johns Hopkins Bayview Medical Center Institutional Review Board for human subjects.

The Walk-Turn-Walk Test
The self-paced test was performed in an air-conditioned nursing unit. Subjects were dressed in street clothes, wearing their own shoes. Subjects were asked to walk 50 ft (15.24 m) down a measured hospital corridor, turn around, and return to the starting point. Participants began the test with both feet touching the starting line and started on command from a stopped position. First, the subjects were asked to walk at their usual pace and then as rapidly as possible, with timed gait measured by stopwatch (to 0.1 second). Steps were counted by the tester.

Measurement of Muscle Strength
Detailed descriptions of the muscle strength testing have been reported ⁽⁸⁾⁽⁹⁾. Lower extremity strength was measured with an isokinetic dynamometer (Kinetic Communicator Model 125E Plus, Chattecx, Chattanooga, TN; Kin-Com) in the dominant leg. Maximal voluntary concentric peak torques of knee extensor and flexor muscle groups were measured at angular velocities of 0.52 rad/s (30°/s). The highest peak torque (N m) value of three maximal tests was used as peak torque.

Leisure Time Physical Activity Questionnaire
Participants were asked to estimate the amount of time spent performing 97 activities on a typical day since their last visit. A description of leisure time physical activity (LTPA) in the BLSA has previously been reported ^(28)(29)(30). The intensity of each activity was expressed in metabolic equivalents of oxygen uptake (MET), that is, 3.5 ml kg^-1 min^-1, based on a coding catalog of physical activity ^(31)(32). The data were normalized to 1440 minutes and then multiplied by the assigned MET value, resulting in the quantitative expression of LTPA into MET-minutes ⁽³⁰⁾. Activities were grouped into three categories: ⁽¹⁾ low-intensity LTPA (activities <4 MET), ⁽²⁾ moderate-intensity LTPA (activities between 4 and 5.9 MET), and ⁽³⁾ high-intensity LTPA (activities 6 MET). In this study, we examined moderate- and high-intensity LTPA.

Data Analysis
Statistical analysis was performed by using SPSS (version 9) for Windows statistical package (SPSS, Chicago, IL). The significance level for all analyses was p < .05. For comparing differences in physical characteristics, gait time, and muscle strength, subjects were grouped by age decade and gender (Table 1 ). A two-way analysis of variance (ANOVA) was used to test for age and gender differences.

As a way to estimate whether a break point occurred in the gait time and strength curve, piecewise linear regression models were applied by using nonlinear regression options in SPSS. The piecewise model identifies a conditional strength level in which a change in slope occurs in the relationship to gait time. The model identifies the most plausible point for where the slope is projected to change. If the resulting slope approaches zero (following a linear decline in gait time in relationship to muscle strength), the model would be consistent with the presence of a reserve capacity threshold level. The model tested was as follows:

where break refers to the strength level where the change in slope optimally occurs. The program uses a maximum likelihood method to estimate the coefficients and break point.

	Results

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Timed Gait Change with Age
Subject characteristics and gait parameters by age and gender are presented in Table 1 . Men were significantly taller, heavier, and stronger than women (p < .01). The concentric peak torque of knee extensor and flexor muscle groups decreased with age, particularly above age 60 years, similar to previous reports from the BLSA ⁽⁸⁾⁽⁹⁾. Because of a high correlation between knee extensor and flexor strength (r = .85, p < .0001), the remaining analyses examine only extensor strength.

For both fast and comfortable gait time there was an increase with age decade (p < .0001). A two-way ANOVA, examining the fast gait time (r² = .28), found no interaction between age decade and gender (p = .47), whereas the main effects for sex (p < .001) and age group (p < .001) were significant. Women on average took 2.0 seconds longer after adjustment for age decade than men to do the walk-turn-walk test at the fast pace. For comfortable gait time (r² = .27), no interaction was found between age and sex (p = .34), while the main effect for sex (p = .12) was not significant and age decade was significant (p < .001).

By linear regression, a quadratic term for age significantly improved the model (p .0001) between age and both fast (r² = .29) and comfortable (r² = .28) gait time, and a significant interaction was found between sex and age (p < .0001; Fig. 1). Women and men had similar gait times at younger ages, whereas women were slower in middle and old age.

View larger version (15K): [in this window] [in a new window]   Figure 1. Fast and comfortable gait speed by age, analyzed separately for women and men: A, fast gait and B, comfortable gait. Regression lines including a quadratic term are fitted separately for women and men.

View larger version (16K): [in this window] [in a new window]   Figure 2. Relationship between muscle strength and gait speed for women and men: A, fast gait and B, comfortable gait. Regression lines are fitted and include a separate quadratic term for women and men.

Contributors to the Muscle Strength, Gait Speed Relationship
We examined contributors to the relationship of gait time to strength by sequentially adding sex, age, height, weight, moderate-intensity physical activity, high-intensity physical activity, and number of steps as predictors (Table 2 ). A sex by age interaction was tested but was not significant, and it deleted from the final models. Table 2 presents the analyses for fast and comfortable gait times. Each line of the table is a regression analysis with corresponding coefficients for all subjects (All), and for women and men separately. Nonsignificant variables are shown by the presence of an asterisk (*). Height and weight improved the prediction to some degree, whereas LTPA (both moderate and high intensity) had no impact. (The model with these two terms added was omitted from Table 2 .) The most important contributor to the regression equations was clearly the number of steps, which for the comfortable gait increased the explained variance from <30% to over 60%. Adjusting the strength–gait speed relationship by age, height, weight, and LTPA did not remove the significant strength and gait time relationship, whereas the addition of number of steps made the relationship between strength and gait time nonsignificant for comfortable gait and for women's fast gait (Table 2 ).

	Discussion

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We found that comfortable and fast gait times in the walk-turn-walk test increased with age, particularly in individuals older than 60 years, which is consistent with other reports that used a linear walk ^{(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)}. The age effect persists for gait times even after strength, sex, and body size differences are accounted for. A difference was found in how gait times change with age for women and men. Young women had similar gait times as young men, whereas older women were slower than older men for both comfortable and fast gait (Fig. 1). A linear relationship was observed between gait time and muscle strength only to a specific strength level (e.g., 190 N m for knee extensor strength and fast gait, and 131 N m for slow gait), above which the relationship plateaued, suggesting the presence of functional reserve at the higher strength levels. However, women never reached this plateau for fast gait, so their performance is linearly related to strength. Even at a comfortable pace, few women were above the threshold level. Being on the linear part of the strength–gait relationship, and not being as strong as men, puts women at risk as they age and lose muscle strength and mass.

The use of the turn in the walk-turn-walk test makes comparisons difficult with other studies that have used a linear walk. When the performance time for the walk-turn-walk test was converted to gait speed (data not reported), the gait speed in BLSA subjects was slower ^(12)(23), similar ^(18)(33)(34), and faster ⁽¹⁵⁾ than speeds reported in other studies on straight walks without the turn. The presence of a turn with its associated deceleration and subsequent acceleration likely caused some slowing of gait speed, but overall seemed to have little impact on speed compared with other reports. Other test differences are harder to consider. For example, differences in specific instructions can lead to substantial differences in performance. Overall, the performance of our subjects appears similar to that given in other reports ^{(12)(18)(23)(33)(34)}.

Physical performance, including walking, is associated with muscle strength ^{(4)(10)(15)(34)(35)}, which declines with age, as demonstrated here and elsewhere ⁽⁸⁾⁽⁹⁾. Less is known about whether the relationship between strength and performance is similar for men and women. A nonlinear relationship was found between muscle strength and both fast and comfortable gait, suggesting that a reserve threshold level could be defined, as others have shown ^(10)(15)(35). The statistical methods used did not lead to a precise estimate for a strength threshold; for example, a 95% CI for fast gait for men was 151.5 to 228.5 N m. Rather, they present a range that could represent the level of uncertainty or that there is no specific value but rather a curvilinear change as represented by a quadratic relationship (Fig. 2). In our piecewise models, knee strength explained 17% of variance of fast gait for men and 12% for women, which is similar to the 17% of gait speed variance found by Buchner and colleagues ⁽¹⁵⁾.

Both women and men showed similar relationships between strength and gait times; however, women and men occupied a different part of the curve. Women were on the linear part of the curve, showing a direct relationship between muscle strength and gait time. For the fast gait time, none of the women reached the plateau level that indicated the presence of functional reserve for knee extensor concentric muscle strength. At comfortable gait, the majority of women were also on the linear part of the curve (Fig. 2). The strength–gait time relationship should have an impact on middle-aged and older subjects, whose muscle strength declines, and in some individuals, particularly women, strength approaches levels below which comfortable performance is difficult to impossible.

Other factors are associated with gait performance and should have an impact on the walk-turn-walk test. We have examined several of these, including height, weight, physical activity, and number of steps to complete the task. A dampening of the strength–gait time relationship occurred when adjusted for body size and physical activity. The main factor accounting for gait time is the number of steps required for the task (Table 2 ), despite being particularly prone to measurement error. For the walk-turn-walk test, the number of steps appears to consolidate a number of important factors contributing to performance, including at least body size, sex, physical activity, and strength. It is likely that women compensate for their smaller body size and strength by means of other factors, including step frequency. With increasing age, as muscle strength declines, women have less ability to compensate for the changes, making it more likely that they will develop functional disability.

Clearly, differences in height, weight, and muscle mass can affect performance on timed gait. Buchner and colleagues ⁽¹⁵⁾ noted that body weight and not height was important. We have observed such contributions in this study. However, after adjustments, strength continues to have an independent, though modest, effect on gait speed. In previous studies ^(7)(8)(9), we have noted that the force generated per unit of muscle is higher in men than women across the adult life span. These findings exist in both upper and lower extremity muscles that act across the knee and elbow. Other factors are important and which likely include age and gender differences in reaction time ⁽³⁶⁾, movement time and accuracy ⁽³⁷⁾, and muscle contraction factors ⁽³⁸⁾.

Conclusions
In summary, the walk-turn-walk test at comfortable and fast paces distinguishes older but not younger men and women. An association exists between knee extensor concentric peak torque and gait time that is nonlinear, with a plateau level above which increasing strength does not lead to faster gait times. Others using a linear walk have seen this pattern. Both men and women show a similar strength-to-gait time relationship but occupy different parts of the curve. A greater number of men occupy the plateau in the relationship than women. The lack of reserve capacity, particularly in older women, is likely an important component of frailty.

	Acknowledgments

 
The BLSA is part of the Intramural Research Program of the National Institute on Aging.

We thank the participants and staff of the Baltimore Longitudinal Study of Aging for their cooperation in the study.

Received September 29, 1999

Accepted April 20, 2001

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