The 6-Minute Walk Test in Mobility-Limited Elders: What Is Being Measured?

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1 Journal of Gerontology: MEDICAL SCIENCES 2002, Vol. 57A, No. 11, M751 M756 Copyright 2002 by The Gerontological Society of America The 6-Minute Walk Test in Mobility-Limited Elders: What Is Being Measured? Jonathan F. Bean, 1,2,3 Dan K. Kiely, 2 Suzanne G. Leveille, 2 Seth Herman, 2 Charlie Huynh, 1 Roger Fielding, 3 and Walter Frontera 1 1 Department of Physical Medicine and Rehabilitation, Harvard Medical School, Spaulding Rehabilitation Hospital, Boston, Massachusetts. 2 Research and Training Institute, Hebrew Rehabilitation Center for Aged, Boston, Massachusetts. 3 Department of Health Sciences, Sargent College of Health and Rehabilitation Sciences, Boston University, Massachusetts. Background. The 6-minute walk (6mw) is a well-established measure of aerobic capacity in elders with cardiorespiratory and peripheral vascular disease and may be an accurate measure of functional performance in healthy elders. In mobility-limited elders, a population at risk for disability, impairments in strength and power are predictive of performance-based measures of function. Though commonly utilized as an outcome measure among otherwise healthy mobilitylimited elders, it is not clear whether the 6mw best represents a measure of functional limitation, aerobic capacity, or both. Methods. We hypothesized that the 6mw would be strongly representative of performance-based measures of function being determined by impairments in muscle strength and power. We performed a cross-sectional analysis of 45 community-dwelling elders with mild to moderate mobility limitations. Results. The 6mw was strongly associated with established functional measures (r.61.83; p.001), but was poorly associated with indirect measures of aerobic capacity (r.25; p.05). Multivariate linear regression models demonstrated that impairments in leg strength and power, especially those at the knee and ankle, were predictive of 6mw performance. Conclusions. These findings emphasize the 6mw as a measure of functional limitation among mobility-limited elders without cardiorespiratory or peripheral vascular disease. A EROBIC capacity is recognized as an important predictor of mortality and morbidity in older adults (1,2). Though the exercise tolerance test is considered the gold standard measure of aerobic capacity, for many older adults, this test is too formidable to complete (3). The 6-minute walk (6mw) test is a well-established, valid, and reliable measure of aerobic capacity in elders with cardiac, peripheral vascular, and respiratory disease (4 7) and has been considered an alternative measure of aerobic capacity. Elders with mobility limitations represent almost 20% of the individuals older than the age of 65, and due to these limitations, they are at increased risk for disability and mortality (8,9). In this population, as well, the 6mw test has been used as a surrogate measure of submaximal aerobic capacity. Though recognized for its reliability (6), its validity for these purposes has yet to be sufficiently determined. In a population of healthy elders, the 6mw test was correlated, however, with performance-based measures of functional limitation such as chair stand time and gait velocity (10). These and other functional measures are predictive of disability and mortality (9) and are determined by impairments in lower extremity power and strength (11,12). These facts taken together confuse the meaning of the 6mw test in the mobility-limited population. Is this established test of aerobic capacity in individuals with cardiopulmonary and peripheral vascular disease a representative measure of functional limitations and impairments in leg strength and power in mobility-limited elders? Our study addressed these questions utilizing the disablement constructs advocated by Jette (13). Specifically, we defined impairments as abnormalities within an organ system (e.g., weakness) and functional limitation as restriction in the performance of basic physical actions (e.g., slowed gait speed). We hypothesized that in mobility-limited elders without cardiovascular or respiratory disease, the 6mw test would be a good measure of functional performance, being determined by impairments in lower extremity strength and power. METHODS Study Design This was a cross-sectional analysis performed on baseline data from a randomized single-blinded intervention trial in mobility-limited elders (14). Study Population Recruitment of subjects (N 45) was conducted in the greater Boston Metropolitan Area for the purpose of participating in the intervention trial. Once consent was obtained, a screening physical performance test was conducted. Eligible subjects received a comprehensive history and physical examination. Inclusion criteria were community-dwelling M751

2 M752 BEAN ET AL. persons age 65 or older, mobility limitations as defined by a score 11 (out of 12) on the Short Physical Performance Battery (SPPB) (15), and the ability to climb a flight of stairs independently. Exclusion criteria included unstable acute or chronic disease (including unstable cardiovascular or respiratory disease), a score 23 on the Folstein Mini- Mental State Examination (MMSE) (16), or a neuromusculoskeletal impairment interfering with independent stair climbing. Subjects who met these criteria underwent a screening submaximal exercise tolerance test conducted according to established guidelines (17). If eligible, subjects completed baseline testing. One hundred and eighty-six inquiries were solicited via advertising in local papers and direct mailings. After initial telephone screening, 57 potential subjects were eligible and were invited to participate in a screening assessment. None of the potential subjects had symptomatic congestive heart failure, coronary artery disease, chronic obstructive pulmonary disease, or peripheral vascular disease. Of the potential subjects, four were excluded for medical reasons and eight chose not to commit to the study. Therefore, a total of 45 subjects (34 women, 11 men) were eligible, representing 79% of the potential subjects. Physical Performance Measures The performance measures included the 6mw test, stair climb time, chair stand time, habitual and maximal gait speed, and the SPPB. All are summarized below and are described in greater detail elsewhere (12,14). Time was recorded by stopwatch. For the 6mw test, subjects were instructed to walk at their normal walking pace for 6 minutes. The distance was recorded by a rolling measuring wheel. For stair climb time, subjects ascended a standard 10-stair flight of stairs as quickly as possible using the handrail if necessary. Though the five-repetition chair stand is a component of the SPPB, we used the 10-repetition chair stand test individually to better discriminate performance differences. Using a chair with a seat height of 0.43 meters, subjects were instructed to stand up and sit back down as fast as they could for 10 repetitions. Both habitual and maximal gait velocities were measured to the nearest 0.01 second. For the SPPB, testing involved an assessment of standing balance, a timed 2.4-meter walk, and a timed test of five chair rises. Each of the three tests was scored between 0 4, leaving a maximum score of 12 for those individuals performing at the highest levels (15). Guralnik and colleagues have characterized mobility limitations and their subsequent disability risk utilizing these scores (mild: 10 12; moderate: 7 9; severe: 4 6) (9,18). Physiologic Measures (Independent Variables) Our physiologic measures included muscle power and strength measurements at the hip, knee, and ankle, and a submaximal treadmill exercise tolerance test. Details of our methods are described elsewhere (11,12). Maximal dynamic strength of the hip and knees was assessed by one repetition maximum (1RM) measures of recumbent double leg press (DLP) (Newtons) and individual knee extensors (KE) (Newton-meters; N-m). The 1RM measurements were conducted using seated KE and recumbent DLP pneumatic resistance machines customized with software and digital displays (Keiser Sports Health Equipment, Inc., Fresno, CA). Following measurement of the 1RM, assessment of DLP and KE peak muscle power was performed using the same pneumatic resistance machines. Peak power was defined as the highest power achieved, regardless of percentage of the 1RM at which it was recorded. For the majority of subjects, this occurred at 70% of the 1RM. Ankle plantar flexion strength and power were measured utilizing a KIN-COM Isokinetic Dynamometer (Chettecx Co., Chattanooga, TN). Strength was recorded as the peak force generation of five repetitions performed at 0 per second (isometric strength). Peak power was recorded as the maximum of five repetitions at a speed of 90 per second. Aerobic capacity was measured by performing a treadmill exercise tolerance test (Woodway, Waukesha, WI) (14). To optimize compliance and reduce test burden, a submaximal test was performed utilizing indirect measures of aerobic capacity. Electrocardiograms were monitored throughout testing, and heart rate, blood pressure, and perceived exertion (Borg scale) (19) were recorded at the termination of each stage. After warm-up, speed was maintained at 80% of the subject s habitual gait speed. Following an initial 3-minute stage at 0% grade, the incline was increased 2% every 2 minutes until termination. Testing was terminated once perceived exertion was very hard ( 16) or heart rate achieved a level greater than 85% of the subject s predicted maximal heart rate, through the entirety of a stage. Heart rate, blood pressure, and test duration were recorded at the last fully completed stage of baseline testing and served as the parameters for comparison. Age, gender, health status, body mass index, depression (Center for Epidemiologic Studies Depression scale [CES- D]) (20), cognition, self-reported physical activity (Physical Activity Scale for the Elderly [PASE]) (21), and falls efficacy (Falls Efficacy Scale) (22) were all factors that could potentially influence our outcomes and were included as covariates (11,12). The number of chronic medical conditions for which subjects were undergoing treatment and were prescribed medications served as measures of health status. Additionally, it was recognized that the use of beta-blockers could influence measures of aerobic performance (17). Use of these medications was used as a covariate in analyses involving these variables. Statistical Analysis Descriptive statistics were calculated for all variables and were inspected for accuracy and external validity. Simple correlations were performed to evaluate the strength of the association between the 6mw test and our measures of submaximal aerobic capacity, the rate-pressure product (RPP), and test duration. Utilizing the 6mw test as the dependent variable and beta-blocker use as a covariate, separate linear regression models were calculated using RPP and test duration as independent variables. To evaluate the influence of impairments in lower extremity power and strength on 6mw test performance, we conducted six separate bivariate linear regression analyses. Utilizing the 6mw test as the dependent variable, models

3 6-MINUTE WALK IN MOBILITY-LIMITED ELDERS M753 were created evaluating the influence of power and strength at the hip, knee, and ankle. For bivariate models that were significant, multivariate linear regression models were performed that included the following covariates: age, gender, body mass index, health status, and falls efficacy. All analyses were performed using SAS (23), (SAS Institute, Cary, NC), and statistical significance was defined as alpha.05. RESULTS Baseline characteristics of the subjects are presented in Table 1. Subjects had a mean age of 72.7, were predominately female (75%), and were cognitively intact (mean MMSE, 29.4). On average, they had more than two chronic medical conditions (2.1) with minimal symptoms of depression (mean CES-D, 14.1). Subjects manifested mild to moderate mobility limitations as evidenced by a mean SPPB score of 9.4 and a habitual gait speed of 1.18 m/s (9). Physical activity levels (mean PASE, 121.8) and measures of lower extremity power (ankle, 25.3 watts; knee, 98.8 watts; hip, watts) and strength (ankle, 41.0 N-m; knee, 54.4 N-m; hip, N) are consistent with moderate-low levels of physical activity (11,21,24). Similarly, measures of submaximal aerobic capacity (RPP, 19,866 beats mm Hg; duration, 9.5 minutes) are consistent with low levels of physical activity (25). The 6mw test was well correlated with all of our measures of functional limitation (Table 2). In every case, the correlations were statistically significant (p.001), with coefficients ranging from r.61 for the SPPB to r.83 for stair climb time (chair stand time, r.62; habitual and maximal gait, r.80). Table 1. Baseline Characteristics for 45 (34 Female, 11 Male) Mobility-Limited Community-Dwelling Elders Without Symptomatic Cardiopulmonary or Peripheral Vascular Disease Characteristic Mean SD Age (years) Body Mass Index (kg/m 2 ) Diagnosis/Chronic Conditions Education (years) Mini-Mental State Exam Depressive Symptoms (CES-D) Falls Efficacy Physical Activity (PASE) SPPB score Habitual Gait (m/s) Minute Walk (m) RPP (beats mm Hg/min) 19, Duration (minutes) Lower extremity power (watts) Ankle plantar flexion peak power Knee extension peak power Hip double leg press peak Power Lower extremity strength (N-m) Ankle plantar flexion isometric strength Knee extension 1RM Hip double leg press 1RM 2, Note: CES-D Center for Epidemiologic Studies Depression Scale; PASE Physical Activity Scale for the Elderly; SPPB Short Physical Performance Battery; RPP rate-pressure product (heart rate systolic blood pressure); RM one repetition maximum. Domain Table 2. Correlations of the 6-Minute Walk Test With Performance-Based Measures of Function and Indirect Measures of Aerobic Capacity Measure Number of Subjects Correlation Coefficient p Value Functional Limitation SPPB Chair stand time Habitual gait Maximal gait Stair climb time Aerobic Capacity RPP Duration Note: SPPB Short Physical Performance Battery; RPP rate-pressure product (heart rate systolic blood pressure); Duration exercise tolerance test duration. In contrast, the 6mw test was poorly correlated with submaximal RPP (r.18, p.30) and test duration (r.21, p.17), in neither case reaching statistical significance. Adjusting for the use of beta-blockers did not materially alter these relationships (data not shown). Table 3 presents the bivariate linear regression models for the association of leg muscle power and strength with the 6mw test. In all cases, these bivariate relationships were statistically significant with power describing between (ankle R 2.37; knee R 2.29; hip R 2.21) and strength describing between (ankle R 2.24; knee R 2.27; hip R 2.20) of the total variance in 6mw. Because statistical significance (p.05) was achieved in all six models, corresponding separate multivariate analyses were conducted for each and are presented in Table 4. After adjusting for effects of age, gender, and other potential confounders, a strong independent association was observed between the 6mw test and measures of muscle power and strength. Only hip strength was found to have marginal significance (p.09). Inspection of the R 2 values indicate that models including power and strength at the ankle (power R 2.60; strength R 2.59) and knee (power R 2.59; strength R 2.57) describe more of the variance in 6mw than the hip (power R 2.50; strength R 2.48). The coefficients, the factor that best describes the ability of the selected power/ strength impairments to predict 6mw performance, were much greater at the ankle (power, 3.03 watts/m; strength, 2.29 N-m/m) and knee (power, 1.25 watts/m; strength, 2.74 N-m/m) as compared with the hip (power, 0.15 watts/m; strength, 0.03 N/m). Table 3. Bivariate Linear Regression Results Relating Measures of Lower Extremity Strength/Power and the 6-Minute Walk Test Lower Extremity Impairment Coefficient SE R 2 p Value Power (watts) Ankle plantar flexion power Knee extension power Hip double leg press power Strength (N-m) Ankle plantar flexion strength Knee extension strength Hip double leg press strength

4 M754 BEAN ET AL. Table 4. Multivariate Linear Regression Results Relating Measures of Lower Extremity Strength/Power and the 6-Minute Walk Test (Covariates Included Age, Gender, Health Status [Chronic Conditions], Body Mass Index, and Falls Efficacy) Lower Extremity Impairment Covariate Coefficient R 2 SE p Value Power (watts) Ankle plantar flexion power Age.003 Gender.18 Chronic conditions.10 Body mass index.27 Falls efficacy.59 Knee extension power Age.15 Gender.35 Chronic conditions.006 Body mass index.02 Falls efficacy.99 Hip double leg press power Age.08 Gender.17 Chronic conditions.03 Body mass index.26 Falls efficacy.85 Strength (N-m) Ankle plantar flexion strength Age.001 Gender.21 Chronic conditions.01 Body mass index.20 Falls efficacy.87 Knee extension strength Age.22 Gender.27 Chronic conditions.005 Body mass index.02 Falls efficacy.75 Hip double leg press strength Age.005 Gender.16 Chronic conditions.09 Body mass index.26 Falls efficacy.65 DISCUSSION In this study involving mobility-limited older adults with no significant cardiovascular or respiratory disease, the 6mw test was strongly associated with performance-based measures of functional limitation, yet was poorly associated with proxy measures of submaximal aerobic capacity. Consistent with other performance-based measures of function, impairments in leg power and strength were predictive of 6mw performance, with the greatest influence being at distal lower extremity sites. The association between the 6mw test and certain performance-based measures of function is supported by previous findings in healthy active older adults. In an observational study, Harada and colleagues had similar findings with chair stand (r.67) and gait speed (r.73) (10). They highlighted the need to examine the relationship between the 6mw test and lower extremity impairments. As evidenced by the findings in Tables 3 and 4, we found that leg power and strength, especially at the ankle and knee, are strongly predictive of 6mw test performance. In a previous report from this study, muscle power was demonstrated to consistently explain more of the variance in physical performance than strength, and these differences were accentuated when curvilinear regression models were utilized (12). In these linear regression models, this also occurred at all leg sites, though the differences were marginal. To more clearly identify the respective influence of strength and power, these findings should be replicated in larger, more functionally diverse population utilizing both linear and curvilinear models. The fact that power and strength at the more distal lower extremity sites (ankle and knee) are the most strongly predictive of 6mw test performance is best explained by recent biomechanical studies evaluating walking. Through a variety of studies on the walking abilities of healthy and mobilitylimited elders, it has been demonstrated that power and strength impairments in the lower leg are major factors influencing gait (26,27). Hip strength and power have less in-

5 6-MINUTE WALK IN MOBILITY-LIMITED ELDERS M755 fluence. Kerrigan and colleagues have specifically emphasized the importance of ankle force generation during walking (27,28), concurring with the findings that ankle power and strength predicted the greatest variance in 6mw test performance. The lack of association with submaximal aerobic capacity is worth noting. During fast walking, the most metabolically active tissue is skeletal muscle, especially within the legs (29). The physiologic measure of aerobic capacity, oxygen consumption ( V O 2 ) is derived by the product of cardiac output (CO) and oxygen extraction from the blood by metabolically active tissues ( V O 2 CO O 2arterial venous ) (29). Taking this into account, it is understandable that diseases that impair cardiac output or peripheral oxygen extraction directly limit the availability of oxygen to lower extremity skeletal muscle during the 6mw. In contrast, it is possible that in mobility-limited individuals without significant cardiovascular or respiratory disease, these relationships are altered. This is supported by the findings in which the 6mw was not significantly associated with indirect measures of aerobic capacity, but strongly associated with measures of functional limitation. Such hypotheses, however, need confirmation. We used indirect measures of V O 2, because elders often have difficulty completing maximal tests of aerobic capacity (3); and recent reports have suggested that submaximal RPP is a valid correlate of V O 2max, even with advancing age (25). Such suggestions regarding our proxy measures have yet to be confirmed. Before any conclusions can be drawn, these findings should be replicated in a larger study utilizing direct measures of aerobic capacity. Other potential limitations of this study exist. It might be suggested that the sample size was too small to detect meaningful differences (type 2 error) and not functionally diverse enough to fully generalize its findings. In the case of our proxy measures of aerobic capacity, this may be the case. However, in regard to the impairment and functional measures, the fact that the key findings were statistically significant, despite a relatively small sample size, suggests that the differences detected were substantive. Our sample did include those with both mild and moderate mobility limitations; however, replication of our findings within a larger, more functionally diverse population is warranted. In this study within mobility-limited elders without significant cardiovascular or respiratory disease, the 6mw is best characterized as a performance-based measure of functional capacity. Like other performance-based measures with which it is correlated, the 6mw is determined by impairments in leg power and strength. Impairments at the ankle and knee are most predictive of 6mw performance. Additionally, the validity of the 6mw as a measure of submaximal aerobic capacity is questioned. These findings should be replicated in larger, more functionally diverse populations utilizing direct measures of aerobic capacity. Acknowledgments This work was supported by the Harvard/Hartford Center of Excellence in Geriatric Medicine, the Harvard Older Americans Independence Center, the American Academy of Physical Medicine and Rehabilitation-Education Research Fund, and the Department of Physical Medicine and Rehabilitation, Harvard Medical School. We acknowledge the assistance of Kelly Mizer, MS, and Damien Callahan, BS, with data acquisition and entry and the Harvard Cooperative Program on Aging, which facilitated recruitment. Some of the findings were presented at the March 2002 annual meeting of the Association of Academic Physiatrists, Las Vegas, NV. Address correspondence to Jonathan F. 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