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To compare associations of physical activity during daily life with treadmill walking performance and corridor-based functional performance measures in persons with lower extremity peripheral arterial disease (PAD).
156 men and women with PAD who completed baseline measurements and were randomized into the Study to Improve Leg Circulation (SILC) exercise clinical trial.
Participants completed a Gardner-Skinner treadmill protocol. Corridor-based functional performance measures were the six-minute walk, walking velocity over four meters at usual and fastest pace, and the Short Physical Performance Battery (SPPB) (0–12 scale, 12 = best). Physical activity during daily life was measured continuously over seven days with a Caltrac accelerometer.
Adjusting for age, sex, and race, higher levels of physical activity during daily life were associated with greater distance achieved in the six-minute walk (p trend =0.001), faster fast-paced four-meter walking velocity (p trend <0.001), faster usual-paced four-meter walking speed (p trend= 0.027) and a higher SPPB (p trend = 0.005). The association of physical activity level with maximum treadmill walking distance did not reach statistical significance (p trend =0.083). There were no associations of physical activity with treadmill distance to onset of leg symptoms (p trend =0.795).
Functional performance measures are more strongly associated with physical activity levels during daily life than treadmill walking measures.
Persons with lower extremity peripheral arterial disease (PAD) have greater functional impairment and faster rates of functional decline compared to persons without PAD (1–3). Identifying new therapies that improve functional performance and prevent functional decline in patients with PAD has the potential to significantly benefit the large and growing number of men and women with PAD.
Treadmill walking performance is typically the primary outcome in clinical trials evaluating medical therapies for improving walking impairment in patients with PAD. However, the degree to which treadmill testing reflects functional performance during daily life is unclear. Treadmill walking performance is measured in a highly controlled setting. Treadmill protocol specifies initial walking velocity and the rate of increase in walking speed and intensity. In contrast, participants set their own pace in corridor-based functional performance measures, such as the six-minute walk test. Therefore, treadmill protocols may represent a relatively artificial measure of walking and may be a less optimal measure of daily or usual walking performance than corridor-based performance measures.
This study determined whether treadmill walking performance and corridor-based functional performance, respectively, were associated significantly with objectively-measured physical activity during daily life among persons with PAD. We hypothesized that corridor-based functional performance measures would be more strongly associated with physical activity during daily life than treadmill walking measures.
The Study to Improve Leg Circulation (SILC) is a randomized controlled clinical trial testing the ability of treadmill exercise and lower extremity resistance training, respectively, to improve walking performance in PAD participants with and without intermittent claudication. Baseline data from SILC were used in the present analyses. The study design is cross-sectional.
The institutional review boards of Northwestern University, Catholic Health Partners Hospital, Evanston Northwestern Hospital, Rush Medical Center, University of Illinois-Chicago, the Jesse Brown Veterans Administration Medical Center in Chicago, and Mt. Sinai Hospital in Chicago approved the protocol. Participants gave written informed consent. The funding source for this study played no role in the design, conduct, reporting of the study, or decision to submit the manuscript.
Potential participants were recruited from among consecutive patients diagnosed with PAD in the non-invasive vascular laboratories and from relevant clinics (vascular surgery, cardiology, general medicine, geriatrics, and endocrinology) at Northwestern Memorial Hospital and other Chicago-area hospitals. Newspaper and radio advertisements, mailings to Chicago residents age 60 and older, fliers, and other community outreach methods were used to identify potentially eligible participants.
The inclusion criterion was an ankle brachial index (ABI) < 0.95. Exclusion criteria and the number of potential participants meeting each criterion are in Table 1.
A hand-held Doppler probe (Nicolet Vascular Pocket Dop II; Nicolet Biomedical Inc, Golden, Colo) was used to obtain systolic pressures in the right and left brachial, dorsalis pedis, and posterior tibial arteries (1–3,4). Each pressure was measured twice: in the order listed and in reverse order. The ABI was calculated by dividing the mean of the dorsalis pedis and posterior tibial pressures in each leg by the mean of the 4 brachial pressures (4). Average brachial pressures in the arm with highest pressure were used when 1 brachial pressure was higher than the opposite brachial pressure in both measurement sets and the 2 brachial pressures differed by 10 mm Hg or more in at least one measurement set, since in such cases subclavian stenosis was possible (5). The lowest leg ABI was used in analyses.
Medical history was obtained from patient-report (see Appendix). Participants were asked whether a physician had ever told them that they had angina, heart failure, diabetes, myocardial infarction, disk disease, spinal stenosis, hip or knee arthritis, cancer, or pulmonary disease.
The six-minute walk test is a measure of walking endurance that has excellent test re-test reliability in persons with PAD (6–8). Following a standardized protocol (6–8), participants walk up and down a 100- foot hallway for six minutes after instructions to cover as much distance as possible. The distance completed during the six-minutes is recorded. The intra-class correlation coefficient for test re-test reliability of the six-minute walk was 0.90 (P<.001) in our laboratory among 155 PAD participants who completed the tests approximately 1–2 weeks apart.
Participants sit in a straight-backed chair with arms folded across their chest and stand five times consecutively as quickly as possible. Time to complete five chair rises was measured (9,10).
Participants were asked to hold three increasingly difficult standing positions for ten seconds each: standing with feet together side-by-side and parallel (side-by-side stand), standing with feet parallel with the toes of one foot adjacent to and touching the heel of the opposite foot (semi-tandem stand), and standing with one foot directly in front of and touching the other (tandem stand) (9,10). Scores range from zero (unable to hold the side-by-side stand for ten seconds) to four (able to hold the full tandem stand for ten seconds) (9,10).
Walking velocity over four meters was measured with a four-meter walk performed at “usual” and “fastest” pace. For the “usual” paced walk, participants were instructed to walk at their usual pace, “as if going down the street to the store.” Each walk was performed twice. The faster walk in each pair was used in analyses (9,10). The intra-class correlation coefficient for test re-test reliability were 0.83 (p<.0001) for the usual paced four-meter walk and 0.88 (p <.0001) for the fast paced four meter walk in our laboratory among 155 PAD participants who completed the tests approximately 1–2 weeks apart.
The Short Physical Performance Battery (SPPB) combines data from the usual paced four-meter walking velocity, time to rise from a seated position five times, and standing balance. Individuals receive a zero score for each task they are unable to complete. Scores of one to four are assigned for remaining tasks, based upon quartiles of performance for over 6,000 participants in the Established Populations for the Epidemiologic Study of the Elderly (9,10). Scores are summed to obtain the SPPB, ranging from 0 to 12 (9,10). Test re-test reliability of the summary performance score was 0.72 (P<.001) in our laboratory among 144 PAD participants who completed the test approximately 1–2 weeks apart.
Maximal treadmill walking distance and distance to onset of ischemic leg symptoms were measured using the Gardner-Skinner protocol (11–13). Treadmill testing was administered by an exercise physiologist blinded to study hypotheses. Participants able to walk at least 2.0 miles per hour (MPH) began exercising at 2.0 MPH. Grade began at zero and was increased by two percent every two minutes. Participants unable to walk at least 2.0 MPH began at 0.5 MPH (modified Gardner protocol). Speed was increased by 0.50 MPH every two minutes until the participant reached 2.0 MPH, after which speed was maintained at 2.0 MPH and grade was increased by two percent every two minutes until the participant was unable to continue. Previous study shows that the test-retest intraclass reliability coefficient is 0.93 for maximum walking distance and 0.89 for distance to onset of leg symptoms (11).
Physical activity levels were measured objectively over seven days using a vertical accelerometer (Caltrac, Muscle Dynamics Fitness Network, Inc., Torrence, CA) (14–16). The Caltrac accelerometers are designed to calculate the number of kilocalories (Kcal) expended by combining data from measured physical activity (vertical movement) with data on each participant’s age, weight, height, and sex. Kilocalories expended are dependent on intrinsic characteristics of the individual, such as age, sex, and body mass index. We aimed to compare actual physical activity levels, rather than Kilocalories expended. Therefore, for each participant we programmed identical values for age, weight, height, and sex, allowing us to compare physical activity levels between participants irrespective of individual variation in age, weight, height, and sex (14–16). Thus, the accelerometers measured “activity units” rather than calories expended (14–16). After wearing activity monitors continuously for 7 days, participants reported the number of activity units and steps displayed on the accelerometer by telephone and mailed their activity monitors back to investigators. Simultaneously with the Caltrac accelerometers, participants wore Digiwalker step counters continuously for seven days (17,18). However, the Caltrac monitors were selected a priori as the primary physical activity outcome measure because the Caltrac monitors have been well validated as measures of physical activity in persons with PAD (14–16).
Height and weight were measured at the study visit. Body mass index (BMI) was calculated as weight/(square of height in meters). Smoking history was obtained by administration of a standardized questionnaire. Leg symptoms were measured with the San Diego claudication questionnaire (19).
Functional performance measures (six-minute walk, four-meter walking velocity at usual and fastest pace) and treadmill outcomes (maximal treadmill walking distance and treadmill distance to onset of leg symptoms) were divided into quartiles. Established categories for the SPPB were used to define SPPB performance (10).
To determine associations of functional performance measures and treadmill walking performance with daily physical activity, mean functional performance for each quartile/category of physical activity were compared using analyses of variance, adjusting for age, sex, and race. Analyses were repeated with additional adjustment for comorbidities, BMI, ABI, leg symptoms, smoking, and treadmill protocol type (Gardner vs. modified Gardner). We also assessed associations of quartiles/categories of functional performance measures with treadmill walking performance (dependent variable), adjusting for age, sex, and race.
Of 485 persons who attended a baseline visit, 329 met one or more exclusion criteria (Table 1), leaving 156 participants. Baseline characteristics of the 156 eligible, randomized participants are shown in Table 2. Comparisons of Caltrac physical activity and Digiwalker step measures showed that the lowest quartile of Caltrac physical activity (<374 activity units) was comparable to < 6,957 steps walked over seven days while the highest quartile of Caltrac physical activity (> 849 activity units) was comparable to > 24,655 steps walked over seven days.
Table 2 shows patient characteristics across quartiles of physical activity. Higher physical activity was associated with younger age (p trend = 0.023), a lower prevalence of diabetes mellitus (p trend = 0.006), and a lower BMI (p trend = 0.004).
Figure 1 shows associations of physical activity levels with corridor-based functional performance measures, adjusting for age, sex, and race. Higher levels of physical activity were associated with greater six-minute walk distance (p trend = 0.001), faster normal-paced four-meter walking velocity (p trend = 0.027), faster rapid-paced four meter walking velocity (p trend <0.001), and a higher SPPB score (p trend =0.005), adjusting for age, sex, and race. The association of physical activity level with maximum treadmill walking distance was nearly statistically significant (Figure 2). Physical activity level was not associated with treadmill distance to onset of leg symptoms, adjusting for age, sex, and race (Figure 2). After additional adjustment for comorbidities, BMI, ABI, leg symptoms, and smoking, only rapid-paced four meter walking velocity remained associated significantly with physical activity levels (1st physical activity quartile – 1.00 meters/second, 2nd physical activity quartile- 1.03meters/second, 3rd physical activity quartile- 1.18 meters/second, 4th quartile- 1.15 meters/second, p trend=0.006).
In additional descriptive analyses, we compared corridor-based walking performance measures with maximum treadmill walking distance and treadmill distance to onset of leg symptoms, adjusting for age, sex, and race (Table 3). Greater six-minute walk distance (p trend <0.001) and faster rapid-paced four meter walking velocity (p trend = 0.005) were associated with greater maximal treadmill walking distance, adjusting for age, race, and sex. Walking velocity at usual pace and the SPPB were not significantly associated with maximum treadmill walking distance (Table 3). Only six minute walk distance was associated significantly with treadmill distance to onset of leg symptoms (p trend = 0.003) (Table 3).
Among 156 persons with PAD, our results show that better performance on each of four measures of corridor-based functional performance: six-minute walk distance, walking velocity at usual speed, walking velocity at fastest speed, and the SPPB were associated significantly with higher physical activity levels during daily life. Maximum treadmill walking distance and treadmill distance to onset of leg symptoms were not associated significantly with physical activity levels during daily life, although associations of maximum treadmill walking distance with physical activity showed a trend toward statistical significance. These findings indicate that performance on corridor-based functional measures are more closely linked to physical activity levels during daily life than treadmill walking performance.
Treadmill walking performance is traditionally used to measure changes in walking performance following therapeutic interventions among persons with PAD. Treadmill testing has potential advantages that include monitoring of study participants in a tightly controlled setting and use of a standardized protocol that specifies initial treadmill speed and grade and rates of increase in treadmill speed and grade at precisely determined intervals, allowing direct comparisons between participants. However, treadmill testing creates a more “artificial” environment compared to corridor-based functional measures, in which walking speed is determined by the patient. Consistent with these differences, previous study in persons without PAD shows that treadmill walking is associated with greater anxiety and requires greater balance than corridor-based walking measures (29-22). For example, a previous study of 12 healthy elderly volunteers (aged 71–80 years) and 12 healthy young volunteers (aged 21–37) compared physiologic responses during treadmill walking vs. the six-minute walk (20). The elderly group, but not the young group, had consistently higher heart rates and slower step rates during treadmill testing as compared to corridor walking (20). In a separate study of 11 patients with COPD, the distance covered during the 6-minute walk test was significantly greater than distance covered during 6 minutes of treadmill walking (530 meters vs. 481 meters, p<0.01) (21). Together, these findings suggest that treadmill testing may be less applicable to usual walking ability in the community than corridor-based functional performance measures.
Of the four corridor-based functional performance measures studied, only the SPPB did not correlate significantly with treadmill walking performance. In contrast to the six-minute walk, and the four-meter walking velocities, the SPPB encompasses more direct measures of balance and leg strength. These components of functioning appear to be distinct from the outcomes of maximum treadmill walking distance and treadmill distance to onset of leg symptoms. Nonetheless, the baseline SPPB score predicts the rate of mobility loss among persons with PAD (23). A baseline SPPB score predicts mobility loss and mortality among persons without PAD (9,10). Cross-sectional data reported here show that better SPPB scores are associated with higher physical activity levels. Together, these data suggest that the SPPB outcome is distinct from treadmill walking measures, but is nonetheless a well validated measure of lower extremity performance in PAD persons.
This study has limitations. First, data are cross-sectional. This study could not determine whether treadmill walking performance is more sensitive to the effects of interventions than walking performance measures. One prior randomized controlled clinical trial demonstrated greater increases in maximum treadmill walking performance than in six-minute walk performance after a six-month supervised exercise intervention (24). Second, the exclusion criteria for this study may limit the generalizability of the findings. Third, participants with PAD included persons with and without intermittent claudication symptoms. In contrast, most clinical trials in persons with PAD are restricted to those with classical symptoms of intermittent claudication.
In conclusion, findings reported here suggest that corridor-based functional performance measures better reflect usual physical activity levels during daily life than treadmill walking performance. Our findings suggest that corridor-based functional performance measures are an important complement to treadmill walking performance and may better reflect certain aspects of usual walking during daily life than treadmill walking performance. Future studies of performance in persons with PAD should include measures of corridor-based functional performance, rather than only treadmill-based performance.
Supported by R01-HL073551 from the National Heart Lung and Blood Institute and by grant #RR-00048 from the National Center for Research Resources, National Institutes of Health (NIH). Supported in part by the Intramural Research Program, National Institutes on Aging, NIH.
ClinicalTrials.gov Identifier: NCT00106327