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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
J Vasc Surg. Author manuscript; available in PMC Jan 31, 2008.
Published in final edited form as:
PMCID: PMC2222553
NIHMSID: NIHMS37748

PATTERNS OF AMBULATORY ACTIVITY IN SUBJECTS WITH AND WITHOUT INTERMITTENT CLAUDICATION

Abstract

Purpose

To compare the patterns of ambulatory activity in subjects with and without intermittent claudication.

Methods

Ninety-eight subjects limited by intermittent claudication and 129 controls who were matched on age, gender, and race participated in this study. Subjects were assessed on their ambulatory activity patterns for one week with a small, lightweight step activity monitor attached to the ankle using elastic velcro straps above the lateral malleolus of the right leg. The step activity monitor recorded the number of strides taken on a minute-to-minute basis, the time spent ambulating, and the time and number of strides measured at low (< 15 strides/min), medium (15–30 strides/min), and high (> 30 strides/min) cadences.

Results

Subjects with intermittent claudication took fewer total strides each day than the controls (3149 ± 1557 strides/day vs. 4230 ± 1708 strides/day; p < 0.001), and fewer strides at both medium (1228 ± 660 strides/day vs. 1638 ± 724 strides/day; p = 0.001) and high (766 ± 753 strides/day vs. 1285 ± 1029 strides/day; p < 0.001) cadences. Subjects with intermittent claudication also had a lower daily average cadence than the controls (11.8 ± 2.9 strides/min vs. 13.5 ± 3.1 strides/min; p < 0.001), and spent less total time ambulating each day (264 ± 109 min/day vs. 312 ± 96 min/day; p = 0.034), primarily at medium (58 ± 30 min/day vs. 75 ± 32 min/day; p < 0.001) and at high (19 ± 17 min/day vs. 30 ± 22 min/day; p = 0.001) cadences.

Conclusion

Intermittent claudication is associated with lower total daily ambulatory activity due both to less time ambulating and to fewer strides taken while ambulating, particularly at moderate and high cadences.

INTRODUCTION

Peripheral arterial disease (PAD) is associated with poor long-term survival.114 The poor prognosis of subjects with PAD is further compounded by sedentary living,1517 as the mortality risk of sedentary subjects is more than three-fold higher than of subjects in the highest quartile of physical activity.18 Additionally, PAD is a leading cause of morbidity due to ambulatory limitations associated with intermittent claudication.1922 Intermittent claudication afflicts 5% of the US population older than 55 years of age,23 and occurs during ambulation when the peripheral circulation is inadequate to meet the metabolic requirement of the active leg musculature. Consequently, intermittent claudication leads to ambulatory dysfunction and a decline in daily physical activities.24

Despite the impact of physical activity on morbidity and mortality in subjects with PAD, relatively few studies have actually quantified physical activity. Several studies found that free-living daily physical activity, measured by an accelerometer, was lower in subjects limited by intermittent claudication than in age-matched controls.2425 Although this is a reliable26 and valid27 method to assess total daily physical activity, it does not provide information about the pattern in which physical activity was performed. Thus, it is unclear whether daily physical activity is lower in subjects with intermittent claudication because of less time spent in physical activity, because of less intense physical activity, or a combination of both. This information is clinically relevant to understanding how intermittent claudication impacts ambulation throughout the day, and may lead to insights into the appropriate exercises for improving daily ambulation.

A major technological advance in assessing daily physical activity occurred with the development of a step activity monitor (StepWatch3, Cyma Inc., Mountlake Terrace, WA) that can be worn at the ankle.2832 The step activity monitor is an ideal tool to quantify daily ambulation because it records ambulatory cadence on a minute-to-minute basis for up to 50 days, as well as time spent ambulating at various cadences. The purpose of this study was to compare the pattern of ambulatory activity in subjects with and without intermittent claudication.

METHODS

SUBJECTS

Recruitment

Subjects between the ages of 50 and 90 years were evaluated in the General Clinical Research Center at the University of Oklahoma Health Sciences Center (HSC)), and in the Health and Exercise Science laboratory at the University of Oklahoma. Subjects were recruited by referrals from the HSC vascular clinic, as well as by newspaper advertisements for a free evaluation of peripheral vascular function and physical activity levels. The procedures used in this study were approved by the Institutional Review Boards at the University of Oklahoma HSC and the University of Oklahoma. Written informed consent was obtained from each subject prior to investigation.

Screening of the Intermittent Claudication Group

Subjects with intermittent claudication secondary to vascular insufficiency were included in this study if they met the following criteria: (a) positive test on the San Diego claudication questionnaire,33 and (b) an ABI < 0.90.23 Subjects were excluded from this study for the following conditions: (a) absence of PAD (ABI ≥ 0.90), (b) inability to obtain an ABI measure due to non-compressible vessels, (c) asymptomatic PAD, (d) use of medications indicated for the treatment of intermittent claudication (cilostazol and pentoxiphylline) within three months prior to investigation, (e) exercise tolerance limited by factors other than leg pain (e.g., severe coronary artery disease, dyspnea, poorly controlled blood pressure), and (g) active cancer, renal disease, or liver disease. A total of 98 subjects with intermittent claudication were deemed eligible for this investigation, whereas 17 subjects were ineligible.

Screening of the Control Group

Control subjects were included in this study if they met the following criteria: (a) negative test on the San Diego claudication questionnaire, (b) no other ambulatory leg pain, and (c) an ABI ≥ 1.00. Controls were excluded from this study for the following conditions: (a) an ABI < 1.00, (b) inability to obtain an ABI measure due to non-compressible vessels, (c) poorly controlled hypertension (resting systolic blood pressure > 200 mm Hg or resting diastolic blood pressure > 120 mm Hg), (d) history of cardiovascular disease, myocardial infarction, or stroke, (e) history of dyspnea, (f) history of angina, and (f) active cancer, renal disease, or liver disease. A total of 129 control subjects were deemed eligible for this investigation, whereas 18 subjects were ineligible.

MEASUREMENTS

Medical History

Demographic information, height, weight, waist and hip circumferences,34 cardiovascular risk factors, co-morbid conditions, claudication history, and a list of current medications were obtained during a medical history and physical examination to begin the evaluation.

ABI

After 10 minutes of supine rest, the ankle and brachial systolic blood pressures were obtained as previously described.35 Briefly, ankle systolic pressure was measured by Doppler technique in the posterior tibial and dorsalis pedis arteries of both legs. The higher of the two arterial pressures from the more severely diseased leg was recorded as the resting ankle systolic pressure. Similarly, brachial blood pressure was taken from both arms, and the arm yielding the higher systolic pressure was recorded as the brachial systolic pressure. The ABI was then calculated as ankle systolic pressure/brachial systolic pressure. The test-retest intraclass reliability coefficient for the measurement of ABI in our laboratory is R = 0.96 for ABI.36

6-Minute Walk Test

Patients performed an over ground, 6-minute walk test, supervised by trained exercise technicians as previously described.37 The pain-free and total distance walked during the test were recorded. Immediately after completion of the test, the technician recorded whether the walking test was done continuously or discontinuously, and asked the subjects to rate their level of perceived exertion while performing the test.38 The test-retest intraclass reliability coefficient is R = 0.75 for distance to onset of claudication pain, and R = 0.94 for the total 6-minute walking distance.37

Walking Impairment Questionnaire (WIQ)

Self-reported ambulatory ability was assessed using a validated questionnaire for PAD patients that assesses ability to walk at various speeds and distances, and to climb stairs.39

Ambulatory Activity Monitoring

Daily ambulatory activity was assessed using a small (75 × 50 × 20 mm), lightweight (38 g) step activity monitor (Step Watch 3, Cyma Inc., Mountlake Terrace, WA) containing a sensor, electronics, and a battery inside a completely sealed and durable polycarbonate case. The step activity monitor was programmed by placing the unit on a USB docking station connected to a computer with StepWatch3 Analysis Software. Ambulatory activity was measured during seven consecutive days in which subjects were instructed to wear the monitor during waking hours and to remove it before retiring to bed. The step activity monitor was attached to the right ankle above the lateral malleolus using elastic Velcro straps, and continuously recorded the number of steps taken on a minute-to-minute basis. Subjects returned the monitor at the end of the seven-day period, and the data were downloaded into a subject file stored in the software program. The accuracy of the step activity monitor exceeds 99% ± 1% compared to hand-tallied step counts during a 6-minute walk test performed by a subset of subjects with intermittent claudication (n = 15) and controls (n = 15). This level of accuracy is similar to previous reports.2832

The step activity monitor quantifies ambulatory stride rate and time of ambulatory activity and inactivity, and characterizes short-duration bursts of activity as well as sustained endurance ambulation. These outcome measures were recorded each day and then averaged over the seven-day monitoring period. Specifically, the outcomes related to stride rate included the total number of strides taken each day, and the daily number of strides taken at investigator-defined low cadence (less than 15 strides per minute), medium cadence (between 15 and 30 strides per minute), and high cadence (more than 30 strides per minute). Time-related outcomes included the daily number of active and non-active minutes, as well as the number of minutes spent ambulating at low, medium, and high cadences. The outcomes related to endurance and burst ambulatory activity included the average strides per minute of the maximum number of strides taken over a sliding window of 60, 30, 20, and 5 continuous minutes each day, as well as the maximum stride rate obtained during the single highest minute. Additionally, the peak activity index was obtained in which the average stride rate for the top 30 minutes of each day was obtained, regardless of whether these minutes occurred continuously or discontinuously. In the subjects with intermittent claudication, the test-retest intraclass reliability coefficient for the measurement of total daily strides and total daily minutes of activity over the 7-day period was R = 0.87 and R = 0.85, respectively. In the controls, the respective intraclass reliability coefficients were R = 0.94 and R = 0.91.

STATISTICAL ANALYSES

Unpaired t-tests were used to compare differences between the subjects with and without intermittent claudication for parametric measures, and Mann-Whitney U-tests were used to compare the groups for non-parametric measures. Analysis of covariance (ANCOVA) was then used to assess group differences in ambulatory function and ambulatory activity after adjusting for baseline clinical characteristics that were significantly different between the two groups. All analyses were performed using the SPSS-PC statistical package. Statistical significance was set at p < 0.05. Measurements are presented as means ± standard deviations.

RESULTS

The clinical characteristics of the subjects with and without intermittent claudication are shown in Table 1. The subjects with intermittent claudication had a lower ankle/brachial index (p < 0.001), lower body mass index (p = 0.031), lower prevalence of obesity (p = 0.025), higher waist/hip ratio (p = 0.033), and higher prevalence of smoking (p < 0.001), hypertension (p < 0.001), and hyperlipidemia (p = 0.005) than the controls. Consequently, all of these measures, except for ankle/brachial index, served as covariates in subsequent analyses to adjust for potential confounding effects when comparing group differences in ambulatory function and ambulatory activity. Furthermore, age and diabetes were added as covariates because group differences in subjects with and without intermittent claudication approached statistical significance for both variables (p = 0.099 and p = 0.070, respectively). Ankle/brachial index was not used as a covariate because it is a measure of PAD severity and is not a confounder.

Table I
Clinical characteristics of subjects with intermittent claudication and controls. Values are means (SD) and percentages.

All of the subjects with intermittent claudication were symptomatic during the 6-minute walk test, whereas the controls were free of leg pain. As shown in Table 2, fewer subjects with intermittent claudication were able to walk continuously during the test than the controls (p < 0.001). Thirty-six percent of the subjects with intermittent claudication walked discontinuously due to leg pain, whereas 13% of controls walked discontinuously due to overall fatigue. The pain-free and 6-minute walk distances were both shorter (p < 0.001) in the subjects with intermittent claudication, and their rating of overall perceived exertion was higher (p = 0.007). Furthermore, subjects with intermittent claudication had lower scores for the walking distance (p < 0.001) and walking speed (p < 0.001) components of the WIQ. Group differences in all of these measures persisted (p < 0.01) after adjusting for body mass index, obesity, waist/hip ratio, smoking, hypertension, hyperlipidemia, age, and diabetes.

Table II
Ambulatory function of subjects with intermittent claudication and controls. Values are means (SD).

Subjects with intermittent claudication took fewer total strides each day than controls (p < 0.001), and fewer strides at both medium (p = 0.001) and high (p < 0.001) cadences (Table 3). On a relative basis, the subjects with intermittent claudication took a higher percentage of strides at low cadence (p < 0.001), and a lower percentage of strides at high cadence (p < 0.001) than the controls. Similar to the stride data, subjects with intermittent claudication also spent less total time ambulating each day than the controls (p = 0.034), and less time ambulating at both medium (p < 0.001) and high (p = 0.001) cadences (Table 4). On a relative basis, the subjects with intermittent claudication spent a higher percentage of time at low cadence (p < 0.001), and a lower percentage of time at medium (p = 0.007) and high (p < 0.001) cadences than the controls. Subjects with intermittent claudication had a lower daily average cadence (Table 5) than the controls (p < 0.001), as well as a lower peak activity index (p < 0.001) and lower maximal stride rates (p < 0.001) for continuous durations of 60, 30, 20, 5, and 1 minute. Group differences in all of these measures persisted (p < 0.01) after adjusting for covariates. Furthermore, several variables that were similar between groups before statistical adjustment were significantly different between groups after statistical adjustment. For example, after adjustment the subjects with intermittent claudication took fewer strides (p = 0.006) and spent less time (p = 0.040) ambulating at low cadence, and spent more time (p = 0.001) being sedentary.

Table III
Ambulatory strides recorded during a one-week monitoring period in subjects with intermittent claudication and controls. Values are means (SD).
Table IV
Ambulatory durations recorded during a one-week monitoring period in subjects with intermittent claudication and controls. Values are means (SD).
Table V
Ambulatory cadences characteristic of bursts of activity and sustained endurance activity in subjects with intermittent claudication and controls. Values are means (SD).

DISCUSSION

The major findings of this investigation were that, compared to controls, subjects with intermittent claudication (1) took 26% fewer strides and spent 15% less time ambulating each day, (2) had an overall 12% slower ambulatory pace, primarily evident at cadences above 15 strides per minute, and (3) had reduced cadences for both short-duration bursts of activity as well as for activities associated with sustained endurance ambulation.

Subjects with intermittent claudication ambulate less each day than controls, as they took 1,081 fewer strides and spent 48 fewer minutes ambulating each day. This is similar to a previous investigation in our laboratory which found that subjects with intermittent claudication took 3,935 fewer steps each day than controls (i.e., 1,967 fewer daily strides), as measured with a pedometer.24 The greater difference found in our previous study may be due to subjects having more severe PAD than in the current investigation, and because pedometers typically overestimate the actual steps taken, due to a spurious recording of non-ambulatory body movements. Our results are also supported by studies showing that subjects with PAD have lower physical activity than controls, as measured by an accelerometer worn for two days and one week, and by questionnaires that assessed activity levels during the previous week, two weeks, and one year.24, 25

The lower total daily physical activity level in the subjects with intermittent claudication was primarily due to less ambulation at moderate and high cadences, and to less time spent ambulating at these paces. This is a novel finding, as previous publications using accelerometers to compare the daily activity of subjects with and without intermittent claudication could not determine the pattern of physical activity.24,25 This is the distinct advantage of the step activity monitor used in this study, as the number of strides and the time spent at various cadences, as well as the cadence during each minute of ambulation, are recorded. Our findings indicate that the amount of daily ambulation performed at a cadence below 15 strides per minute is similar between subjects with and without intermittent claudication. However, subjects with intermittent claudication either chose to ambulate less at higher cadences, or they are less capable of doing so. Thus, subjects with intermittent claudication have difficulty ambulating at paces exceeding one stride every four seconds, which may further increase their risk of morbidity40 and mortality,114 and worsen their health-related quality of life.41,42

The pattern of reduced ambulation at medium and high cadences in subjects with intermittent claudication agrees with our previous report that total leisure-time physical activity decreased progressively with more severe PAD, primarily due to less intense activities performed for a similar duration of time.43 Thus, the self-reported measure of physical activity agrees with objectively measured activity in the present study, indicating that subjects with intermittent claudication adopt slower ambulatory paces throughout the day.

Subjects with intermittent claudication also had patterns of slower ambulation at paces characteristic of bursts of activity, as well as of sustained endurance activity. The maximum cadence attained for a one-minute duration throughout the day was 8% lower in the subjects with intermittent claudication than in controls, and the maximum cadence for a five-minute duration was 16% lower. Ambulation performed for five minutes or less is characteristic of activities requiring short bursts of activity rather than of more sustained, endurance-related activities. The slower paces in the subjects with intermittent claudication suggest that ambulatory pattern is altered preceding the onset of symptoms. This agrees with our previous report that an altered gait pattern in subjects with intermittent claudication favored greater stability at the expense of slower ambulation even before the onset of claudication.44

The maximum cadence attained for a 20-minute duration throughout the day was 21% lower in subjects with intermittent claudication than in controls, and the maximum cadence for 30-minute and 60-minute durations were also 21% lower. Continuous ambulation for 20 minutes to one hour reflects endurance-related activities. The slower paces in the subjects with intermittent claudication suggest that subjects with intermittent claudication must slow their cadence to complete activities performed for relatively long durations. The slower rate of ambulation may be an adopted strategy to prolong the onset and development of intermittent claudication during sustained activities. Although the pace during sustained daily activities is slower in subjects with intermittent claudication, it may still provide an adequate stimulus to elicit ambulatory benefits, as low-intensity exercise training results in improvements similar to high-intensity training, provided that the same amount of work is accomplished.45

The cadence associated with the peak activity index was 15% lower in subjects with intermittent claudication. The peak activity index represents the average stride rate for the highest 30 minutes of each day, regardless of whether they occurred continuously or discontinuously. Thus, the peak activity index represents the highest 30 minutes of ambulatory stride rates accumulated throughout the day rather than the highest 30-minute period. The lower peak activity index of the subjects with intermittent claudication is consistent with the findings for the maximum cadence at 20, 30, and 60 continuous minutes, and indicates that subjects with intermittent claudication perform their daily ambulatory activities at slower paces.

This study has several clinical implications for subjects with intermittent claudication to improve their ambulation using a home-based exercise program. The decreased daily ambulation above a relatively slow threshold pace of 15 strides per minute suggests that not only do subjects with intermittent claudication spend less time ambulating above this pace, but they also perform much of their ambulation discontinuously on a minute-to-minute basis. For example, it is quite likely that subjects with intermittent claudication ambulate and rest for portions of each minute of daily activity rather than performing continuous ambulation at a slower pace throughout each minute. The clinical implication is that subjects with intermittent claudication may improve their ambulation through home-based exercise training if they simply ambulate in a more continuous manner during daily activities, thereby increasing the number of strides taken each minute and increasing the total number of active minutes, even if ambulation is done at relatively slow paces that do not elicit leg pain. A second key finding is that subjects with intermittent claudication ambulate less in activities requiring bursts of activity. The clinical implication to this finding is that subjects with intermittent claudication may further improve their ambulation in a home-based exercise program by adding several minutes of rapid ambulation as a supplement to their increased continuous, slow ambulation. This combination should improve both ambulatory endurance and ambulatory power required during many activities of daily living.

There are several limitations to this study. The cross-sectional design comparing subjects with and without intermittent claudication does not allow causality be established, as it is possible that subjects with intermittent claudication had slower ambulatory paces prior to the development of symptoms. Additionally, it is possible that the subjects did not wear the step activity monitor during portions of their waking hours, thereby resulting in an underestimate of daily ambulation. We believe that this possibility is unlikely because long durations in which no active minutes were recorded during daytime hours were rarely evident from the software graphs depicting the number of strides taken on a minute-to-minute basis. Even during several hours of being sedentary, a few minutes in which some strides occurred were typically evident, indicating that the step activity monitor had not been removed. Another limitation is that the type of sedentary activity which took place during non-active minutes cannot be determined by the step activity monitor. Consequently, it is not possible to quantify the time spent in various sedentary activities such as watching television, taking naps, and sitting while eating. Furthermore, the step activity monitor does not quantify non-ambulatory physical activity, and therefore it underestimates the total amount of daily physical activity accomplished to some extent. The present findings are also limited to PAD patients with intermittent claudication, and may not be generalized to patients with less severe and more severe symptoms. However, subjects with intermittent claudication had a good proportion of women and African-Americans, and they had typical risk factors for PAD, including smoking, diabetes, hypertension, hyperlipidemia, and obesity. Thus, the findings of the present study are generalizable to the majority of subjects with intermittent claudication who typically have numerous co-morbid conditions.

Subjects with intermittent claudication ambulate less each day than controls, and the difference in total daily activity is primarily due to less ambulation above a threshold pace of 15 strides per minute by the subjects with intermittent claudication. Additionally, subjects with intermittent claudication had patterns of slower ambulation at paces characteristic of bursts of activity, as well as of sustained endurance activity. We conclude that intermittent claudication is associated with lower total daily ambulatory activity due both to less time ambulating and to fewer strides taken while ambulating, particularly at moderate and high cadences. Future research is needed to determine whether the pattern of daily ambulatory activity changes following interventions designed to improve claudication distances, such as exercise rehabilitation, medication therapy, and interventional procedures. Using this approach, it can be determined whether the reduced ambulation of subjects with intermittent claudication is primarily due to symptomatology, or whether it is due to complications that develop secondary to chronic ischemic leg pain.

Acknowledgments

This research was supported by grants from the National Institute on Aging (NIA) (R01-AG-16685; AWG), by a Oklahoma Center for the Advancement of Science and Technology grant (HR04-113S; AWG), and by the University of Oklahoma Health Sciences Center General Clinical Research Center grant (M01-RR-14467), sponsored by the National Center for Research Resources from the National Institutes of Health.

Footnotes

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