For researchers interested in quantifying the energy expended in physical activity, measurement of walking behavior is an important concern. Walking is the one of the most common forms of physical activity for Americans (10), and recent public health initiatives have emphasized this activity (23,26). Walking is usually performed during recreational activities, transportation, occupational tasks, and activities of daily living. Though physical activity questionnaires often include questions about walking (15), few studies have examined the accuracy with which people are able to report daily walking distances (4).
One of the better-known physical activity surveys is the College Alumnus questionnaire (CAQ) of Paffenbarger et al. (22). This instrument was one of the first to quantify physical activity by estimating energy expenditure in kcal·wk−1. The questionnaire asks about the types of daily physical activities that he/she has done. Respondents are asked to recall how many city blocks they walk each day, how many flights of stairs they climb each day, and the frequency and duration of their sports, recreation, and other activities. By using established estimates of the energy cost of various activities (3), the number of kcal expended each week in physical activity is computed. A recent study by Ainsworth et al. (4) compared participants’ responses on the questionnaire with 48-h physical activity diaries. One of their findings was that although participants recalled walking 1.0 km·d−1 on the questionnaire, in their diaries they reported walking 5.3 km·d−1 (4). This indicates that these instruments measured two separate things. The CAQ estimated blocks normally walked each day, whereas the diary measured actual walking for that day.
The present study was designed to further investigate the question regarding walking distance on the CAQ. Specifically, we were interested in the question that asks, “How many city blocks or their equivalent do you regularly walk each day? (Let 12 blocks = 1 mile)” (21,22). In the present study, estimates of daily walking distance on the CAQ were compared to data from the electronic pedometer. The pedometer served as a criterion measure for steps taken, and walking distance was computed by assuming an average stride length.
METHODS
Subjects.
Participants in the study were 48 men and 48 women, between the ages of 25 and 70 yr (mean ± SD, 39.9 ± 11.3 yr). Subjects were recruited by word-of-mouth and posted announcements on bulletin boards in Knoxville, TN. Most of the participants recruited were college graduates, making the study population similar in terms of educational status to that of Paffenbarger et al. (22). Before taking part in the study, the nature of the study was explained to the participants, and they were asked to read and sign an informed consent form approved by the University of Tennessee’s Institutional Review Board. The physical and demographic characteristics of the subjects are shown in Table 1.
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Table 1:
Physical and demographic characteristics of study participants (mean ± SD).
CAQ.
Each participant filled out a self-administered questionnaire on physical activity, developed by Paffenbarger et al. (21,22). This questionnaire includes sections on: (a) background information, (b) past and present health status, (c) physical activities, (d) dietary and social habits, and (e) miscellaneous. Although the original questionnaire was mailed to study participants, we handed it to the participants and let them take it home to fill it out. This slight alteration in the method of administering the questionnaire should not have altered the results.
A physical activity index (PAI-CAQ) was computed as the sum of walking, stair climbing, and “sports, recreation, or other physical activity.” Respondents were asked to identify the number of city blocks walked each day, the number of flights of stairs climbed, and the frequency and duration of “sports, recreation, or other physical activity.” Adjustment for seasonal variability in participation in sports and recreational activities was accounted for by having the subjects identify the number of times per year they engaged in these activities. For each sports and recreation activity listed, the estimated level of energy expenditure (METs) was determined from the Compendium of Physical Activity (3). One MET is equal to an average human (3.5 mL·kg−1·min−1 resting metabolic rate. The Compendium is a published report summarizing the energy cost of different activities. Using a 5-digit code for each activity (which links the MET level to the activity), the subject’s weight, and the duration of the activity, the energy cost was computed.
Two minor modifications were made in the scoring of the CAQ. The questionnaire asks respondents to identify how many flights of stairs they climb up each day (one flight = 10 steps), and it is understood that subjects usually ascend and descend an equal number of flights. The caloric cost of going up and down one flight of stairs was originally assumed to be 4.0 kcal in the publication of Paffenbarger et al. (22). However, the energy cost of going up and down one flight was recently shown to be 1.63 MET-min (which equals 2.0 kcal for a 70-kg person) (6). Hence, we used the more recent value to compute the energy cost of stair climbing. The second modification concerned the energy cost of walking one city block. The caloric cost was originally assumed to be 8 kcal·block−1 for all individuals. However, since the energy cost of walking is proportional to body weight, we used standard metabolic prediction equations that take body weight into account (1). This method yielded values equivalent to 7.1 kcal·block−1 for a 70-kg person.
Electronic pedometer.
Participants were not told that the study involved the use of a pedometer until after they had completed the questionnaire. On the informed consent form, we stated only that they would wear a physical activity monitor to estimate their caloric expenditure. If we had administered the CAQ at the end of the 1-wk measurement period, their responses to the question about walking distance might have been biased by knowledge of the pedometer readings. Although the measurement periods were not identical, the CAQ asks about habitual, as opposed to past week, activity. Furthermore, the CAQ shows good repeatability for walking and PAI variables, when administered within a 1-month period (4).
Once they returned the questionnaire, participants were given an electronic pedometer (Yamax model DW-500, Yamasa Corporation, Tokyo) and instructed to position it on their belt or waistband in the midline of the thigh. In a previous study, we compared the accuracy of five electronic pedometers (5). The Yamax DW-500 was found to be very accurate, yielding values within 1–2% of the actual distance during a 4.88-km walk. It also demonstrated superior accuracy over a range of walking speeds, providing distance estimates within 3% of actual at 67 and 80 m·min−1, and within 10% of actual at 54 and 94 m·min−1 (5). We have found that this pedometer provides an accurate measure of total daily walking distance. In a pilot study, we asked 17 participants to wear the pedometer for 24 h. For validation purposes, they pushed a calibrated 10-cm measuring wheel (affixed to an aluminum shaft) wherever they walked. The correlation between the Yamax DW-500 and measuring wheel values was r = 0.977. The average distances for the pedometer and measuring wheel were 5.42 ± 2.63 and 5.21 ± 2.58 km, respectively.
To determine stride length, participants were instructed to take ten strides at their normal walking speed. The total distance covered (m) was then divided by 10 to yield stride length. This method produced stride lengths that averaged 41.5% of height for men and 41.3% of height for women. These values are consistent with stride lengths determined during treadmill walking at 80 m·min−1 (5). They are also consistent with two studies by Japanese investigators reporting that stride length is 42% of height (12,20).
Because the goal was to determine how far the participants walked in their everyday lives, they were instructed not to change their level of physical activity during the study. In addition, the following instructions were given:
- • The pedometer should be worn at all times for exactly 7 d, except for sleeping, showering, and sports/recreation.
- • We are interested to know how many steps you take each day, excluding sports, recreation, or other physical activity.
- • As soon as you wake up each morning, press the yellow reset button.
- • Put the pedometer on your belt or waistband. Take it off when you do sports, recreation, or other physical activity.
- • Just before you go to bed each night, please remove the pedometer and write down the number of steps for that day.
A list of examples of sports, recreation, or other exercise related physical activities was provided for purposes of clarification. We emphasized activities that fit Caspersen’s (9) definition of exercise as “planned, structured, and repetitive bodily movement done to improve or maintain one or more components of physical fitness.” To avoid duplication when computing the PAI, participants were told to remove the pedometer if they performed any walking that fell under the category of sports and recreation. The pedometer instruction sheet included a table for recording the number of steps per day.
Calculation of Walking Distance from Pedometer.
The Yamax pedometer registers steps taken during stair climbing and stair descending, as well as during level walking. Hence, the pedometer data had to be adjusted to reflect only level walking. This was done by subtracting the estimated number of stairs climbed per day from the total steps walked:MATH 1 where SL is the stride length in meters and flights represents the number of flights of stairs climbed (assuming 10 steps per flight going up and 10 steps per flight coming down). This provided a measure of horizontal walking distance per day, exclusive of stair climbing or descending.
The energy cost of walking was computed from standard energy cost formulas (1). We took the subject’s body weight into account, and assumed a walking speed of 80 m·min−1 (4.8 km·h−1). It should be noted that the energy cost of walking a given distance is practically independent of speed, within the range of 50–100 m·min−1. This is because as walking speed increases, the increase in rate of energy expenditure is offset by the decreased time it takes to walk the distance. The energy expenditure of walking was computed from the pedometer data as follows:MATH 2 The MET·min·wk−1 values were converted to kcal·wk−1 using the formula : where BW is the body weight. The pedometer-derived scores for distance and energy expenditure were then compared with the responses provided on the CAQ.
Statistical analysis.
A two-way ANOVA (gender × method) was carried out on the following variables: daily walking distance, walking energy expenditure (MET·min·wk−1), and walking energy expenditure (MET·min·wk−1). Gender differences in stair climbing and sports and recreation were analyzed with a Student’s unpaired t-test. The relationship between estimated and measured daily walking distance was analyzed by fitting a linear regression line to the data and determining the correlation coefficient. Finally, to analyze the steps·d−1 data, a two-way ANOVA (gender × days) was used. For all comparisons, a significance level of P < 0.05 was used.
RESULTS
The average number of steps taken per day are shown in Figure 1. Women tended to accumulate slightly more steps per day than men (6413 ± 2267 vs 5569 ± 2093 steps·d−1), though the difference was not statistically significant. An unexpected finding was that subjects took fewer steps on Sunday than on other days of the week (P = 0.01).
Figure 1:
Number of steps per day measured by the Yamax pedometer, according to the day of the week. Values shown are mean ± SD (exclusive of sports and recreation). * Significant difference between Sunday and other days of the week (P = 0.01)
The estimates of daily walking distance derived from the CAQ and the electronic pedometer are shown in Figure 2. Men under-reported their walking distance on the CAQ, compared with the pedometer values (1.56 ± 0.98 vs 4.02 ± 1.56 km·d−1, mean ± SD) (P = 0.0001). Women also under-reported their walking distance (1.30 ± 1.03 vs 4.32 ± 1.69 km·d−1) (P = 0.0001).
Figure 2:
Average distance walked per day, as reported on the College Alumnus questionnaire (CAQ) and measured by the pedometer (PEDOM). Pedometer values have been adjusted by eliminating stair climbing. Values are mean ± SD * Significant difference between CAQ and PEDOM (P = 0.0001).
The relationship between daily walking distances computed from the CAQ and measured by the pedometer are shown in Figure 3. For men, the correlation coefficient between these two measures was r = 0.346 (P = 0.02). For women, the correlation coefficient between the variables was r = 0.481 (P = 0.001).
Figure 3:
Reported College Alumnus questionnaire (CAQ) distance versus measured pedometer distance. Pedometer values have been adjusted by eliminating stair climbing. Regression equation for males y = 0.220x ± 0.427 (r = 0.346), for females y = 0.254x ± 0.080 (r = 0.481).
Table 2 shows the weekly energy expenditure estimates for each component of the PAI (walking, stair-climbing, sports/recreation) for both the questionnaire and pedometer-derived scores. The energy expenditure of walking computed from the CAQ was significantly lower than the pedometer-derived estimates (555 vs 1608 kcal·wk−1) (P = 0.0001). Men reported expending more energy (MET·min·wk−1) in stair climbing on the CAQ than women (P = 0.02). When energy expenditure was expressed in kcal·wk−1, there were significant gender differences in walking (P = 0.001), stair climbing (P = 0.001), and sports and recreation (P = 0.02). The greater likelihood of finding gender differences when data were expressed in kcal·wk−1 was due to the larger average body size of men.
Table 2:
Physical Activity Index from the College Alumnus questionnaire (PAI-CAQ) and the electronic pedometer data (mean ± SD).
DISCUSSION
The results showed that participants underestimated their daily walking distance on the CAQ, compared with the pedometer values. This could be related to the wording of the question, which asks, “How many city blocks or their equivalent do you regularly walk each day?” (21). This question may give subjects a mental image of walking on city sidewalks (4). The original intent of the question, however, was to capture all walking done during the day (Ralph Paffenbarger, personal communication). If participants had been asked to recall all walking done during the day (i.e., in and around the house, at work, shopping, etc.), this might have improved the accuracy of their responses (4). However, it is also possible that recall of walking (a ubiquitous, moderate activity) is less accurate than recall of more strenuous pursuits (7). Indeed, the validity of physical activity questionnaires in assessing structured, vigorous bouts (e.g., running, swimming) is greater than for light-to-moderate activities (24,27).
The energy expenditure of walking computed from the CAQ data (555 kcal·wk−1) was lower than the pedometer-derived values (1608 kcal·wk−1). This was especially true in women, who tended to underestimate their daily walking distance to a greater extent than the males. This supports the findings of Ainsworth et al. (4), who found that PAI-CAQ underestimated physical activity as compared with physical activity records, mostly due to differences in walking scores.
Previous studies have suggested that expending 2000 kcal·wk−1 in physical activity may be optimal for coronary heart disease (CHD) prevention. However, if the respondents in the 1978 College Alumnus Study (22) also underestimated their daily walking distance, then the amount of physical activity needed to confer protection against CHD may be greater than this (4). In addition, because walking usually falls into the “moderate intensity” range (3–6 METs), moderate activity may be underestimated to a greater degree than vigorous activity on the CAQ.
In defense of the CAQ, the overall validity and reliability of this instrument is well established (4,14,19). The CAQ has provided vital information on the dose-response relationship between physical activity and health outcomes. The general nature of these relationships has been verified in numerous other studies (8,17). The present study simply indicates that the walking component of physical activity may be underestimated, compared with a recently developed technology (i.e., pedometer). However, this error is partly offset by the fact that the energy cost of stair-climbing was originally overestimated (6). In conjunction with the high validity of the CAQ for sports and recreation, this indicates that the validity of the overall PAI is better than that of the walking component.
Several studies have examined whether walking, by itself, is associated with a lower risk of CHD (11). The 1978 paper by Paffenbarger et al. (22) examined walking independent of other activities in 16,936 college alumni. They found that men who walked less than 5 blocks per day had a 26% increase in heart attack risk, compared with men who walked more. Likewise, a 1999 study by Hakim et al. (11) examined 2678 retired men in the Honolulu Heart Program. They found that men who walked less than 0.25 miles·d−1 had a two-fold increase in CHD risk compared with those who walked more than 1.5 miles·d−1. Manson et al. (16) examined the relationship between walking and coronary events in 72,488 women. Women in the highest quintile, who walked the equivalent of 3 or more h·wk−1 at a brisk pace, had a relative risk of 0.65, compared with women in the lowest quintile. These studies all indicate that walkers have a lower risk of CHD. However, there is a need for devices (e.g., pedometers, accelerometers) to validate questions about walking distance on physical activity questionnaires (7).
Montoye et al. (18,19) concluded that the older mechanical-style pedometers had insufficient accuracy for epidemiological research. However, the Yamax electronic pedometer used in this study has greater accuracy than old-fashioned mechanical pedometers (5). This pedometer uses a spring-suspended horizontal pendulum arm that moves up-and-down in response to vertical accelerations of the body. With each step an electrical contact is made, and one electrical event is recorded. The Yamax DW-500 pedometer has been shown to measure distance to within ±10% at speeds ranging from 54 to 94 m·min−1 (5). In addition, we found it to be a valid tool for assessing total daily walking distance.
In 1995, the CDC and ACSM (23) issued a joint position statement recommending that “Every U.S. adult should accumulate 30 min or more of moderate intensity physical activity on most, preferably all, days of the week.” Moderate intensity activities were defined as those eliciting an energy expenditure of 3.0–6.0 METs. It was further noted that, “One specific way to meet the standard is to walk two miles briskly” (2). Seventy percent of the people in our study walked at least this far. However, because walking speed was not determined, it is not known whether the walking they accumulated during “activities of daily living” met the minimum criteria for moderate intensity exercise (67 m·min−1 or 2.5 mph = 3.0 METs). Further studies are needed to determine what proportion of Americans are meeting the current activity recommendations.
Japanese researchers have recently begun to recommend 10,000 steps·d−1 for optimal health (12,28). This recommendation has the advantage of setting a standard for total physical activity (which recognizes the wide variability in housework, transportation, and occupational physical activity). However, it is restrictive in the sense that it does not include other types of physical activities such as swimming, rowing, and bicycling. Hatano (13) recently reported on walking (steps·d−1) in 500 Japanese citizens aged 30–79 yr. There was an age-related decline from 8240 to 4652 in men and from 7233 to 3930 in women for the youngest to oldest age groups. The number of steps·d−1 was inversely correlated to systolic blood pressure and skin-fold thickness in this population. Sequira et al. (25) examined Swiss residents aged 25–74 yr. These authors noted a decline from 11,900 to 6700 for men and from 9300 to 7300 for women in the youngest to oldest age groups. These studies suggest that pedometers might be useful in examining international differences in physical activity.
In summary, we found that participants under-reported their daily walking distance on the CAQ. This may be due to the wording of the walking question, which asks subjects about the number of “city blocks” walked per day. The wording may cause subjects to think about walking only in the context of city blocks and not to consider walking done in other settings (4). Physical activity questionnaires will continue to be a mainstay of epidemiologic investigation because they allow researchers to sample large populations with relative ease. The electronic pedometer provides a useful new method of examining questions about walking on these survey instruments.
This study was supported by a Professional Development Award to D. R. Bassett from The University of Tennessee, Knoxville, and a grant from the American Heart Association Southeast Research Consortium (9810204SE).
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