Epidemiologic evidence demonstrating a strong relation between a sedentary lifestyle and risk of cardiovascular and other diseases has prompted several government and scientific organizations to develop physical activity guidelines (25). Two guidelines are widely used: (a) the cardiorespiratory and muscular fitness guideline established by the American College of Sports Medicine (ACSM) in 1990 recommends vigorous exercise 3–5 d·wk−1 for at least 20 min per session (2), and (b) the health-related guideline established by the Centers for Disease Control and Prevention (CDC) and ACSM in 1995 (17) recommends that all adults accumulate at least 30 min or more of moderate-intensity physical activity on most, preferably all, days of the week. Because national policies are based on prevalence estimates of meeting these guidelines, it is essential that prevalence estimates are accurate.
To determine the prevalence of meeting the guidelines, valid, reliable, and low-cost measures of physical activity are needed. Self-report questionnaires remain the most commonly used method of assessing physical activity in epidemiologic studies. However, the extent to which they accurately quantify physical activity, particularly moderate intensity physical activity, remains questionable (25).
Prevalence rates may depend on the instrument used to assess physical activity. The characteristics of a questionnaire, such as how physical activity is defined, how the questions are worded, and how the response sets and scales are used, may affect estimates of physical activity (5,22,25). In addition, how the raw data are summarized or scored may affect the prevalence rates.
The problem is illustrated in the 1996 Surgeon General’s Report, Physical Activity and Health, which found varying rates of physical activity and sedentary lifestyles based on different measures applied to various samples (25). However, comparing rates across studies assumes that both samples are representative of the population and makes it difficult to conclude whether any differences are related to the sample or to the questionnaire. To our knowledge, no studies have compared prevalence rates using different questionnaires within the same sample. The purpose of this study was to compare prevalence estimates determined by multiple measures collected on the same sample in the same time period. Three commonly used measures of physical activity were selected for comparison. Because walking is known to be the most popular physical activity (26), prevalence rates were also assessed using only walking activity. A secondary purpose was to evaluate the effects of specific scoring protocols on the estimated prevalence of meeting the ACSM fitness and CDC health-related guidelines for physical activity.
METHODS
Participants
The study sample was recruited from the senior class at a large public university in Southern California. Informed consent was obtained from 575 men and women. The demographic and physical fitness characteristics of the participants are presented in Table 1. The mean age was 24.5 ± 1.9 yr, and 56% were women. The sample was 46% non-European-Americans and was representative of the university population in terms of age, ethnicity, and work status. During an initial telephone contact, participants were screened for physical limitations that would exclude them from participating in physical activity. Recruitment methods and results are described in detail elsewhere (21).
Table 1: Physical and demographic characteristics of participants.
Measures
The Seven-Day Physical Activity Recall (PAR) (3) was administered via telephone by trained research assistants. Interviewers followed the protocol outlined in a previously published article (20). The PAR was designed to examine total weekly energy expenditure. It assesses frequency, intensity, and duration of physical activity. Occupational and leisure-time physical activity were coded separately. The PAR has been found to be a valid and reliable measure of physical activity (16). A test-retest reliability of R = 0.86 was found when the PAR was administered by two independent interviewers to the same person on the same day (10).
The PAR interview asked participants to recall the time they spent sleeping and doing physical activities that were at least as intense as a normal-paced walk over the past 7 d. To aid in recall, participants were asked to recall day-by-day, starting with the previous day, and to segment the day into morning, afternoon, and evening time periods. Participants rated the intensity of their physical activities as moderate, hard, or very hard by comparing “moderate” activity to a normal-paced walk, and “very hard” to jogging or running. Those activities that fell in between the moderate and very hard were categorized as “hard.” Participants were also asked to determine the duration of each physical activity excluding rest periods. Physical activities were recorded if they added up to at least 10 min in one intensity category and occurred during one segment of the day. Intermittent activity (e.g., 10 min of walking, 10 min of gardening) was recorded if it met these requirements.
Physical activity was also assessed with self-report measures used in national surveys. One of these surveys was the CDC’s Youth Risk Behavior Survey (YRBS) which was adapted for college students (18,24). The YRBS has been used to study nationally representative samples of adolescents in high school, grades 9 through 12 (14). Questions examined the frequency of aerobic exercise done during the past 7 d. Participants reported the number of days per week (1–7) in which they engaged in these activities. Participants were asked “on how many of the past 7 d did you exercise or participate in sports activities for at least 20 min that made you sweat and breathe hard, such as basketball, jogging, fast dancing, swimming laps, tennis, fast cycling, or similar aerobic activities?”
A portion of the National Health Interview (NHIS) Survey was self-administered to collect specific information about quantity and types of physical activity participation (6). Participants were asked to report the number of times in the past 2 wk they engaged in any of the following 25 activities: walking, hiking, running/jogging, aerobic dance, other dance, calisthenics, golf, tennis, bowling, biking/exercise cycle, swimming, yoga or stretching, weight lifting, basketball, baseball/softball, football, soccer, volleyball, handball/racquet ball/squash, skating, skiing, rowing, surfing, housework, and other. For the present study, three activities were added to the list of 22 activities on the 1990 NHIS; surfing, rowing, and housework. Duration was assessed by asking how many minutes they did the activity each time. Test-retest reliability of this measure in our laboratory was R = 0.77. The validity of this survey has not been published.
Walking, which is one of the most common physical activities for women and men (25), was assessed with two survey measures. Time spent walking in the last 2 wk was measured in a question derived from the NHIS survey. The adapted question asked “about how many days in the past 2 wk did you walk for exercise, and about how many minutes did you walk for exercise each time in the past 2 wk?” Walking was also examined with four items that assessed the number of minutes walked to and from work and/or school, during lunch or breaks between classes, as part of errands performed outside the household, and walking for exercise (12,19). Test-retest reliability in an unpublished study was R = 0.67.
Physiological Measures
The Kasch Three-Minute Step Test (9), was used as an estimate of cardiovascular fitness. Participants stepped on and off a 12-inch bench at a rate of 94 steps·min−1 for 3 min. Five seconds after the completion of the stepping task, a 1-min recovery pulse count was taken at the radial artery with the participant in a seated position. Interobserver reliability of the pulse counts was determined on a randomly selected subsample of participants. Radial pulse was palpated on each arm simultaneously, and the interobserver reliability was R = 0.96. Participants were categorized as either “above average fitness” or “average fitness and below” based on their 1-min recovery heart rate (9). Body mass and height were measured without shoes on a calibrated Health-O-Meter balance beam scale. All tests were administered by trained research assistants in quiet settings.
Scoring Measures to Define Meeting the Physical Activity Guidelines
There is no well-accepted standardized method of scoring physical activity data to determine whether participants meet health or fitness guidelines. Table 2 describes the various scoring protocols applied to each measure to examine their effects on the prevalence estimates. The scoring protocols varied by the inclusion or exclusion of frequency data and the range of intensities included.
Table 2: Descriptions of scoring protocols for each questionnaire and criteria used to define meeting CDC/ACSM
a health-related (
18) and ACSM fitness guidelines (
2).
A MET value, a multiple of resting metabolic rate (RMR) (1 MET = 3.5 mL O2·kg−1·min −1), was assigned to each of the physical activities reported using the Compendium of Physical Activities (1). Physical activities were classified as moderate or vigorous intensity using MET ranges used in previous literature (17). Moderate intensity was defined as 3–5.9 MET and vigorous intensity was defined as 6 MET or greater. Six MET correspond to approximately 50% of V̇O2max and 60% of maximal heart rate for a young adult 20–39 yr of age (25).
ACSM fitness guidelines.
Two scoring protocols were applied to the data to determine whether the fitness guidelines were met. For the first protocol, the participant had to report at least three 20-min bouts of vigorous activity per week. However, these bouts did not necessarily occur on separate days of the week. For the second protocol, the participant had to report at least 60 min of vigorous activity per week. This distinction is relevant because it is unclear in epidemiologic studies whether the pattern of activity or total energy expenditure is more important for health outcomes (11).
CDC/ACSM health-related guidelines.
Compared with the ACSM fitness guidelines, the CDC/ACSM health-related guidelines leave more room for interpretation. The guidelines allow people a choice of accumulating minutes of physical activity over the course of the day or doing 30-min bouts of physical activity. In addition, there is a choice of whether to do moderate intensity activity or moderate to vigorous activity. To determine the prevalence of meeting the health-related guidelines, four scoring protocols were applied to the data. The protocols included one that considered a frequency of five times per week and another that considered an accumulation of 150 min·wk−1 instead of frequency. To these scoring protocols, two intensity ranges were applied: (a) moderate intensity (3 to 5.9 MET), and (b) moderate intensity and above (≥3 MET).
Statistical Analyses
Analyses were done using the Statistical Package for the Social Sciences, Version 7.5 (23). Frequencies were computed for the percentage of male and female participants meeting ACSM and CDC guidelines, according to various criteria noted. Cochran’s Q was used to determine whether the distributions were different among the self-report measures.
To assess construct validity of the physical activity scores, logistic regression tested the relation between meeting the CDC and ACSM guidelines and cardiovascular fitness, as estimated by the step test. Participants were divided into either average and below-average fitness, or above-average fitness, according to their recovery heart rate. Participants who met the ACSM and CDC guidelines were expected to be in the above average fitness category.
RESULTS
Table 3 illustrates the percentage of participants who met the current ACSM and CDC physical activity guidelines. For men and women combined, the proportion of participants meeting the ACSM fitness guideline for vigorous exercise 3 d·wk−1, 20 min per bout ranged from 32 to 40% using the NHIS and YRBS, respectively. When total weekly minutes of vigorous exercise were computed without considering frequency, the NHIS and PAR were similar.
Table 3: Percentage of participants meeting the ACSM and CDC guidelines.a
The proportion of participants meeting the CDC/ACSM health-related guidelines varied by questionnaire and scoring protocol. The proportion meeting the guideline ranged from 3.5 to 17% when only moderate intensity activity was considered with the 5 d·wk−1, 30 min per bout scoring protocol. When moderate to vigorous intensity activity was included, NHIS and PAR produced similar percentages. Applying the 150 min·wk scoring protocol resulted in 7.7–43.7% meeting the guideline if only moderate intensity activity was included. The PAR reported over twice the number of participants meeting the guideline compared with the NHIS questionnaire. The ratio was similar for the PAR and NHIS results when moderate to vigorous activity was included in the 150 min·wk−1 protocol.
The Type of Walking item resulted in a much higher estimate of those meeting the CDC/ACSM health-related guideline compared with the NHIS Walking item. For the NHIS Walking item, the 150 min·wk−1 protocol doubled the percentage meeting the guidelines compared with the 5 d·wk−1 30 min per bout scoring protocol.
Results of the Cochran’s Q showed that the questionnaires produced significantly different proportions for each scoring protocol. The proportions meeting the guidelines were compared within each scoring protocol. The NHIS, PAR, and YRBS produced significantly different percentages for the ACSM guideline of 3 d·wk−1, 20 min per bout (P < 0.001). However, the NHIS and PAR were not significantly different when the 60 min·wk−1 scoring protocol was applied (P < 0.233). The distributions among the questionnaires for the CDC/ACSM guidelines were significantly different for the 5 d·wk−1, 30 min per bout (P < 0.0001) and for the 150 min·wk−1 (P < 0.0001) scoring protocols.
Table 4 compares different scoring protocols for the PAR and its effects on percentages meeting the ACSM and CDC guidelines. Leisure-only and occupational-plus-leisure physical activity were analyzed. Including occupational physical activity increased the percentage meeting the ACSM and the CDC/ACSM guidelines. The percentage meeting the ACSM fitness guidelines was increased by approximately 5 to 6 percentage points for the 3 d·wk−1, 20 min per bout scoring protocol and 60 min·wk−1 protocol. The percentage meeting the CDC/ACSM guidelines was increased by 13 to 14 percentage points when occupational activity was included in the 5 d·wk−1, 30 min per bout protocols. When occupational physical activity was included in the 150 min·wk−1 scoring protocols, the percentages went up by 12 and 9 percentage points for moderate intensity only and moderate to vigorous intensity protocols, respectively.
Table 4: Comparison of percentages of participants meeting physical activity guidelines with occupational-plus-leisure versus leisure-only activity, from the Seven-day Physical Activity Recall Interview (PAR).
Table 5 illustrates the results of the logistic regression analyses. In general, the odds were over three times greater of being above average in fitness for participants who met the ACSM fitness guideline. Odds ratios for the ACSM fitness guidelines showed little variation among the self-report measures. Although the odds ratio were lower, meeting the CDC/ACSM health guideline was also a significant predictor of above average fitness. The NHIS, NHIS Walking item, and the measures that included physical activities ≥ 3 MET resulted in the highest odds ratios. Including only moderate intensity (3–5.9 MET) physical activities in the scoring protocol produced lower odds ratios.
Table 5: Odds of being in the “above average” fitness category based on meeting the ACSM and CDC/ACSM guidelines.
DISCUSSION
The results indicate that the prevalence of meeting physical activity guidelines are dependent on the physical activity measure and the scoring protocol used. Different scoring protocols appear to have more of an effect on the prevalence of meeting the CDC/ACSM health-related guidelines compared with the ACSM fitness guidelines.
ACSM fitness guidelines.
Compared with the NHIS and the PAR, the YRBS elicited the highest prevalence of meeting the ACSM fitness guidelines when both frequency and duration were considered in the scoring. It is not unexpected that an interval response option would produce a higher prevalence compared with open-ended responses. It is possible that the YRBS response options (0–7 d·wk−1) increased the percent reporting 3 d or more per week. Respondents may have been more likely to select the numbers of days in the middle of the range, i.e. 3, 4, and 5, than the extreme options of 1 and 2. It also appears that men may over-report on the YRBS (43%) compared with the prevalence rates elicited by the PAR and the NHIS recall questions (34%, and 35%, respectively). Women had similar percentages on the YRBS and PAR (37 vs 40%), yet the NHIS produced a much lower rate (29%). This sex difference may be related to the list of activities on the NHIS survey, which may be more representative of men’s than women’s activities.
The proportion of participants meeting the fitness guideline using the YRBS question (37.4% of women, 43.1% of men) was similar to previous studies that used the YRBS with university students. Patrick et al. (18) surveyed 3,810 university students in California and found 40.3% of the women and 49.2% of the men met the fitness guideline. In a nationally representative sample of university students (N = 4,838), the same question produced a similar proportion, 33% women and 43.7% men (8). These results suggest that the present sample is similar in vigorous exercise habits to other university samples.
Prevalence rates for meeting the ACSM fitness guideline were about two times higher than 1991 NHIS national data on men and women 18–29 yr of age. Present rates, based on NHIS, were 35.3% for the men and 29.7% for the women compared with national data of 19.7% of men and 16% of women. The 1992 BRFSS produced even lower rates among those 18–29 yr of age (8% of men, 11% of women) (25). The difference in rates may be related to the differences in the samples’ level of education, age, or other demographics (15) with the present sample being better educated than the general population (25). Another possible reason for this discrepancy is that the 1991 NHIS national data were summarized with a relative-intensity criterion of physical activities that were 50% of age and sex-specific maximum cardiorespiratory capacity. The present study used an absolute intensity criterion of physical activities classified as 6 MET or more. If an intensity of 6 MET is less than 50% of maximal cardiorespiratory capacity for these young adults, then this may account for some of the differences.
For the 3 d·wk−1, 20 min per bout protocol, the difference between high and low prevalence estimates was 10 percentage points for women and 9 percentage points for men. This is a large discrepancy and indicates that it is not advisable to compare results of different studies using different measures.
CDC/ACSM health-related guidelines.
Comparing the various measures in terms of the CDC/ACSM health-related guidelines, a much greater variation is apparent in the prevalence rates. Prevalence estimates ranged from 4 to 38% using the 5 d·wk−1, 30 min per bout scoring protocol. Again, the NHIS and PAR gave remarkably similar results for both the moderate intensity only protocol and the moderate to vigorous intensity protocol. However, the NHIS Walking for exercise item produced an unexpectedly low prevalence rate in comparison with the PAR and NHIS (list of 25 activities). It was expected that walking would constitute a larger portion of physical activity, because it is the second most common physical activity done for exercise among a national sample of adults ages 18–29 yr (25). It is possible that subjects in a university sample perform a wider variety of activities compared with a national sample of same-aged adults. The results suggest that walking should not be considered an adequate sample of moderate intensity activities, at least in young adults.
For the health-related guidelines using the 150 min·wk−1 definition, the PAR produced rates that were approximately 22–27 percentage points higher than the NHIS. This range is most likely related to the specificity of the questionnaires. The NHIS and Type of Walking item are limited by number of activities, whereas the PAR is limited by the duration of the activity. Measuring only walking activities (Type of Walking item) or sport and exercise-related activity (NHIS) restricts the response options and may not capture other relevant types of physical activity, consequently lowering the prevalence rates. On the PAR, any type of activity that is at least moderate in intensity and has a duration of at least 10 min is recorded. These features suggest that the PAR is more appropriate for determining the prevalence of meeting the CDC/ACSM 1995 health-related guidelines.
Few studies have determined the prevalence rate of meeting the CDC/ACSM health-related guidelines. Jones et al. (13) applied the criteria of 5 d or more per week at 30 min per bout and an intensity of 3–6 MET to the NHIS 1990 survey data on 30,582 adults and found that 42.6% met the health-related guidelines. Applying a similar scoring protocol (5 d·wk−1, 30 min per bout, 3–5.9 MET) to the NHIS data in the present study, only 17% of the participants met the guideline. It is possible that this discrepancy is related to not including 6 MET in the intensity range. The cut point for vigorous intensity activities is often set at 6 MET. A range of 3–5.9 MET may not capture vigorous intensity activities whereas including 6 MET may. When the intensity criterion was increased to include activities that were ≥ 3 MET, the prevalence rates for the present sample were more similar to the results of Jones et al. (13).
The prevalence rates of meeting the CDC/ACSM health-related guidelines varied greatly, mainly because of the inclusion of the NHIS Walking item and the various scoring protocols. Measures that include a variety of activities as opposed to a single activity, such as walking, may more accurately reflect the prevalence of meeting the health-related guidelines. The PAR is the most inclusive measure, and it consistently produced the highest estimates.
Scoring protocols.
Scoring protocols for physical activity measures affect prevalence rates. Several scoring protocols that can be derived from the physical activity guidelines were tested. In this study, the percentage of participants meeting the guidelines was lower when the scoring format required meeting specific frequency and duration criteria. When frequency was not considered in the scoring protocol for the fitness guideline (60 min·wk−1), prevalence rates increased by 23.5 percentage points for the NHIS and by 22.2 points for the PAR. In addition, the rates generated by the NHIS and PAR were not significantly different (55.7 vs 59.4%). These results suggest that university students may exercise vigorously for longer durations but lower frequency than that recommended by the ACSM. This could result from extensive sport or recreation activity on weekends only. Moreover, if frequency is not considered in the scoring protocol, prevalence rates could be vastly overestimated.
Prevalence estimates were also higher when frequency was not included in the criteria of the health-related guidelines. The 150 min·wk−1 protocol increased rates by 3.7 to 32.5 percentage points over that of the 5 d·wk−1, 30 min per bout protocol depending on the instrument and range of intensity used. The exclusion of frequency from the scoring protocol appeared to affect the PAR more than the NHIS. Again, this suggests that the PAR may be more sensitive to lifestyle physical activities that are done intermittently throughout the day, whereas the NHIS maybe a better measure of physical activities that are completed in distinct bouts.
These results suggest that studies reporting prevalence rates of meeting guidelines should be interpreted carefully. Further epidemiologic studies of health outcomes are necessary to resolve the controversy of whether total energy expenditure or specific physical activity regimens are most related to health and fitness (11). At the present time, the scoring protocols should include frequency and duration of physical activity because this reflects the major guidelines more closely.
The CDC/ACSM health-related guidelines have been interpreted in various ways since their inception in 1993 (7). One interpretation is that the guideline encourages only moderate intensity activity, whereas another interpretation includes moderate to vigorous intensity physical activity. In addition, the guideline recommends 30 min of activity on most to all days of the week with the option of accumulating the minutes in as low as 10-min bouts. Because people are able to choose how they will meet the guideline, it is difficult to determine how to summarize the data. In this study, including only moderate intensity (3–5.9 MET) activity in the scoring protocol reduced the prevalence estimates of meeting the health-related guidelines by approximately 50% for all measures. A discrepancy of this magnitude suggests that prevalence estimates should be determined for each interpretation.
To our knowledge, no data exist that compare prevalence rates of meeting the CDC/ACSM health-related guidelines among different instruments or scoring protocols. The 1991 NHIS and 1992 BRFSS data were summarized with a scoring protocol of 5 times per week for 30 min per bout of any type or any intensity of physical activity (25). Because these prevalence rates include activities of light intensity, it is not reasonable to compare these findings with those of the present study. It is important to obtain national estimates of meeting the CDC/ACSM guidelines, and scoring protocols should be standardized to permit comparisons across samples.
Because the PAR has the capacity of measuring both leisure-time and occupational physical activity, it was of interest to compare how the inclusion and exclusion of these modes affected the prevalence rates. It is likely that excluding occupational physical activity underestimates the prevalence of meeting health and fitness guidelines. In Table 4, the addition of occupational activity increased the prevalence rates by about 5–6 percentage points for the fitness guidelines and 9–14 percentage points for the health-related guidelines. The addition of occupational activity appeared to have its greatest effect on the health-related guidelines 5 d·wk−1, 30 min per bout protocol that included only moderate intensity activity, with the prevalence rate nearly doubling (16 to 29.2%). This finding suggests that a substantial portion of moderate-intensity activity is done during work hours for this sample of university students. The influence of including occupational physical activity on prevalence rates is likely to vary by the age and socioeconomic status of the population.
Construct validity of physical activity self-reports.
It was of interest to examine the construct validity of the physical activity self-reports. Meeting the ACSM fitness guidelines has been shown to result in physical fitness gains (2). If the self-report is a valid measure of meeting the guidelines, then those who met the recommendations should have corresponding fitness levels. Although enhancing fitness is not the primary purpose of the CDC/ACSM health-related guidelines (17), it is possible that those who meet the recommendations may have subsequent increases in cardiovascular fitness (4). As expected, the odds of being in the above average fitness category were about three to four times higher for participants who met the ACSM fitness guidelines. The odds ratios were similar whether the frequency of exercise was at least three days per week or whether the total duration was at least 60 min. Moreover, the odds ratios were similar for each of the questionnaires used.
As expected, the measures used to assess meeting the CDC/ACSM health recommendations were less predictive of fitness levels. Scoring protocols that included moderate to vigorous intensity activities (≥3 MET) predicted fitness better than scores that only included moderate intensity physical activity (3–5.9 MET). The NHIS, including the NHIS Walking item, significantly predicted fitness whether or not the frequency of physical activity was considered.
Interestingly, the NHIS Walking item was a strong predictor of fitness, yet the Type of Walking item was not. The inconsistency may be related to the type of walking behavior assessed and how the question is formatted. The NHIS asks participants to report “walking for exercise.” The Type of Walking item asks participants to report various forms of lifestyle walking and walking for exercise. These lifestyle-types of walking are most likely done at lower intensities compared with walking for exercise. Although the Type of Walking item includes walking for exercise, it is a small proportion of total walking. The NHIS Walking item may be a marker for participation in other exercise-related activities that would affect the odds of being in the above average fitness category.
CONCLUSIONS
This study of multiple physical activity self-reports in the same sample demonstrated large differences in physical activity prevalence estimates. The major implication is that it is invalid to compare prevalence estimates that are based on different measures. There is a critical need to standardize measures of physical activity used in population studies. The type of measure affects prevalence of meeting the CDC/ACSM health-related guidelines more than the ACSM fitness guidelines. Choice of a scoring protocol also has effects on prevalence estimates. A consensus needs to be developed for specific operational definitions of the physical activity guidelines and inclusion of occupational activities.
Strengths of the study included the evaluation of multiple widely-used measures and evidence of construct validity. A limitation is the narrow age and education characteristics of the sample, so the findings need to be replicated in other groups.
Accurate prevalence estimates of physical activity participation are needed to guide health policy. Application of varying measures and scoring protocols in different samples renders comparisons problematic. The present study defines the extent of discrepancies but does not identify the most valid methods. If self-reports continue to be used for estimating population prevalence of physical activity, selection of standard consensus measures should be based on solid evidence of validity.
This work was supported by National Institutes of Health HL 49505.
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