To the Editor
Recent studies suggest a high volume of sedentary behavior may be an independent risk factor for adverse health outcomes.1 However, there are few data on how this behavior is patterned, (e.g., does most sedentary behavior occur in a few long bouts or in many short bouts?), and whether sedentary patterns are relevant for health. Thus, we examined details of sedentary behavior, assessed by accelerometers, among older women. Because physical activity is influenced by age, body mass index (BMI), and smoking status, we further examined sedentary behavior in relation to these characteristics.
Methods
The Women’s Health Study is a completed randomized trial (1992-2004) of aspirin and vitamin E for preventing cardiovascular disease and cancer among 39,876 healthy women throughout the United States, with continued observational follow-up (2004-present).2 An observational ancillary study, begun in 2011, is assessing physical activity using accelerometers. Women provided written consent to participate and the study was approved by the Brigham and Women’s Hospital’s institutional review board committee. Women were mailed an accelerometer (ActiGraph GT3X+) and detailed instructions, and asked to wear it for 7 days during waking hours. They also completed a wear time diary, indicating which days the monitor was worn.
Accelerometer data were screened for wear time using standard methods.3 Briefly, non-wear time was defined as 90 consecutive minutes of zero counts, with an allowance of up to 2 minutes of nonzero counts, if there were 30-minute consecutive zero counts up- and down-steam. Wear days were identified using the wear time diary. A bout of sedentary behavior was defined as consecutive minutes where the accelerometer registered less than 100 counts per minute.4 A break in sedentary behavior was defined as at least 1 minute where counts registered at least 100 following a sedentary bout. Using least-squares regression (SAS 9.3), we tested for differences in mean values among subgroups using two-sided tests with a significance level of p<0.05.
Results
Cross-sectionally, we examined 8,373 women who returned the accelerometer by March 2013. We excluded 723 women who did not return a diary and 403 women who did not have at least 4 days of at least 10 hours of wear per day (standard convention);5 resulting in 7,247 women (87%) with mean age 71.4 (SD = 5.8) years. On average, women wore the accelerometer for 14.8 (1.2) hours per day over 6.8 (0.57) days. The mean percent of wear time spent in sedentary behavior was 65.5% (9.0), equivalent to 9.7 (1.5) hours per day (Table 1). The mean number of sedentary bouts per day was 85.9 (16.1), with 9.0 (2.4) breaks per sedentary hour. Adjusting for wear time and smoking status, total sedentary time and the sedentary bout duration differed by age and BMI (p<0.001).
Table 1.
Characteristics of accelerometer-assessed sedentary behavior among all women and in subgroups, Women’s Health Study, 2011−2013
| Percent of Wear Time Spent in Sedentary Behaviora |
Time Spent per Day in Sedentary Behavior (Hours) |
No. of Bouts per Day of Sedentary Behaviorb |
No. of Breaks per Sedentary Hourc |
||||||
|---|---|---|---|---|---|---|---|---|---|
| No. of Women (%)d |
Mean (95% CI) | P-valuee | Mean (95% CI) | P-valuee | Mean (95% CI) | P-valuee | Mean (95% CI) | P-valuee | |
|
|
|||||||||
| All Women | 7247 | 65.5 (65.3, 65.7) | 9.7 (9.7, 9.7) | 85.9 (85.5, 86.3) | 9.0 (9.0, 9.1) | ||||
| Women categorized byf Age, y |
|||||||||
| <65 | 844 (11.6) | 63.5 (62.9, 64.1) | 9.4 (9.3, 9.5) | 88.2 (87.2, 89.3) | 9.6 (9.5, 9.8) | ||||
| 65 to <70 | 2721 (37.5) | 65.1 (64.7, 65.5)* | <0.001 | 9.6 (9.6, 9.7)* | <0.001 | 86.2 (85.6, 86.9)* | <0.001 | 9.1 (9.0, 9.3)* | <0.001 |
| ≥70 | 3682 (50.8) | 69.1 (68.7, 69.5)* | 10.2 (10.2, 10.3)* | 82.7 (82.0, 83.3)* | 8.2 (8.1, 8.3)* | ||||
|
| |||||||||
| BMI, kg/m2 | |||||||||
| <25 | 3300 (45.6) | 63.9 (63.5, 64.3) | 9.5 (9.4, 9.5) | 90.1 (89.4, 90.7) | 9.7 (9.6, 9.8) | ||||
| 25 to <30 | 2518 (34.8) | 65.4 (64.9, 65.9)* | <0.001 | 9.7 (9.6, 9.7)* | <0.001 | 85.8 (85.0, 86.5)* | <0.001 | 9.1 (8.9, 9.2)* | <0.001 |
| ≥30 | 1425 (19.7) | 68.3 (67.8, 68.9)* | 10.1 (10.0, 10.2)* | 81.3 (80.4, 82.1)* | 8.2 (8.1, 8.4)* | ||||
|
| |||||||||
| Smoking Status | |||||||||
| Never | 3569 (49.3) | 65.0 (64.7, 65.4) | 9.6 (9.6, 9.7) | 85.8 (85.2, 86.3) | 9.1 (9.0, 9.2) | ||||
| Former | 3362 (46.4) | 64.7 (64.4, 65.1) | <0.001 | 9.6 (9.5, 9.6) | <0.001 | 85.9 (85.3, 86.4) | 0.53 | 9.2 (9.1, 9.3) | 0.005 |
| Current | 310 (4.3) | 67.9 (66.9, 68.9)* | 10.0 (9.9, 10.2)* | 85.5 (84.0, 87.0) | 8.7 (8.5, 9.0)* | ||||
Abbreviations: BMI, body mass index; 95% CI, 95% confidence interval.
Women were considered to be sedentary if their accelerometer registered <100 counts per minute.
A bout of sedentary behavior is defined as consecutive minutes where the accelerometer registered <100 cpm.
A break in sedentary behavior is defined as at least one minute where counts registered ≥100 following a bout of sedentary behavior.
BMI and smoking status were not reported for all women.
P-value tests for equal means across all levels of subgroup.
Means are adjusted for wear time and other subgroup characteristics (age, BMI, and smoking status).
Pairwise mean difference p<0.05 when compared to age <65, BMI <25, or Never Smokers.
Most sedentary time occurred in bouts of shorter duration (Table 2). Among the total number of sedentary bouts, the mean percent of bouts of at least 30 minutes was 4.8% (2.9), representing 31.5% (12.4) of total sedentary time.
Table 2.
Number of Sedentary Bouts per Day, Percent of Sedentary Bouts, and Percent of Sedentary Time of Various Bout Durations, Women’s Health Study, 2011−2013
| Bout Duration (minutes) |
No. of Sedentary Bouts per Day (Mean [SD]) |
% of Sedentary Bouts (Mean [SD]) |
% of Sedentary Time (Mean [SD]) |
|---|---|---|---|
| 1+ | 85.9 (16.1) | 100 | 100 |
| 5+ | 29.8 (4.7) | 35.5 (6.7) | 81.6 (6.4) |
| 10+ | 15.9 (3.2) | 19.4 (5.9) | 65.5 (10.1) |
| 20+ | 7.0 (2.2) | 8.7 (4.1) | 44.5 (12.5) |
| 30+ | 3.8 (1.6) | 4.8 (2.9) | 31.5 (12.4) |
| 40+ | 2.2 (1.2) | 2.9 (2.1) | 22.7 (11.3) |
| 50+ | 1.4 (0.9) | 1.8 (1.6) | 16.5 (10.0) |
| 60+ | 0.9 (0.7) | 1.2 (1.2) | 11.9 (8.6) |
Abbreviations: SD, standard deviation.
Women were considered to be sedentary if their accelerometer registered <100 counts per minute. A bout of sedentary behavior is defined as consecutive minutes recorded on the accelerometer where counts per minute <100.
Discussion
This study provides a detailed analysis of sedentary behavior patterns among a large sample of older women, more than 6 times the size of previous similar studies.6 We found that older women spent about two-thirds of waking time in sedentary behavior, most of which occurred in bouts lasting less than 30 minutes. Previous studies have shown a similar proportion of time spent in sedentary behavior, however, these did not report on patterns.4,6
While accelerometers provide objective measures, they cannot convey postural information (i.e., sitting vs. standing still). However, these older women are unlikely to be standing still for long periods. These data are limited to women from the Women’s Health Study, who are primarily White and of higher socioeconomic status; however, total sedentary time was similar to a national sample.4 If future studies confirm the health hazards of sedentary behavior and guidelines are warranted, these data will be useful to inform recommendations on how to limit such behavior.
Acknowledgements
We are grateful to the staff of the Women’s Health Study (Brigham and Women’s Hosptial), particularly Ara Sarkissian, MM; Bonnie Church, BA; Colby Smith, and Jane Jones, MEd. None of the persons named in the acknowledgments were compensated for manuscript preparation.
Funding/Support and Role of Sponsor
This research was supported by research grants CA154647, CA047988, CA121005, HL099557, HL043851, HL080467, HL099355, and HL007575 from the National Institutes of Health. The National Institutes of Health played no role in the design and conduct of the study; the collection, management, analysis, and interpretation of the data; or the preparation, review, or approval of the manuscript.
Footnotes
Conflicts of interest
EJ Shiroma and IM Lee report no conflicts of interest. PS Freedson and SG Trost are members of the Actigraph Scientific Advisory Board (Pensacola, FL).
Contributor Information
Eric J Shiroma, Harvard School of Public Health 900 Commonwealth Ave 3rd Floor Boston, MA 02215 (843) 408 – 6167.
Patty S Freedson, University of Massachusetts - Amherst.
Stewart G Trost, University of Queensland.
I-Min Lee, Harvard Medical School.
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