Letters RESEARCH LETTER
Patterns of Accelerometer-Assessed Sedentary Behavior in Older Women Recent studies suggest a high volume of sedentary behavior may be a risk factor for adverse health outcomes.1 However, few data exist on how this behavior is patterned (eg, does most sedentary behavior occur in a few long bouts or in many short bouts?) and whether sedentary patterns are relevant for health. We examined details of sedentary behavior among older women. Because physical activity is influenced by age, body mass index (BMI; calculated as weight in kilograms divided by height in meters squared), 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 continuing observational follow-up (2004-present).2 An observational ancillary study, begun in 2011, is assessing physical activity using accelerometers. This cross-sectional study included all women who returned the accelerometer by March 2013. Women provided written consent to participate and the study was approved by the institutional review board of Brigham and Women’s Hospi-
tal. Women were mailed an accelerometer (ActiGraph GT3X+, ActiGraph Corp) and detailed instructions and were 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, nonwear 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 upstream and downstream. Wear days were identified using the diary. A bout of sedentary behavior was defined as consecutive minutes in which the accelerometer registered less than 100 counts per minute.4 A break in sedentary behavior was defined as at least 1 minute in which counts registered at least 100 following a sedentary bout. Using leastsquares regression (SAS version 9.3, SAS Institute Inc), we tested for differences in mean values among subgroups using 2-sided tests with a significance level of P < .05. Results | Of 8373 women who returned the accelerometer by March 2013, we excluded 723 who did not return a diary and 403 who did not have at least 4 days of at least 10 hours of wear per day (standard convention)5; resulting in 7247 women (87%) with a mean (SD) age of 71.4 (5.8) years. Women wore the accelerometer for a mean (SD) of 14.8 (1.2) hours per day over a mean (SD) of 6.8 (0.57) days. The mean (SD) percentage of wear
Table 1. Characteristics of Accelerometer-Assessed Sedentary Behavior Among All Women and in Subgroups in the Women's Health Study, 2011-2013 Wear Time Spent in Sedentary Behaviora No. (%) Women All women
7247
Mean (95% CI), %d
Time/d Spent in Sedentary Behavior Mean (95% CI), hd
P Valuee
65.5 (65.3-65.7)
P Valuee
9.7 (9.7-9.7)
Bouts/d of Sedentary Behaviorb Mean (95% CI), No.d
P Valuee
85.9 (85.5-86.3)
Breaks/Sedentary Hourc Mean (95% CI), No.d
P Valuee
9.0 (9.0-9.1)
Age, y
Patterns of Accelerometer-Assessed Sedentary Behavior in Older Women Recent studies suggest a high volume of sedentary behavior may be a risk factor for adverse health outcomes.1 However, few data exist on how this behavior is patterned (eg, does most sedentary behavior occur in a few long bouts or in many short bouts?) and whether sedentary patterns are relevant for health. We examined details of sedentary behavior among older women. Because physical activity is influenced by age, body mass index (BMI; calculated as weight in kilograms divided by height in meters squared), 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 continuing observational follow-up (2004-present).2 An observational ancillary study, begun in 2011, is assessing physical activity using accelerometers. This cross-sectional study included all women who returned the accelerometer by March 2013. Women provided written consent to participate and the study was approved by the institutional review board of Brigham and Women’s Hospi-
tal. Women were mailed an accelerometer (ActiGraph GT3X+, ActiGraph Corp) and detailed instructions and were 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, nonwear 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 upstream and downstream. Wear days were identified using the diary. A bout of sedentary behavior was defined as consecutive minutes in which the accelerometer registered less than 100 counts per minute.4 A break in sedentary behavior was defined as at least 1 minute in which counts registered at least 100 following a sedentary bout. Using leastsquares regression (SAS version 9.3, SAS Institute Inc), we tested for differences in mean values among subgroups using 2-sided tests with a significance level of P < .05. Results | Of 8373 women who returned the accelerometer by March 2013, we excluded 723 who did not return a diary and 403 who did not have at least 4 days of at least 10 hours of wear per day (standard convention)5; resulting in 7247 women (87%) with a mean (SD) age of 71.4 (5.8) years. Women wore the accelerometer for a mean (SD) of 14.8 (1.2) hours per day over a mean (SD) of 6.8 (0.57) days. The mean (SD) percentage of wear
Table 1. Characteristics of Accelerometer-Assessed Sedentary Behavior Among All Women and in Subgroups in the Women's Health Study, 2011-2013 Wear Time Spent in Sedentary Behaviora No. (%) Women All women
7247
Mean (95% CI), %d
Time/d Spent in Sedentary Behavior Mean (95% CI), hd
P Valuee
65.5 (65.3-65.7)
P Valuee
9.7 (9.7-9.7)
Bouts/d of Sedentary Behaviorb Mean (95% CI), No.d
P Valuee
85.9 (85.5-86.3)
Breaks/Sedentary Hourc Mean (95% CI), No.d
P Valuee
9.0 (9.0-9.1)
Age, y
