The data

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The data are from the 2007 to 2009 Canadian Health Measures Survey (CHMS), which collected physical measurements for the household population aged 6 to 79. The survey excluded residents of Indian Reserves, Crown lands, institutions and certain remote regions, and full-time members of the Canadian Forces. Approximately 96% of Canadians were represented. Data were collected at 15 sites across Canada from March 2007 through February 2009. Ethics approval to conduct the survey was obtained from Health Canada's Research Ethics Board.¹¹ Details about the CHMS are available elsewhere.³

Participants were interviewed at home before visiting a mobile examination centre for a series of physical measurements. Upon completion of that visit, ambulatory respondents were asked to wear an Actical accelerometer (Phillips – Respironics, Oregon, USA) over their right hip on an elasticized belt during their waking hours for seven consecutive days. The Actical (dimensions: 2.8 x 2.7 x 1.0 centimetres; weight: 17 grams) measures and records time-stamped acceleration in all directions, providing an index of physical activity intensity. The digitized values are summed over a user-specified interval of 1 minute, resulting in a count value per minute. Accelerometer signals are also recorded as steps per minute. The Actical has been validated for measuring physical activity in adults¹² and children,^13,14 and step counts in adults and children.¹⁵

The monitors were initialized to start collecting data at midnight following the mobile examination centre visit. The data being recorded were not accessible by the respondents who were wearing the devices. The monitors were returned to Statistics Canada in a prepaid envelope, where the data were downloaded and the monitor was checked to determine if it was still within the manufacturer's calibration specifications.⁴

A total of 4,440 respondents returned the accelerometer with at least four valid days. A valid day was defined as having 10 or more hours of wear time. Wear time was determined by subtracting nonwear time from 24 hours. Nonwear time was defined as at least 60 consecutive minutes of zero counts, with allowance for 2 minutes of counts between 0 and 100. For each minute, the level of movement intensity—sedentary, light, and moderate-to-vigorous (MVPA)—was based on cut-points corresponding to intensity level. The MVPA threshold was set at 1,500 for children and youth aged 6 to 19,¹³ and at 1,535 for adults aged 20 to 79.¹⁶ Minutes were summed by time period, defined as 7:00 to 8:59 a.m., 9:00 to 10:59 a.m., 11:00 a.m. to 12:59 p.m., 1:00 to 2:59 p.m., 3:00 to 4:59 p.m., 5:00 to 6:59 p.m., 7:00 to 8:59 p.m. and 9:00 p.m. to 6:59 a.m. and averaged over the number of valid days. The final response rate for having a minimum of four valid days was 41.8% (69.6% for selected household x 88.1% for selected person x 84.2% for mobile examination centre visit x 80.8% for valid accelerometer results).

Weekend estimates result from average minutes of MVPA on valid Saturday and Sunday, and weekday estimates, from average minutes of MVPA on valid Monday through Friday. Terciles of MVPA represent the weighted 33.3% and 66.6% of daily average of MVPA. Terciles were calculated by age group and sex.

Height was measured to the nearest 0.1 cm using a ProScale M150 digital stadiometer (Accurate Technology Inc., Fletcher, USA), and weight was measured to the nearest 0.1 kg with a Mettler Toledo VLC with Panther Plus terminal scale (Mettler Toledo Canada, Mississauga, Canada).¹⁷ Body mass index (BMI) was calculated as weight (kg) divided by squared height (m) and was classified according to published BMI thresholds: normal (18.5 to 24.9 kg/m²), overweight (25.0 to 29.9 kg/m²), and obese (30.0 kg/m² or more) for adults.^18,19 

To account for survey design effects of the CHMS, standard errors, coefficients of variation, and 95% confidence intervals were estimated using the bootstrap technique.^17,20,21 Statistical significance was set at a p value of 0.05.  Bonferroni adjustments were performed when comparing the different time periods. The number of degrees of freedom was specified as 11 to account for the CHMS sample design.¹⁷