Association Between Cardiorespiratory Fitness and Accelerometer-Derived Physical Activity and Sedentary Time in the General Population
Does sedentary behavior reduce cardiorespiratory fitness independently from exercise activity?
A total of 2,223 participants from the National Health and Nutrition Examination Survey (NHANES) 2003-2004, were included in the present study. Participants were free of known heart disease, had completed cardiovascular fitness testing, and had accelerometer data for 1 or more days (up to 7 days). Accelerometer data were used to quantify exercise bouts in each participant. Bouts of exercise were defined as at least 8-10 minutes above prespecified count threshold and quantified as the mean minutes of activity bouts per day for each participant. Sedentary time was defined as <100 counts/min of wear time in mean minutes/day. Sedentary time was quantified in three different ways: average daily sedentary time (hours/day); proportion of total wear time that was sedentary; and average count intensity during sedentary time, an indicator of stillness during sedentary time. Fitness was assessed by submaximal treadmill testing. Fitness was categorized into three levels. A low level of cardiorespiratory fitness was defined as an estimated maximum oxygen consumption at or below the 20th percentile of the Aerobics Center Longitudinal Study data for the same sex and age; moderate fitness is defined as a value between the 20th and the 59th percentile; and high fitness is defined as a value at or above the 60th percentile. For adolescents ages 12-19 years, standards were based on criteria from the FITNESSGRAM program.
The mean age of the study population was 22.4 ± 10.3 years, with 1,053 (47%) females. The duration of accelerometer wear time did not differ across fitness groups for all participants. Women were more sedentary than were men (7.0 ± 2.1 hours/day vs. 6.6 ± 2.4 hours/day, respectively; p < 0.001). Women also had less average total daily moderate and vigorous activity time than did men (0.13 ± 0.22 hours/day vs. 0.28 ± 0.37 hours/day, respectively; p < 0.001). An additional hour of daily exercise activity time was associated with a 0.88 (0.37-1.39; p < 0.001) metabolic equivalent of task (MET) higher fitness level for men, and a 1.37 (0.43-2.31; p = 0.004) MET higher fitness level for women. An additional hour of sedentary time was associated with a -0.12 (-0.02 to -0.22; p = 0.03) and a -0.24 (-0.10 to -0.38; p < 0.001) MET difference in fitness for men and women, respectively.
The investigators concluded that after adjustment for exercise activity, sedentary behavior appears to have an inverse association with fitness. These findings suggest that the risk related to sedentary behavior might be mediated, in part, through lower fitness levels.
These findings suggest that sedentary behaviors are not healthy even if patients are exercising at recommended levels (i.e., 150 minutes/week). The independent inverse associate of sedentary behavior and cardiovascular fitness suggests the need for finding methods for reducing sedentary time during working hours (for example, standing desks) and in leisure time.
Keywords: Research Personnel, Exercise, Physical Fitness, Sedentary Lifestyle, Metabolic Equivalent, Nutrition Surveys
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