Methods:

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.