ASSESSING INTENSITY OF PHYSICAL ACTIVITY WITH THE TRITRAC ACCELEROMETER257

Because of the limitations in self-report measures of physical activity, it is important to use objective measures whenever possible in population studies. The Tritrac triaxial accelerometer is a promising instrument because it stores minute-by-minute movement data. To assess physical activity patterns and adherence to national guidelines, it is necessary to know the relationshhip between raw data (vector magnitude) and various intensities of physical activity. The purpose of this study was to identify the Tritrac vector magnitudes corresponding to light, moderate, and vigorous physical activity. Sixty healthy, non-obese college men and women (23.4±2.9 yr, x±SD) served as subjects. Walking and jogging at speeds of 2, 4 and 6 mph at 0% grade were selected to correspond to light (2-3.9 METS), moderate(4-7 METS), and vigorous (>7 METS) physical activity, based on data in the 1996 Surgeon General's Report. Five minute bouts of treadmill exercise at each of the three speeds were performed continuously by the same subject in randomized order. Actual MET values were determined for each speed by indirect calorimetry with open-circuit spirometry (V max 229). The relationship between vector magnitude and METS across all speeds was highly linear (r = 0.95, SE 477), with little overlap between light, moderate, and vigorous categories. The mean vector magnitudes at 2, 4, and 7 METS were 741, 1824, and 3448, respectively. These data indicate that the Tritrac accurately distinguishs various intensities of walking and jogging on level ground. Evaluation of its performance with other types of weight-bearing and non-weight-bearing activity is warranted. These cut-points can be used to categorize the physical activity intensity level of each Tritrac data point.