Validation of the RT3 triaxial accelerometer for the assessment of physical activity

The aims of this study were to assess and compare the validity of the RT3 accelerometer for the assessment of physical activity in boys and men, to compare RT3 and Tritrac accelerometer counts, and to determine count cut-off values for moderate (> or =3 < 6 METs) and vigorous (> or =6 METs) activity.

Nineteen boys (age: 9.5 +/- 0.8 yr) and 15 men (age: 20.7 +/- 1.4 yr) walked and ran on a treadmill, kicked a ball to and fro, played hopscotch, and sat quietly. An RT3 was worn on the right hip; boys also wore a Tritrac on the left hip. Oxygen consumption was expressed as a ratio of body mass raised to the power of 0.75 (S VO2).

RT3 counts correlated significantly with S VO2 in boys (r = 0.87, P < 0.01) and men (r = 0.85, P < 0.01). However, during treadmill activities, RT3 counts were significantly higher for boys (P < 0.05). RT3 counts corresponding to "moderate" and "vigorous" activity were similar for boys and men for all activities (moderate = 970.2 for boys and 984.0 for men; vigorous = 2333.0 for boys and 2340.8 for men) but approximately 400 counts lower in men when only treadmill activities were considered. Tritrac counts correlated significantly with S VO2 in boys (r = 0.87, P < 0.01), but were significantly lower than RT3 counts across most activities (P < 0.05).

The RT3 accelerometer is a good measure of physical activity for boys and men. However, moderate and vigorous intensity count thresholds differ for boys and men when the predominant activities are walking and running. RT3 counts are significantly higher than Tritrac counts for a number of activities. These findings have implications when comparing activity counts between studies using the different instruments.