Effect of monitor placement and of activity setting on the MTI accelerometer output

Purpose: To examine the effect of monitor placement (hip vs back) and of activity setting (treadmill vs track) on the output from the Manufacturing Technology Inc. (MTI), activity monitor (model WAM 7164). Methods: In a laboratory study, 28 subjects (14 men, 14 women) walked at a normal pace, walked at a fast pace, and jogged at a comfortable pace on an indoor track. These activities were repeated on a treadmill using the individual speeds from the track locomotion. Oxygen uptake was measured simultaneously using a portable metabolic system. One activity monitor was worn on the hip and one on the lower back. In a field study, 34 subjects (18 men, 16 women) each wore two monitors (hip and low back placement) for seven consecutive days. Results: In the laboratory study, ANOVA showed significant effects of placement (P = 0.009) and setting (P < 0.001), indicating that activity counts differ between different body sites and different settings (track vs treadmill). Gross energy expenditure predictive equations were developed and thereafter evaluated in the field study. Time spent at moderate and vigorous intensity of physical activity was 38% and 85% (P < 0.001) higher when calculated from the treadmill-based equations as compared to the track-based equations. Free-living physical activity estimates were not affected by the placement. Conclusion: The relationship between activity counts and energy expenditure during laboratory locomotion is placement and setting-specific. When habitual physical activity is assessed in free-living subjects, the treadmill derived relationship between energy expenditure and activity counts may overestimate time spent at moderate intensity of physical activity, whereas the placement of the monitor does not influence on the interpretation of the data.