Methods to assess physical activity with special reference to motion sensors and accelerometers

The evolution of motion sensors is reviewed from the mechanical pedometer to the electronic accelerometer. It is concluded from this review that for accurate assessment of physical activity under free living conditions the recently introduced accelerometer looks most promising. The reliability of an accelerometer with a three-directional sensor was examined. Intrainstrument variation in a bench test was less than 8% during four measurements over a week. Interinstrument variation during treadmill experiments, while subjects wore two accelerometers at the same time, was on average 22% and did not improve after adjustment for differences found in the bench test. Reproducibility in the treadmill experiment was approximately 76, 85. and 95% at 3, 5, and 7 km/h, respectively. Bench testing revealed that the sensitivity of a piezoelectric element was prone to shifts, probably due to mechanical. electromagnetic, and/or temperature shock which may be encountered during outdoor application. However, the results of the bench test did not correspond with the findings in subjects.