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

Motion sensors may be applied for the assessment of physical activity. This paper reviews the evolution of these instruments from the mechanical pedometer to the electronic accelerometer. We conclude that for accurate assessment of physical activity under free living conditions the recently introduced accelerometer looks most promising, although little information was available regarding the reliability of these instruments. Subsequently, 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 was not improved 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 is prone to shifts, probably due to mechanical, electromagnetic, and/or temperature shock, which may be encountered during outdoor application. However, the relevance of the bench test in this study may be questioned, as results did not correspond with the findings in subjects. This needs further investigation.

Assessment of energy expenditure by recording heart rate and body acceleration.

The feasibility of a portable accelerometer equipped with a three-directional sensor for the assessment of physical activity and the consequences for energy expenditure was examined under laboratory conditions and during normal daily life. Heart rate monitoring was also conducted to allow comparison of both techniques. In the laboratory study 16 healthy subjects performed a number of specified exercises within a range of activity levels that may be expected in normal life. Accelerometer output was compared with energy expenditure measured by continuous respirometry. A linear relationship was found between accelerometer output and energy expenditure for the pooled data. The standard error of estimate is 79.1 J.min-1.kg-1. In the field study four subjects were observed during a week under free living conditions. Energy expenditure was calculated from food intake registered over the whole period. Energy expenditure calculated from accelerometer output and heart rate exceeded the energy intake figures by 30% and 33%, respectively. Possible explanations for this discrepancy are discussed. Despite this discrepancy, accelerometer output appeared to correlate highly with energy intake (r = 0.99, P less than 0.025), which suggests accurate performance of the accelerometer under free living conditions. The heart rate method gave much poorer results in estimating individual energy expenditure.

Ambulatory sleep-wake polygraphy in narcolepsy.

We have presented an ambulatory monitoring and off-line data analysis system for sleep-wake polygraphy and have illustrated its use for the diagnosis of narcolepsy. The advantages over conventional polysomnography have been discussed.