Abstract
BackgroundFor many patients clinical prescription of walking will be beneficial to health and accelerometers can be used to monitor their walking intensity, frequency and duration over many days. Walking intensity should include establishment of individual specific accelerometer count, walking speed and energy expenditure (VO2) relationships and this can be achieved using a walking protocol on a treadmill or overground. However, differences in gait mechanics during treadmill compared to overground walking may result in inaccurate estimations of free-living walking speed and VO2. The aims of this study were to compare the validity of track- and treadmill-based calibration methods for estimating free-living level walking speed and VO2 and to explain between-method differences in accuracy of estimation.MethodsFifty healthy adults [32 women and 18 men; mean (SD): 40 (13) years] walked at four pre-determined speeds on an outdoor track and a treadmill, and completed three 1-km self-paced level walks while wearing an Actigraph monitor and a mobile oxygen analyser. Speed- and VO2-to-Actigraph count individual calibration equations were computed for each calibration method. Between-method differences in calibration equation parameters, prediction errors, and relationships of walking speed with VO2 and Actigraph counts were assessed.ResultsThe treadmill-calibration equation overestimated free-living walking speed (on average, by 0.7 km · h−1) and VO2 (by 4.99 ml · kg−1 · min−1), while the track-calibration equation did not. This was because treadmill walking, from which the calibration equation was derived, produced lower Actigraph counts and higher VO2 for a given walking speed compared to walking on a track. The prediction error associated with the use of the treadmill-calibration method increased with free-living walking speed. This issue was not observed when using the track-calibration method.ConclusionsThe proposed track-based individual accelerometer calibration method can provide accurate and unbiased estimates of free-living walking speed and VO2 from walking. The treadmill-based calibration produces calibration equations that tend to substantially overestimate both VO2 and speed.
Highlights
For many patients clinical prescription of walking will be beneficial to health and accelerometers can be used to monitor their walking intensity, frequency and duration over many days
Determination of individual calibration equations A linear relationship between Actigraph counts and speed of walking during the calibration trials was supported by individual scatter plots with superimposed fitted lines
For the ‘average’ subject walking at a speed of 5.7 km · h−1, which was the average speed of free-living walks across all individuals, the treadmill-based calibration equation overestimated actual speed by 0.7 km · h−1
Summary
For many patients clinical prescription of walking will be beneficial to health and accelerometers can be used to monitor their walking intensity, frequency and duration over many days. Clinical prescription and monitoring of post-diagnosis physical activity may benefit patients with specific disease states [3] and may be used for morbidity prevention. Due to between-individual variability in exercise capacity, health status and pre-intervention levels of physical activity, prescribed exercise intensity (e.g., walking speed) and duration should be determined on an individual basis. Accelerometers are small, non-intrusive physical activity monitors appropriate for the clinical setting [2]. They are suited for objectively monitoring walking intensity, frequency and duration over many days. Actigraph is the most widely used brand of accelerometer [7]
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