Comparison of Time in MVPA Between MTI Actigraph Accelerometer Activity Counts and GPS Walking Speed
James J. McClain, Mark D. Peterson, Catrine Tudor‐Locke · Medicine & Science in Sports & Exercise · 2006
Prior to the development of accelerometer-based technologies, walking speed was considered one of the best measures of free-living physical activity intensity. Recently a number of instruments, including Global Positioning Systems (GPS) devices, have been developed that allow for objective measurement of walking speed on a continuous basis similar to accelerometer technology. In contrast to GPS technology, accelerometer technology is generally of high cost, less user friendly, and is not widely marketed to the general public. GPS instruments may provide an objective measure of physical activity intensity determined from walking speed that is comparably similar to derived accelerometer outputs. PURPOSE: This study compared objectively measured time in moderate to vigorous physical activity (MVPA) from MTI Actigraph (MTI; MTI Health Services, Fort Walton Beach, Florida) accelerometer activity counts and the Delorme Earthmate Blue Logger GPS (GPSL; Delorme, Yarmouth, Maine) walking speed. METHODS: Ten male participants (age = 24.2 years) wore both monitors while playing golf (and walking between holes) on a level, 18 hole golf course. Data were collected at five second sampling intervals for both instruments. MTI activity counts were recoded into time in MVPA intensity using a sampling rate adjusted Freedson cutpoint (i.e., MVPA ≥ 163 counts/5 seconds). GPSL walking speed data was recoded to time in MVPA intensity using: 1) the Compendium-indicated walking speed associated with moderate intensity or 3 METs (COMP; 2.5 mph or 4.02 kph); and, 2) a 3-MET floor value for moderate intensity based on the ACSM energy expenditure prediction equation for walking speed (ACSM; 2.6 mph or 4.2 kph). Descriptive statistics and repeated measures ANO VA were used to evaluate differences between MTI and GPSL estimates of time spent in MVPA. An alpha level of 0.05 was used to evaluate significance. RESULTS: Mean MVPA values for MTI, COMP, and ACSM respectively were 101.9, 83.2, and 78.6 minutes. Time in MVPA was significantly different between MTI vs. COMP (Δ= 18.7 min, p<.001) and MTI vs. ACSM (Δ= 23.3 min, p<.001). Additionally, time in MVPA between COMP vs. ACSM was significantly different (Δ= 4.6 min, p<.001). CONCLUSION: Determining time spent in MVPA from the data detected by the MTI resulted in approximately 22.5% and 29.6% more time respectively compared to COMP and ACSM estimates derived from objectively measured walking speed. Since both instruments have established evidence of measurement accuracy, discrepancies in time in MVPA are likely due to incongruence of walking speed and accelerometer activity counts cut points used to interpret moderate intensity.