Assessment of speed and position during human locomotion ushg nondifferential GPS

Purpose: To validate a nondifferential global positioning system (GPS) to measure speed, displacement, and position during human locomotion. Methods: Three healthy participants walked and ran over straight and curved courses for 59 and 34 trials, respectively. A nondifferential GPS receiver provided speed data by Doppler shift and change in GPS position over time, which were compared with actual speeds determined by chronometry. Displacement data from the GPS were compared with a surveyed 100-m section, and static positions were collected for 1 h and compared with the known geodetic point. Results: GPS speed values on the straight course were closely correlated with actual speeds (Doppler shift: r = 0.9994, P < 0.001, A GPS position/time: r = 0.9984, P < 0.001). Actual speed errors were lowest using the Doppler shift method (90.8% of values within +/- 0.1 m.s(-1)). Speed was slightly underestimated on a curved path, though still highly correlated with actual speed (Doppler shift: r = 0.9985, P < 0.001, A GPS distance/time: r = 0.9973, P < 0.001). Distance measured by GPS was 100.46 +/- 0.49 m, and 86.5% of static points were within 1.5 m of the actual geodetic point (mean error: 1.08 +/- 0.34 m, range 0.69-2.10 m). Conclusions: Nondifferential GPS demonstrated a highly accurate estimation of speed across a wide range of human locomotion velocities using only the raw signal data with a minimal decrease in accuracy around bends. This high level of resolution was matched by accurate displacement and position data. Coupled with reduced size, cost, and ease of use, this method offers a valid alternative to differential GPS in the study of overground locomotion.