Measurement of the mechanical power of walking by satellite positioning system (GPS)

Purpose: This descriptive article illustrates the application of Global Pos itioning System (GPS) professional receivers in the field of locomotion stu dies. The technological challenge was to assess the external mechanical wor k in outdoor walking. Methods: Five subjects walked five times during 5 min on an athletic track at different imposed stride frequency (from 70-130 st eps.min(-1)), A differential GPS system (carrier phase analysis) measured t he variation of the position of the trunk at 5 Hz. A portable indirect calo rimeter recorded breath-by-breath energy expenditure. Results: For a walkin g speed of 1.05 +/-0.11 m.s(-1), the vertical lift of the trunk (43 +/- 14 mm) induced a power of 46.0 +/- 20.4 W. The average speed variation per ste p (0.15 +/-0.03 m.s(-1)) produced a kinetic power of 16.9 +/-7.2 W. As comp ared with commonly admitted values, the energy exchange (recovery) between the two energy components was low (39.1 +/- 10.0%), which induced an overes timated mechanical power (38.9 +/- 18.3 W or 0.60 W.kg(-1) body mass) and a high net mechanical efficiency (26.9 +/-5.8%). Conclusion: We assumed that the cause of the overestimation was an unwanted oscillation of the GPS ant enna. It is concluded that GPS (in phase mode) is now able to record small body movements during human locomotion, and constitutes a promising tool fo r gait analysis of outdoor unrestrained walking. However, the design of the receiver and the antenna must be adapted to human experiments and a thorou gh validation study remains to be conducted.