1. Philippe Terrier*, Q. Ladetto º, B. Merminod º, Y. Schutz* * Institute of Physiology, University of Lausanne, Switzerland º Institute of Geomatics, Swiss Federal Institute of Technology, Lausanne, Switzerland Course of the experiment. A subject (woman, 25yr) walked 5 runs at increased stride frequencies. The upper panel presents raw vertical position. The middle panel shows the instantaneous horizontal speed as measured by Global Positioning System (GPS) at 5Hz. The lower panel displays the vertical body acceleration sampled at 40Hz by a portable accelerometer. Results 150 consecutive steps in each subject were analyzed for each different Stride Frequency (SF) (i.e. 6000 steps analyzed). Average walking speed was computed dividing the distance traveled after the 150 steps by the time. Step duration was calculated for each step using a peak detection algorithm, which assessed time duration between two consecutive peak heights of the trunk. The same method was applied to vertical accelerometric signal. Instantaneous speed was obtained by derivation of position in horizontal plane. Vertical lift of the trunk per gait cycle and speed variation around average walking speed per gait cycle was averaged for 150 steps. A perfect correlation between Step Duration (StD) measured by DGPS and accelerometer was found: StD DGPS=1.0·StD Accelerometer+0.0, r=1.000, N=40, indicating that the two independent devices recorded the same gait parameter. Introduction Different methods have been developed to study the biomechanics of human locomotion, such as reaction force analysis using force- plates or kinetic analysis using video. These techniques can be applied only under laboratory conditions. Furthermore, it is not possible to record a substantial number of strides. So, until recently, there was no objective and practical means to analyze the different parameters of the gait during outdoor walking, and to record an extended number of strides. GPS (Global Positioning System) is a satellite localization system developed by the US Department of Defense. The complete satellite constellation consists of 24 satellites and provides navigation and surveying capability worldwide. Using two receivers – one as fixed base station and the other as moving antenna (rover) – allows to work out relative positions with a sub-centimeter accuracy measuring phase. The present study explores whether GPS device can be used to record basic biomechanical parameters of outdoor walking, namely walking speed, stride length and stride frequency. We also tested whether it was possible to record basic parameters for studying the external mechanical power of the locomotion, namely the vertical lift of the trunk and the variation of velocity per gait cycle. Statement of Clinical Significance We think that GPS technology will permit to analyze gait in more physiological conditions than laboratory experiments. New indications about normal or pathological gait would be obtained with a minimal disturbance for the patient. Data about the energetics of locomotion during unrestrained walking might be useful for disease prevention and treatment, such as obesity or cardiovascular diseases. The Leica-500 GPS System combined with a protable inderect calorimeter (Cosmed k4b2) The subjects were asked to regulate their walking pace by following the beep of an electronic metronome. Selected frequencies were 70, 90, 110 and 130 steps per minute. This method made possible to record a broad range of walking speed (2.8km/h to 6.30km/h) while keeping a constant gait style. The fifth run was walked at free pace: the instructions were to choose the most comfortable stride rhythm and keep a constant gait style. The GPS logger was placed in a rucksack tightly attached to the person. The receptive antenna, placed just over the head of the subject, was mounted on a metal bar fixed onto the dorsal pack. The precise positioning of the persons was obtained with two Leica System 500 double frequency GPS receivers, measuring at a 5 Hz rate. It was possible to perform differential carrier phase localization between the fixed base station and the antenna mounted on the walking person. In order to measure body accelerations, a high precision triaxial accelerometer (Physilog, BioAGM, Lausanne, Switzerland) was used. It was then possible to compare stride frequency measured by DGPS and accelerometers. Basic gait parameters assessed by Global satellite Positioning System Methodology Eight subjects walked 5 different runs on a standard outdoor athletics track. Each run lasted about 5 minutes. A two minutes break was imposed between two runs in order to clearly differentiate the different phases in differential GPS recorded signal. Discussion In differential mode, GPS allows measuring average walking speed with a relative accuracy of 0.1%. Our results demonstrated that stride frequency could be retrieve with reliability comparable to accelerometric method, provided that the sampling rate is sufficient. Stride length can be calculated from these two parameters. GPS can be therefore considered as the ideal tool for measuring basic walking parameters under totally unrestraine and open sky conditions, The vertical lift of the trunk we found was higher than expected. Previous studies indicated typical values of about 3.5cm. We believe that perturbing movements of the antenna, which oscillated up and down during each gait cycle, might cause this overestimation. In future work, shorter and more ergonomic antenna may solve this issue. This research has been sponsored by the Swiss National Science Foundation High precision satellite positioning system as a new tool to study the biomechanics of human locomotion