ACQUISITION AND ANALYSIS OF GROUND REACTION FORCES AND FOOT ORIENTATION ON IN-LINE SKATES DURING TRACK SPEED SKATING

METHODS The thin curved arches of a marathon roller-skate frame were directly strain gauged for a total number of 8 full bridges in order to collect information about the vertical load FZ and the lateral load FY acting on each of the four wheels (100 mm diameters). A total number of 32 strain gauges was applied to each frame: cables were properly grouped and joined to a rear plug as shown in Figure 1. The 8 channels were statically and cyclically calibrated with a servohydraulic testing machine loading each wheel and with a calibrated force platform, using a dummy silicon foot as a support. Two additional inertial platforms X-sense Mti were applied to the skate frame and acquired at 100 Hz: three accelerations, three gyro meters and the magnetic heading of the platform were synchronously recorded. Track field measures were taken at 400 Hz by means of a portable data acquisition system installed in a backpack: tests were performed by an Olympic gold medallist (700 N B.W.) during speed skating in an oval track of 200 m and video recorded with two commercial video cameras. Recorded data were analysed in order to estimate the peak values of the loads, the load distribution at the wheels and the force pattern in straight skating or cornering events. RESULTS AND DISCUSSION The peak values of the total resultant loads in the vertical Z and lateral Y directions are reported in Table 1: peak values appeared at the outer skate during a curve, higher than twice the body weight. The loads at each wheel were plotted and wheels 1 and 2 (anteriors) showed highest loads. The skating pattern was steadily repeated by the skilled tester and allowed to clearly distinguish between external and internal foot. Kinematical data, properly combined with mechanical data, allowed to transform the load recorded in the skate reference system into an absolute reference system.