Accuracy of PARTwear inertial sensor and Optojump optical measurement system for measuring ground contact time during running Brief running head: Ground contact time accuracy in portable measurement systems Authors:

The aim of this study was to validate the detection of ground contact time (GCT) during running in two differently working systems: a small inertial measurement sensor, PARTwear (PW), worn on the shoe laces, and the optical measurement system, Optojump (OJ), placed on the track. Twelve well-trained subjects performed 12 runs each on an indoor track at speeds ranging from 3.0 9.0 m·s-1. GCT of one step per run (total 144) was simultaneously obtained by the PW, the OJ, and a high-speed video camera (HSC), whereby the latter served as reference system. The sampling rate was 1,000 Hz for all methods. Compared to the HSC, the PW and the OJ systems underestimated GCT by -1.3 ±6.1% and -16.5 ±6.7% (p-values < .05), respectively. The intraclass correlation coefficients between PW and HSC and between OJ and HSC were .984 and .853 (p-values < .001), respectively. Despite the constant systematic underestimation of GCT, analyses indicated that PW successfully recorded GCT over a wide range of speeds. However, results showed only moderate validity for the OJ system, with increasing errors when speed decreased. In conclusion, the PW proved to be a highly useful and valid application, and its use can be recommended not only for laboratory settings but also for field applications. In contrast, data on GCT obtained by OJ during running must be treated with caution, specifically when running speed changes or when comparisons are made with GCT data collected by other measurement systems.