Validación de sensores inerciales para el análisis técnico de tenistas

The aim of the study was to analyze the validity of three well-known commercial sensors (Zepp1, Zepp2 and Qlipp) by comparing the speed data they provide with a speed radar and a 3D photogrammetric system. Thirteen tennis players of different levels were part of the present study: In the first experiment, performed in the tennis field, 4 players executed a total of 77 strokes (serves and groundstrokes), in the groundstrokes using a ball throwing machine to standardize throws at a speed of 70 km/h and with the minimum spin effect allowed by the machine. The ball speed measured with the Zepp1 sensor and with the Qlipp sensor was compared with the speed recorded by a radar (Stalker Pro II, USA) and with a photogrammetric system composed by 4 USB cameras (ELP, China) recording at 100 Hz. The ball and the end of the racket frame were digitized on the video using the freeware Kinovea and their real 3D coordinates were obtained by applying the DLT algorithm, using the Kinemat tool in the mathematical analysis software GNU Octave. The velocity was calculated by deriving the 3D coordinates using a fifth degree spline. In the second experiment, performed inside the laboratory, 9 players executed 20 forehand and backhands each one (n = 360 groundstrokes). Ball speed was computed with the Zepp2 device and with an highly accurate photogrammetric device (Qualisys), considered as the reference. The data of the present work indicate that the hitting kinematics of each player and the speed of the stroke affects the accuracy of the sensor, so further studies are required to evaluate the error in players of different levels and playing styles. The Zepp1 and Zepp2 inertial sensors evaluated in this work seem adequate to measure ball speed in intra-subject studies and the Lin CCC values in the first study and the adjusted values in the second study were almost all greater than 0.75.