MATHEMATICAL MODEL FOR THE ESTIMATE OF GRAVITY CENTER KlNEMATICS IN SPRINT START

Abstract

Short distance sprinting events may be divided in two phases. accelesarion pllasc and maintenance phase. Reaction time, velocity of the g1-avity centel- at the enti of the push off and body geometry in the first step are important variables influencing the acceleration phase and the final pel-fosnlance. Several studies concerning sprint s t a t have been pul-sued in 01-del- to tietecl information from a bio~nechanical point of view. hut these filldings 81-e IIIOI-e oriented toward an improvement in the knowledst. rath her- than towal-d a practical application. The present paper describes a mathematical m nod el designed to estimate the kinematics of the gravity center of an athlete, in order to cluantih the performance of the start and to identify relationships among other available kinematic and kinetic variables. The model considers the human body as a set of rigid segments connected by ideal hinges, and it has been designed in order to estimate the 3D coordinates of segments' center of gravity by means of a reduced number of anatomical landmarks. The model is constrained to applications in which large rotations of the segments around sagittal axes are not too large, like in sprint starts Weight of the segments and relative posit'~on of their gravity centers are computed by using the parameters and the regression equations proposed by Zatjorsky and Seluyanov. Input of the model are the 3D coordinates of eighteen anatomical landmarks, nineon each side of the subject, and some antropometric measurements made on the athlete. Due to the motion analizer utilized (ELlTE System, BTS srl), the markers were passive retroreflective hemispheres of 10 mm. in diameter Given that the velocity at start is a factor that probably does not vary dramatically during the season in skilled athletes, particular care has been paid in verifying the accuracy of the computed data in order to detect significant small differences Measurements of the accuracy, filtering procedure and model validation have been the subjects of this step. Experimental sessions have been utilized in order to validate the model The athletes performed squat and long jumps on a force plate These movements allow the comparison of the take-off velocities computed through the integration of the ground reaction force and those estimated by means of kinematic records Preliminay results concerning the sprint start will also discussed.