Enregistrements vidéos et moments de force mesurés sur cycloergomètre : étude du transfert d'énergie entre membre inférieur et manivelle à vitesse élevée

Abstract

Introduction. – Previous studies suggested the occurrence of an energy transfer between leg and crank at the bottom dead centre at high pedalling frequency. We propose to verify this hypothesis by the comparison between kinematic and dynamic data obtained at the peak velocity of a maximal sprint on a Lode Excalibur cycle ergometre. Fact synthesis. – Leg kinematic data are calculated from video frames collected at the peak velocity of a high speed sprint (198 rpm) in a former competitive cyclist. The sum E of the translational and rotational kinetic energies and the potential energies of the thigh, lower leg and foot is calculated during one revolution at peak velocity. Torque exerted on the crank (T) is measured at peak velocity by means of the strain gauges of the Lode Excalibur cycle-ergometre. Crank angular velocity can be considered as constant during one revolution at peak cycling rate, provided that peak velocity is very high. Consequently, external power (dW Lode/dt) is equal to the product of T and average angular velocity of the crank. Muscle power (dW muscle/dt) is equal to the algebraic sum of dW Lode/dt and dE/dt. At peak velocity, an opposition between dW Lode/dt and dE/dt is observed before the bottom dead centre, in agreement with the hypothesis of a transfer of leg kinetic energy into crank work in this cyclist. Positive values of dW muscle/dt between 210 and 270°, whereas dW Lode/dt is negative in this cyclist, indicate that the right leg muscles participate in the upward movement of the leg, but that the force produced by these muscles is not sufficient to accelerate the leg as fast as the left leg muscles push on the opposite pedal.