A MECHANICAL MODEL FOR LEG STIFFNESS AND AVERAGE AND MAXIMUM FORCE ESTIMATIONS IN SPRINTERS

Abstract

In running the leg's complex system of muscle, tendon and ligament has a spring like behaviour, which can be considered as a single non-linear spring. A single spring-mass model, consisting of a single non-linear leg spring and a mass in running has been considered to estimate the leg stiffness and muscle force in running remarkably well. The model has shown that in running, the stiffness of the leg spring is proportional to the squared cosine of the leg angle relative to the axis perpendicular to ground and also to the displacement of CG (centre of gravity) and finally to the squared vertical velocity component. The variation of the leg stiffness with CG displacement and the angle swept by the leg spring, when sprinters alter their supporting leg from braking phase to propulsive phase, at their maximal speed. A 20m-switch pad in conjunction with an electronic interface and a laptop computer has been used for touch down time measurement. The angle swept by sprinter's leg was determined by video filming. The proposed mathematical model enabled us to estimate leg stiffness and muscle force satisfactorily.