The bounce of the body in running, trotting and hopping

The bouncing mechanism of running, trotting and hopping has never been substantiated by an analysis of the vertical motion of the centre of mass of the body during the stride. In this study, the stride period was divided, as in a simple harmonic motion, into four quadrants delimited by the instants of zero and maximal (upward and downward) vertical velocity of the centre of mass. During most speeds of trotting and at low speeds of running, the stride is symmetric, i.e. the duration of the two lower quadrants of the vertical oscillation equals that of the two upper quadrants. Stride frequency and natural frequency of the bouncing system coincide. During hopping and at high speeds of running, the stride is asymmetric, i.e. the duration of the two upper quadrants is greater than the duration of the two lower quadrants and the stride frequency is smaller than the natural frequency of the system. At a given speed, the asymmetric mechanism requires a greater power to sustain the motion of the centre of mass. It is possibly adopted to contain the power necessary to reset the limbs.