Mechanical energy transfer by internal force during the swing phase of running

Running is a cyclic motion in which each leg has two phases, stance and swing. The energy generated by the muscular forces are dominant in the phase between initial contact and mid-stance. However, during the swing phase, other internal forces act to transfer the mechanical energy between segments. In this research, we focused on the energy transfer mechanism during the swing phase and investigated how non-muscular (e.g. centrifugal and gravity) forces of each link generate, absorb, and transfer the mechanical energy. A mathematical analysis of the lower limb's movement in the swing phase of running at different speeds was carried out. A multi-body power analysis was performed using dynamical equations of a three-dimensional double pendulum to find the mechanism to increase the speed of the leg swing efficiently. Results show that the internal force at the knee joint work throughout the swing phase to generate centrifugal force in the shank and that the knee joint moment is very small in comparison to the internal force. Understanding the transfer mechanism using the internal forces could lead to economical running forms that prevent injuries and enhance the performance of the runners.