There is no trade-off between speed and force in a dynamic lever system

Abstract

Lever systems within a skeleton transmit force with a capacity determined by the mechanical advantage,A. Ais the distance from input force to a joint, divided by the distance from the joint to the output force. A lever with a relatively highAin static equilibrium has a great capacity to generate force but moves a load over a small distance. Therefore, the geometry of a skeletal lever presents a trade-off between force and speed under quasi-static conditions. The present study considers skeletal dynamics that do not assume static equilibrium by modelling kicking by a locust leg, which is powered by stored elastic energy. This model predicts that the output force of this lever is proportional toA, but its maximum speed is independent ofA. Therefore, no trade-off between force and velocity exists in a lever system with spring-mass dynamics. This demonstrates that the motion of a skeleton depends on the major forces that govern its dynamics and cannot be inferred from skeletal geometry alone.