Meeting Abstract
Levers impose a force-velocity trade-off. In static conditions, a larger moment arm increases a muscle’s force capacity while a smaller moment arm enhances velocity at the end of the lever. However, muscle force is influenced by contractile velocity and fiber length. Additionally, the rate at which the muscle contracts is influenced by the inertial properties of the mass it accelerates. We hypothesize that these dynamic effects constrain the functional output of a muscle-lever system. We predict that there is an optimal moment arm for either force or velocity for any given muscle-lever configuration. Here we test this hypothesis by building and systematically modifying a simple lever system in OpenSim. The model consists of a mass on the end of a lever that pivots around a pin joint, driven by a Millard2012EquilibriumMuscle muscle with a non-compliant tendon. The muscle’s moment arm was defined by the radius of a cylinder around which it wraps. We generated 3600 modifications of this model with different muscles with varying optimal fiber lengths, moment arms and starting normalized muscle lengths. For each model we simulated the motion that results from 100% muscle activation and extracted the maximum velocity of the driven mass as well as the total impulse applied to the mass from the onset of movement until the time of maximum velocity. In contrast to the common notion of a tradeoff between force and velocity in a lever system, we found that there was, instead, an optimal moment arm which maximized both velocity and total impulse. From this we conclude that in a dynamic lever system where muscle activation is held constant, there is no tradeoff between force and velocity.
