Smooth upper extremity reaching movements may be explained by metabolic cost

People reach with smooth hand trajectories, which are thought to facilitate accuracy [1]. Smoothness predicts trajectories well, but appears incompatible with physiological criteria such as metabolic cost, whose known contributors tend to predict overly jerky movements. This incompatibility might be resolved by a cost associated with rapid force production (“force-rate”), not previously appreciated in movement. During free reaching, this cost would be expected to increase with movement jerkiness. We therefore tested whether the cost for cyclic reaching movements would increase with higher (jerkier) movement frequencies, as predicted by force- rate. Adult participants (n=10) expended nearly four times more metabolic energy with increasing force-rate, even when controlling for mechanical work and other known contributors to cost. A force-rate metabolic cost provides a physiological and effort-based justification for favoring smooth movements, other than accuracy alone.