Henneman's ‘size principle’: current issues

Despite initial formulation over a quarter of a century ago, Henneman's ‘size principle’ remains a provocative concept. Evaluation of the size principle has focused on testing the possibility that motoneuron size is the basis for orderly motor-unit recruitment during the graded development of muscle force. Although the results have been largely inconclusive, these efforts have been central to our understanding of motor-control mechanisms. This article provides an assessment of the relevance of the ‘size principle’ to our current understanding of motor control.