Rotational equilibrium during multi-digit pressing and prehension.

We review a series of studies that show stabilization of the moment of a couple produced by a set of digits in many maximal and submaximal accurate force production tasks that have no requirements for the moment. In particular, an unusual and novel multi-digit force production task shows stabilization of the total moment while the total force requires extensive practice to be stabilized. Similar results were obtained in persons with Down syndrome during easier tasks. During prehension, changes in digit forces and coordinates of their points of application suggest the presence of two multi-digit synergies whose purpose is to assure a certain grip force and a certain total moment, respectively. Elderly persons show impaired production of both maximal and submaximal moments that goes beyond their documented loss of muscle force. We conclude that moment production (keeping rotational equilibrium) is a central constraint in a variety of multi-digit tasks that has received little attention. Analysis of digit interaction for moment production during handwriting could signify a major step towards understanding the control of this action.