Internal forces during static prehension: effects of age and grasp configuration.

The authors studied effects of healthy aging on 3 components of the internal force vector during static prehensile tasks. Young and older subjects held an instrumented handle using a 5-digit prismatic grasp under different digit configurations and external torques. Across digit configurations, older subjects showed larger internal normal (grip) and tangential (load-resisting) digit force components and larger internal moment of force. In contrast to earlier reports, safety margin values were not higher in the older subjects. The results show that the increased grip force in older persons is a specific example of a more general age-related problem reflected in the generation of large internal force vectors in prehensile tasks. It is possible that the higher internal forces increase the apparent stiffness of the hand+handle system and, hence, contribute to its stability. This strategy, however, may be maladaptive, energetically wasteful, and inefficient in ensuring safety of hand-held objects.