The incidence of stroke-induced hemiparesis among veterans is likely to rise as this population ages. Post-stroke hemiparesis is often marked by persistent hand impairment, which adversely affects both a person's ability to work and his/her quality of life. We believe that impairment is primarily due to neural, rather than biomechanical, factors. At the muscle level, these factors relate either to the inability to activate muscles (i.e., low muscle activation) or to activate them appropriately (i.e., abnormal muscle co-activation). Currently it is unclear as to which one is largely responsible for weakness in the hand as the net mechanical effect, e.g., reduced fingertip force production, could be the same. Determination of voluntary muscle force generation could help to explain deficits in fingertip force production in specific directions, as well as to customize treatment approaches in which force generation ability of some muscles is decreased and others increased. The goal of this work is to explain the source of neurally-mediated weakness at the fingertip following hemiparetic stroke, and to design and experimentally test rehabilitation interventions that attempt to offset this weakness.