Neurological problems affecting hand dexterity.

The first objective of this review is to summarize how grip force and load force (holding and transporting forces) are coordinated. Usually, the two forces vary in parallel, thereby resulting in a constant force ratio. Departures from this rule have been observed, however, depending on dynamic task constraints. The second objective is to summarize some of the pathophysiology of grasping in movement disorders. By means of a drawer-pulling task, regulation of grip force was analyzed when pulling was perturbed either by self-induced or externally applied load disturbances. Normal subjects automatically increased grip force in anticipation to the expected load. In the same situation, hemiparetic patients failed to generate proactive grip force and frequent slips were observed. Cerebellar patients were shown to adopt a 'default' strategy in producing high grip force output when the drawer had to be pulled up to its mechanical stop. This differed from the more flexible normal mode of raising grip force in accord with the pulling speed. In patients with Huntington's Chorea, grip/load force coordination differed from that of normal subjects, as expressed in an overscaled grip force. This might be a secondary, less flexible 'default' strategy to overcome the failure in adapting grip force to upcoming disturbances. Writer's cramp patients overscaled grip force in both the dominant and non-dominant hand, and grip force further increased when hand muscles were vibrated, suggesting an abnormal sensorimotor integration. The results illustrate the degrading consequences of cortical and subcortical pathology on manual dexterity, which is sometimes partly compensated for by new, less flexible default strategies.