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 f orces vary in parallel, thereby resulting in a constant force ratio. Depart ures from this rule have been observed, however, depending on dynamic task constraints. The second objective is to summarize some of the pathophysiolo gy of grasping in movement disorders. By means of a drawer-pulling task, re gulation of grip force was analyzed when pulling was perturbed either by se lf-induced or externally applied load disturbances. Normal subjects automat ically increased grip force in anticipation to the expected load. In the sa me 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 flexib le normal mode of raising grip force in accord with the pulling speed. In p atients with Huntingtons Chorea, grip/load force coordination differed from that of normal subjects, as expressed in an overscaled grip force. This mi ght be a secondary, less flexible 'default' strategy to overcome the failur e 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 abno rmal sensorimotor integration. The results illustrate the degrading consequ ences of cortical and subcortical pathology on manual dexterity, which is s ometimes partly compensated for by new, less flexible default strategies. ( C) 2001 Elsevier Science B.V. All rights reserved.