Impaired proactive and reactive grip force control in chronic hemiparetic patients

Objectives: The aim of the study was to test manipulative capacities of hemiparetic patients with partial recovery in a drawer task. The main objective was to assess adjustments of grip force in the face of load perturbations. Methods: The task was to pull and to hold the drawer manipulandum during predictable or unpredictable perturbations with short (90 ms) load pulses (factor set). Results: The following novel observations were made. (1) Load pulses elicited, at a latency of about 70 ms, a transient grip force response and a corresponding phasic EMG response. These reactive adjustments were larger during holding than during pulling (factor task). In patients, the reactive grip force adjustments and the EMG response in the grip muscles were reduced. (2) The above deficit was set-dependent. (3) With regular perturbations, grip force was scaled already before perturbation onset. This proactive adjustment was greatly reduced in the patient group. (4) Coordination between grip force and pull force before onset of the perturbation was also disturbed in the patients who generated less grip force per unit pull force than control subjects. Conclusions: It is concluded that the patients had difficulties in adapting proactively and reactively to external load disturbances, in addition to their hand weakness.