SPATIAL ZONES FOR MUSCLE COACTIVATION AND THE CONTROL OF POSTURAL STABILITY

It is hypothesized that, depending on the motor task, the angular rang e of a joint may be subdivided into zones in which agonist and antagon ist muscles are coactive, only one group of muscles is active or neith er group is active. It is further hypothesized that central commands m ay change the size and location of these spatial zones. We investigate d whether spatial zones are used by the nervous system and how they ma y be changed to provide postural stability of the elbow. We compared r esponses to sudden unloading of the elbow flexors in neurologically no rmal subjects with those in patients with postural control deficits du e to unilateral hemispheric and/or subcortical lesions. By studying re sponses in patients, we sought to determine whether the specification of zones of agonist/antagonist muscle coactivation (''coactivation zon es'') may be essential for postural stability. At an initial elbow ang le (130 degrees; full extension is 180 degrees), flexors were pre-acti vated by compensating an initial load which was equal to approximately 30% of the subject's maximal isometric voluntary contraction effort. Subjects were instructed not to correct the arm displacement elicited by a sudden decrease in the load. Data from 10 trials were collected a t each of 4-6 final load levels (separated by 1.5-2 Nm) in order to ma p out the relationship between torque and angle in each subject. The p rocedure was repeated from a more flexed initial position of the elbow (100 degrees). EMG activity from two elbow flexors and two elbow exte nsors, as well as torque, velocity and joint position were recorded. H ealthy control subjects and patients with mild clinical symptoms had c oactivation zones or small silent zones around the final positions est ablished after unloading. In these subjects, final positions of the li mb were stable. Voluntary movement, i.e., transition of the limb from one initial position to another, was associated with a change in the l ocation of the zone in articular space. The presence of large silent z ones in patients with moderate or severe symptoms was correlated with postural instability and oscillations about the final position of the arm after unloading. The comparison of results from healthy and hemipa retic subjects implies that the central specification of the size and the location of a coactivation zone may be fundamental for the control of posture and movement.