Force as the controlling muscle variable in limb movement

A potential mechanism underlying the voluntary suppression of tardive dyskinesia

Many clinicians have observed that patients with tardive dyskinesia (TD) can voluntarily suppress the movements to some extent. The mechanism by which voluntary control is exerted most likely involves peripheral feedback. Quantitative analyses of isometric and isotonic jaw stability were conducted in patients with and without TD to test the general hypothesis that involuntary movements of TD may be brought under voluntary control via one of two receptor mechanisms; muscle length and/or muscle force. Results indicated that in the absence of length feedback patients with TD exhibited significantly greater instability while patients without TD performed within normal limits. This suggests that patients with TD utilized muscle length feedback to suppress motor instability but were unable to utilize muscle force feedback to suppress instability. Implications for the use of laboratory procedures in the measurement and clinical management of TD are discussed.

Peripheral regulation of stiffness during arm movements by coactivation of the antagonist muscles

Two experiments investigated whether unexpected and differential loading of a rapid, unsighted arm movement resulted in the central nervous system (CNS) regulating limb stiffness by modifying the associated neuromuscular activity. In Experiment 1, subjects completed multiple, spring-loaded training trials until a prespecified criterion of learning was attained. On selected trials, the spring load was unexpectedly replaced by an inertial load. Results indicated that to maintain positional accuracy during this inertial load trial, limb stiffness was increased by coactivating the antagonist muscles, i.e. by changing the associated neuromuscular activity from a predominantly triphasic pattern to one of coactivation. In Experiment 2, the sequence of loading was reversed producing a change in the required limb stiffness from a relatively high to low level. This change was observed as a pattern of coactivation being replaced by a triphasic activity pattern. These results support the notion that limb stiffness is regulated primarily through modification of the neuromuscular activity pattern prior to movement termination. It was also demonstrated that the size of the unexpected load did not affect the basic activation pattern selected by the CNS. It is proposed that the signal which triggers the CNS to regulate limb stiffness is based on peripheral information generated as a result of agonist activity occurring during the first part of the movement.