IMPLICATIONS OF POSITIVE FEEDBACK IN THE CONTROL OF MOVEMENT

In this paper we review some theoretical aspects of positive feedback in the control of movement. The focus is mainly on new theories regard ing the reflexive role of sensory signals from mammalian tendon organ afferents. In static postures these afferents generally mediate negati ve force feedback. But in locomotion there is evidence of a switch to positive force feedback action. Positive feedback is often associated with instability and oscillation, neither of which occur in normal loc omotion. We address this paradox with the use of analytic models of th e neuromuscular control system. It is shown that positive force feedba ck contributes to load compensation and is surprisingly stable because the length-tension properties of mammalian muscle provide automatic g ain control. This mechanism can stabilize control even when positive f eedback is very strong. The models also show how positive force feedba ck is stabilized by concomitant negative displacement feedback and, un expectedly, by delays in the positive feedback pathway. Other examples of positive feedback in animal motor control systems are discussed, i ncluding the beta-fusimotor system, which mediates positive feedback o f displacement. In general it is seen that positive feedback reduces t he sensitivity of the controlled extremities to perturbations of postu re and load. We conclude that positive force feedback can provide stab le and effective load compensation that complements the action of nega tive displacement and velocity feedback.