MUSCLE HYSTERESIS AND MOVEMENT CONTROL - A THEORETICAL-STUDY

In this study we have tried to elaborate necessary theoretical approac hes for the adequate analysis of the central motor commands to a mamma lian muscle in the equilibrium states and during transition movements between these states. At present, the equilibrium point hypothesis has obtained a wide distribution in this field. The muscle is considered in the framework of the theory as an executive element of the reflex c ircuits originating in the muscle proprioceptors and being closed at t he level of spinal cord and the supraspinal motor centres. The main pa rameter defining the muscle state is supposed to be the threshold of t he stretch reflex-the minimal length value at which muscle begins to r esist to the externally applied force. We have attempted to show that the theory has an essential shortcoming because it does not take into account such important non-linearity in the muscle behaviour as hyster esis. In the framework of the equilibrium point hypothesis, the muscle behaviour within the stretch reflex system does not depend on movemen t direction. The stretch and unloading reflexes are supposed to have t he same length-tension dependencies when the muscle is stretching or c ontracting with a rather slow velocity. However, powerful hysteresis o f the stretch reflex system requires taking into account the direction of the current movement, the after-effects of previous movement led t o a principal uncertainty in the muscle steady-stale. We would like to stress that any process of active muscle shortening should be control led by dynamic components in efferent inflow. At the same time, the re sulting steady-state develops by using effective hysteresis mechanism for its maintenance. The following hypothesis was proposed to explain the length clamping mechanisms in shortening transition movements. A s ignificant decrement of the arrived efferent activity at the phase of the length fixation can evoke an internal elongation of the contractil e elements within the muscle and corresponding hysteresis-like enhance ment of the contractile effectiveness. Hence, instead of considering t he quasi-static and dynamic components of movement commands as in the equilibrium point hypothesis, it is preferable to adopt a model of the shared coding of both the final position and movement velocity. The d ynamic component of the efferent discharge seems to be required for a complete definition of the final steady-state, but maintenance of the state is closely associated with energetically advantageous hysteresis mechanisms. It was concluded that the dynamic phase of efferent activ ity should play an extremely important role in the central coding of t he real movements produced, in particular, by contraction of agonists in the absence of antagonist activation. (C) 1997 IBRO. Published by E lsevier Science Ltd.