Hysteresis of Muscle Contraction and Effects of Uncertainty in Proprioceptive Activity and Motor Performance

Abstract

Accuracy in estimation of mechanical properties of skeletal muscles is of primary importance for the adequate analysis of various problems of motor control. Based on a well-known family of length-tension curves obtained on a nerve-muscle preparation in isometric conditions, the spring model of muscle and the equilibrium point hypothesis were suggested for treatment of single-joint movements (Feldman, 1986). However, it is shown that efficiency of muscle contraction is strongly movement-dependent: a muscle generates stronger forces at lengthening vs shortening and such a hysteresis leaves long-term aftereffects (Kostyukov, 1987). Some nonlinear effects in muscle dynamics have been evaluated in our study on the ankle extensors of anaesthesised cats (Figure 1). The muscles were activated by a distributed stimulation of the motor fibres supplying them; a servo-controlled linear motor was used as a muscle stretcher. Length-tension hysteresis was well expressed even at very slow movements and resulted in strong after-effects. In spite of the identity of stimulation parameters and applied loads, the steady lengths differed substantially depending on the direction of the previous movement (Figure 1A). At the same time pronounced uncertainty could be observed after the movements in the same direction as well (Figure 1B). In this case, powerful hysteresis after-effects connected with oppositely directed conditioning movements were not fully eliminated during the subsequent identical test load steps. The increase of these steps brought a divergence between the steady values of length down to its full disappearance.