Muscle history, fusimotor activity and the human stretch reflex

Abstract

has established that the responses of muscle spindles and their spinal reflex action are dependent on the muscle's immediate previous history of contraction and length changes, because muscle exhibits thixotropy (for a review see Proske et al. 1993). To demonstrate extremes of behaviour of spindles and spindle reflexes in relation to muscle history effects, we have devised two forms of conditioning, termed hold-long conditioning and hold-short conditioning. Hold-long conditioning consists of stretching the muscle to a length longer than that at which the test measurements are to be made, then contracting the muscle and holding it at that length for a few seconds before returning it to the test length. Similarly, hold-short conditioning is where the muscle is shortened, contracted and then stretched back to the test length. The reasoning underlying these forms of conditioning is as follows. As the muscle relaxes following a contraction, a number of stable cross-bridges form between actin and myosin filaments in sarcomeres of muscle fibres (Hill, 1968). The presence of stable cross-bridges gives the muscle a degree of passive stiffness. As a result, when the stiffened muscle fibres are shortened, as occurs after hold-long conditioning, they fall slack. By contrast, slack does not develop if the contraction is carried out at a shorter length and the muscle is then stretched back to the test length, that is, after hold-short conditioning. We hypothesize that slack is able to form in both extrafusal and intrafusal fibres. The presence of slack in intrafusal fibres has a large effect on the responses of muscle spindles.

A Novel Approach that Ameliorates Motor Disabilities and Improves Range of Motion of Joints; Comparison with Proprioceptive Neuromuscular Facilitation

“…Stretching of muscles (by maintaining a timed passive position of the body segments) is currently used to decrease excessive stiffness and to improve ROM of joints [1]. This contrasts however with previous studies which have established that, when a muscle is held extended for several seconds, the muscle spindles lose their sensitivity as a result of slackness on the spindles themselves, a phenomenon termed thixotropy [2]. Thixotropy is detrimental to the body's proprioception and to the myotatic reflex [3].…”

Effects of muscle action type on corticospinal excitability and triceps surae muscle-tendon mechanics

“…These efferents provide the equivalent of alpha-gamma coactivation; when skeletofusimotor neurons are activated, unloading of the spindle by activation of extrafusal fibers is at least partially compensated by loading due to intrafusal contraction. (Gregory et al 1998;Kandel et al 2000, 725;Vallbo 1971;Wilson et al 1997) Stretch reflex. Mechanical deformation of the intrafusal muscle fibers generates APs by activating mechanically gated ion channels in the afferent axons coiled around the spindle.…”

Movement Reduces the Dynamic Response of Muscle Spindle Afferents and Motoneuron Synaptic Potentials in Rat

“…For example, the strength of the stretch reflex modulates over different phases of a step cycle (Kearney et al 1999;Misiaszek et al 2000; see reviews by Sinkjaer et al 2001;Zehr and Stein 1999). The stretch reflex is also modulated by movement history, with changes in strength resulting when the reflex is immediately preceded by muscle contraction or by changes in muscle length (Gregory et al 1998;Hagbarth et al 1995;Nakazawa et al 1998;Ogiso et al 2002;Wallace and Miles 1998;Wood et al 1996; see review by Proske et al 1993). Stretch reflex modulation is variously proposed in these reports to provide resistance to unexpected perturbations, compensation for ground irregularities, additional neural drive for muscle force, and stabilization of limb trajectory.…”