Movement-related and steady-state electromyographic activity of human elbow flexors in slow transition movements between two equilibrium states.

The electromyograms were recorded in healthy human subjects by surface electrodes from the mm. biceps brachii (caput longum et. brevis), brachioradialis, and triceps brachii (caput longum) during slow transition movements in elbow joint against a weak extending torque. The test movements (flexion transitions between two steady-states) were fulfilled under visual control through combining on a monitor screen a signal from a joint angle sensor with a corresponding command generated by a computer. Movement velocities ranged between 5 and 80 degrees/s, subjects were asked to move forearm without activation of elbow extensors. Surface electromyograms were full-wave rectified, filtered and averaged within sets of 10 identical tests. Amplitudes of dynamic and steady-state components of the electromyograms were determined in dependence on a final value of joint angle, slow and fast movements were compared. An exponential-like increase of dynamic component was observed in electromyograms recorded from m. biceps brachii, the component had been increased with movement velocity and with load increment. In many experiments a statistically significant decrease of static component could be noticed within middle range of joint angles (40-60 degrees) followed by a well expressed increment for larger movements. This pattern of the static component in electromyograms could vary in different experiments even in the same subjects. A steady discharge in m. brachioradialis at ramp phase has usually been recorded only under a notable load. Variable and quite often unpredictable character of the static components of the electromyograms recorded from elbow flexors in the transition movements makes it difficult to use the equilibrium point hypothesis to describe the central processes of movement. It has been assumed that during active muscle shortening the dynamic components in arriving efferent activity should play a predominant role. A simple scheme could be proposed for transition to a steady-state after shortening. Decrease of the efferent inflow can evoke internal lengthening of the contractile elements in muscle and, as a result, hysteresis increase in the muscle contraction efficiency. Effectiveness in maintenance of the steady position seems to also be enhanced due to muscle thixotropy and friction processes in the joint. Hysteresis after-effects in elbow flexors were demonstrated as a difference in steady-state levels of electromyograms with oppositely directed approaches to the same joint position.

Interaction of the movement-dependent, extrafusal and fusimotor after-effects in the firing of the primary spindle endings.

The after-effects of the firing of the primary spindle endings were studied in ankle extensor muscles of cats under Nembutal anesthesia. The activities of 27 primary endings of the muscle spindles from mm. soleus, plantaris, and gastrocnemius have been analysed in various combinations of fusimotor and extrafusal stimulation and application of the mechanostimulation to the spindle bearing muscle. Short-term simulation of static gamma-axons evoked a post-stimulation increase in the spindle ending firing, which can be recorded under both isometric and isotonic conditions on applying a weak extrafusal stimulation or without it. The movement-dependent after-effects were tested with a double-trapezoid pattern of muscle length (or load) changes. The after-effects consisted of the difference of firing rates at the same values of muscle length (or load) with opposite direction of movement to the steady states; these uncertainties were also present during constant stimulation of static gamma-axons. The rate difference showed a tendency to a certain decrease with stimulation rate increment. For diapason of the stimulation rates up to 125 impulses/s a small negative correlation (r = -0.61) between the firing rate differences and the gamma-stimulation rate has been registered in the population of primary endings tested under length servo-control conditions. Using a frequency-modulated intrafusal stimulation, a clockwise hysteresis dependence of the spindle firing rate upon stimulation rate was demonstrated. The pronounced after-effects were shown to exist for steady rates of stimulation: the discharge rates were always higher after stimulation rate increase and lower after its decrease. Fusimotor after-effects were effectively destroyed by both the extrafusal stimulation and the cyclic length (load) changes evoked lengthening-shortening movements of the muscle. The results obtained can be considered as evidence for a hypothesis that history-dependent behavior of muscle spindles is mainly connected with hysteresis of the intrafusal muscle fibers and the whole spindle bearing muscle.

After-effects of preceding movement on dynamic responses of spindle primary afferents during passive muscle lengthening in the cat.

After-effects of preceding movement on the activity of primary spindle afferents of de-efferented cat hindlimb muscles were examined during the dynamic phases of slow linear test movements. These dynamic after-effects were compared with the static after-effects observed in the spindle activity during steady-state of the parent muscle and with hysteresis after-effects of the muscle proper. According to their pattern, the dynamic phases of the spindle and muscle reactions can be divided into two parts. During the first part (at the beginning of movement), both spindle responses and muscle state (either length or load depending on the mode of the test movement) strongly depended on the previous history of movement, being completely independent of the direction of preceding movement in the second part of the dynamic phase. These two parts were treated in terms of interaction of the movement-dependent after-effects in muscles and muscle spindle afferents. These findings allowed us to suppose that, during rather slow single-joint movements, the spindle afferents from the passive antagonist muscles can provide signals free from the effects of preceding movement.

Movement-dependent after-effects in the firing of the spindle endings from the de-efferented muscles of the cat hindlimb.

The muscle spindle reactions evoked by the servo-controlled changes in the muscle length or external load were studied on the de-efferented muscles in experiments on cats under Nembutal anaesthesia. The activity of 39 primary and 10 secondary endings of the muscle spindles from four hindlimb extensor muscles: soleus, plantaris, lateral and medial heads of gastrocnemius has been recorded during servo-controlled changes in muscle length (L-control) or external load (P-control). Slow linear reciprocating (triangular) signals and their modification with fixation of the controlled parameter at the same level at forward and reverse phases were used as commands for the muscle stretcher. The steady firing rates in primary and secondary endings during fixation of muscle length or external load were shown to be strongly dependent on the direction of previous changes in these parameters. The firing rates were always higher after preceding lengthening (loading) and lower after shortening (unloading). These rate uncertainties in the steady firing of spindle endings at the same level of controlled parameter could be as much as 15-20 p.p.s. The revealed dependence of the spindle ending firing on the past history of the muscle movement was denoted as movement-dependent after-effects. Two kinds of movement-dependent after-effects in the firing of the spindle endings can be observed: (1) the movement-dependent ones being studied in the present work, and (2) those connected with the conditioning fusimotor stimulation and described elsewhere. Their common origin is supposed to be connected with the hysterical (thixotropic) properties of the intrafusal muscle fibres. Similarity of the movement-dependent after-effects in L- and P-control conditions was shown to be connected with a resemblance of the length-firing rate [F(L)] and load-firing rate [F(P)] hysterical loops registered during reciprocating movements. Both kinds of loops were in a clock-wise direction, their comparison in normalized form showed that F(L) loops were always broader than F(P) ones. Isotonic and isometric uncertainty vectors were introduced to analyse quantitatively the hysterical effects in the spindle firing and their relation to the muscle hysteresis proper. Uncertainty vectors connect the points of equal load (isotonic uncertainty vectors) and equal length (isometric uncertainty vectors) on F(L) and F(P) loops correspondingly. The projections of both uncertainty vectors onto the Y-axis give the rate uncertainty coinciding in sign for both cases, whereas their projections onto the X-axis differ in sign, being positive for isometric uncertainty vectors (the tension uncertainty) and negative for isotonic uncertainty vectors (the length uncertainty).(ABSTRACT TRUNCATED AT 400 WORDS)