Changes in muscle afferents, motoneurons and motor drive during muscle fatigue

Abstract

Fatigue is a reduction of maximal muscle force or power that occurs with exercise. It is accompanied by changes at multiple levels in the motor pathway and also by changes in the discharge patterns of muscle afferents. Changes in afferent firing can lead to altered perceptions and can also act on the efferent pathway. Changes in the motor pathway include slowing of motor unit firing rates during sustained maximal voluntary contractions (MVCs). Muscle responses to stimulation at different levels of the motor pathway also change. Transcranial magnetic stimulation of the motor cortex and stimulation of descending tracts in the spinal cord in human subjects show an increase in the response of the cortex and a decrease in response of the motoneuron pool during sustained MVCs. In addition, the silent period following magnetic stimulation is prolonged. During relaxation after fatiguing exercise, muscle responses to stimulation of the motor cortex are initially facilitated and are then depressed for many minutes, whereas responses to descending tract stimulation are initially depressed but recover over about 2 min. Although some of the loss of force of fatigue does occur through inadequate drive to the muscle, it is not clear which, if any, of the changes described in the cortex or the motoneurons are responsible for loss of maximal voluntary force and thus contribute to fatigue. Changes may be associated with muscle fatigue without causing it.

Postural Stability and Visual Verticality Perception of Neck Disturbance of the Middle-aged during Quiet Standing

“…Generally, prolonged and low-level static contraction of the muscles increases EMG amplitude and decreases the mean spectral frequency of the EMG power spectrum 39) . In submaximal contractions, an increase in amplitude and a decrease in frequency of the EMG are markers of muscle fatigue 13) . The increase in EMG amplitude is related to the increase in the number of motor units compensating for the loss of contractility 40) .…”

“…Vibration of the neck muscles produces an increase in body sway and an inclination of the body toward the side contralateral to the vibrated muscle 43,44) . A possible mechanism affecting postural balance during neck muscle fatigue is blood flow impairment and inflammatory substance accumulation, for example, potassium, lactic acid, bradykinin, and arachidonic acid 13) . Muscle fatigue alters the discharges of muscle spindles 45) .…”

“…For submaximal contractions, muscle fatigue shows an increase in the amplitude of electromyograms (EMG) and a decrease in the median frequency of EMG 13) . Neck muscle fatigue is reported to influence postural control and may disturb subjective postural reference.…”

“…A study reported that after 5 minutes of intense contraction of approximately 35% of maximal voluntary contraction of the dorsal neck muscles the body sway increased during quiet standing 15) . A load of 25% maximal isometric contraction of the neck extensor muscles for 10 and 15 minutes duration produced significant changes in center of pressure (COP) sway 14) , which is believed to be caused by metabolic accumulation in the neck muscles 13,18) and decreased neck joint position sense 19,20) .…”

The effect of high-intensity cycling training on postural sway during standing under rested and fatigued conditions in healthy young adults

“…The maintenance and control of a stable upright stance represents an essential prerequisite for many sporting, physical and daily activities (Lepers et al 1997). Muscle fatigue represents an inevitable consequence of physical, occupational and recreational tasks (Vuillerme, Sportbert and Pinsault 2009) which is known to alter the quality of proprioceptive information and/or integration within the central nervous system (Paillard 2012) and reduces the muscles force-generating capacity (Taylor, Butler and Gandevia 2000). In recent years, the immediate post-exercise effects of cycling on postural stability have received growing interest.…”