Impairment of motor output by fatigue-related firing of small-diameter muscle afferents
School of Medical and Health Sciences, Edith Cowan University & Neuroscience Research Australia
During fatiguing exercise, firing of small-diameter muscle afferents, which are sensitive to the accumulation of metabolites, leads to sensations of burning and fatigue from the exercising muscles. As well as eliciting reflex cardiorespiratory responses, these afferents impair voluntary motor performance. Occlusion of blood flow to the muscle after fatiguing exercise can trap metabolites and prolong firing of these afferents to allow investigation of their actions. Such studies have shown that voluntary activation of both upper and lower limb muscles is reduced by firing of afferents that innervate the tested muscle, as well as by firing of afferents from antagonist muscles, and also from more distal muscles in the same limb (1,2). For example, voluntary activation of the elbow flexor muscles is reduced by feedback from the elbow flexors, the elbow extensors or the hand. By contrast, afferents from the contralateral limb do not reduce voluntary activation (3). Reductions in motoneurone excitability may contribute to reduced voluntary activation for some muscle groups but some motoneurone pools are facilitated by fatigue-related sensory feedback (4). Therefore, supraspinal actions of the afferents are likely to feature in impairment of voluntary activation. However, no robust effects on responses to motor cortical stimulation have been reported. Thus, the supraspinal site of action and the mechanism by which fatigue-sensitive small-diameter muscle afferents impair motor output remain uncertain.
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