Effect of fatigue‐sensitive afferents on human extensor and flexor motoneurons

Activity in fatigue‐sensitive group III and IV muscle afferents does not inhibit motoneurons of human elbow flexor muscles (Butler et al. 2003), but flexors and extensors behave differently in animals. Hence, we investigated the influence of these afferents on motoneurons innervating extensor muscles of the elbow. In two experiments, subjects ( n =8) performed brief maximal voluntary contractions (MVCs) of elbow extensors before and after a 2‐min MVC of the extensors. During MVCs, EMG responses from triceps were evoked by stimulation of the corticospinal tracts (cervicomedullary motor evoked potentials [CMEPs]) and by supramaximal stimulation of the brachial plexus (M max ). In one experiment, input from fatigue‐sensitive afferents was prolonged after the fatiguing contraction by maintained muscle ischemia. In additional experiments, responses were evoked in triceps during brief extensor MVCs before and after a 2‐min sustained flexor MVC ( n =7) or in biceps during brief flexor MVCs before and after a sustained extensor MVC ( n =7). Ischemia was maintained for 2 min following the sustained contractions. During sustained MVCs of the extensors, CMEPs in triceps decreased by ~30% and, without ischemia, recovered within 15 s. However, when ischemia was maintained, CMEPs evoked during brief extensor MVCs remained depressed (by ~28%). CMEPs in triceps were also depressed (by ~20%) after fatigue of the flexors, whereas CMEPs in biceps were facilitated (by ~25%) following fatigue of extensors. During fatigue, inputs from group III and IV afferents of homonymous or antagonist muscles depress extensor motoneurons but facilitate flexor motoneurons.