Modulation of motor unit discharge rate and H-reflex amplitude during submaximal fatigue of the human soleus muscle

Abstract

Declining motor unit discharge rates and H-reflex amplitude have been observed in separate experiments during fatiguing submaximal contractions in humans. The purpose of this experiment was to investigate motor unit discharge rate, H-reflex amplitude, and twitch contractile properties concurrently during a fatiguing submaximal isometric contraction of the ankle plantarflexors. Eleven healthy subjects performed fatiguing contractions of low force (25% maximal voluntary contraction (MVC)) or high force (42-66% MVC). Hoffmann (H)-reflexes, muscle compound action potentials (M-waves), twitch contractile properties, and motor unit discharges were recorded from the soleus muscle. In the low-force fatigue task, motor unit firing rate increased gradually over time, whereas the resting H-reflex was significantly depressed at 15% of endurance time and remained quasiconstant for the rest of the task. This suggests that the processes mediating the resting H-reflex depression are relatively independent of those modulating the motor unit firing rate during a low-force fatigue task. In the high-force fatigue task, a decline in the average motor unit discharge rate was accompanied by a decrease in the resting H-reflex amplitude and a prolongation of the twitch half-relaxation time (HRT) at the completion of the fatigue task. Overall, motor unit firing rate was modulated in parallel with changes in the twitch HRT, consistent with the muscle wisdom hypothesis.

Fatigue-Sensitive Afferents Inhibit Extensor but Not Flexor Motoneurons in Humans

“…Much of the indirect evidence favoring a reflex inhibitory pathway mediated by group III and IV muscle afferents has been obtained for other physiological extensors. For example H-reflexes evoked in ankle plantarflexors are depressed after fatigue (Garland and McComas, 1990;Garland, 1991;Loscher et al, 1996;Walton et al, 2002;Kuchinad et al, 2004). We suggested previously that the discrepancy between results based on the H-reflex and that for the CMEP in elbow flexors (Butler et al, 2003) could be attributable to presynaptic inhibition of group Ia afferent volleys by group III and IV muscle afferents (Pettorossi et al, 1999;Rossi et al, 1999;Rudomin and Schmidt, 1999).…”

“…When firing of group III and IV muscle afferents is maintained after fatiguing exercise by muscle ischemia, the recovery of the discharge rates of motoneurons is prevented (Bigland-Ritchie et al, 1986;Woods et al, 1987). The tendon jerk and H-reflex are also reduced by fatigue (Garland and McComas, 1990;Garland, 1991;Duchateau and Hainaut, 1993;Walton et al, 2002;Kuchinad et al, 2004), and a contraction-induced spinal inhibition may operate between synergists (Sacco et al, 1997). These studies suggest that group III and IV muscle afferent feedback from the fatiguing muscle can reflexly depress or inhibit motoneuronal firing rates.…”

Effect of Bone Tissue on Vibration-Induced Electrical Activity of Muscles

“…Vib ra ti on su pe rim po sed MVC may eli cit ne u ro mus cu lar fa ti gu e and a dec li ne in EMG ac ti vity. 10,26,27 For so me of our sub jects, MVC-EMGrms did not in cre a se, rat her they dec re a sed. In the se ca ses, it might ha ve be en du e to ex ces si ve vib ra ti on lo ad that ca u sed musc le fa ti gu e. In this study, the vib ra ti on lo ad was equ al to 1/3 of the ide al body we ight of the sub jects.…”

“…Ide al body we ight was cal cu la ted as (Ide al body mass in dex) x (body he ight) 2 . The mean vib ra ti on lo ad was 20.2 (18-23) kg in fe ma les and 23.8 (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28) kg in ma les.…”

“…Se ve ral stu di es ha ve shown that the vib ra ti on sup pres ses the H-ref lex amp li tu de. 27,[35][36][37] In the pre sent study, FCR H ref lex was fo und to be sup pres sed or depres sed du ring vib ra ti on, as well. H-ref lex sup pressi on/dep res si on ca u sed by vib ra ti on is exp la i ned by presy nap tic in hi bi ti on of musc le spind le gro up Ia af fe rents.…”