Firing of antagonist small-diameter muscle afferents reduces voluntary activation and torque of elbow flexors.

Key points Maintained firing of fatigue-sensitive small-diameter muscle afferents is reported to reduce voluntary activation of the homonymous (fatigued) muscle., Our study determined if firing of fatigue-sensitive afferents from elbow extensor muscles reduces voluntary activation and torque of the non-fatigued elbow flexors., We examined voluntary activation of the elbow flexors by measuring changes in superimposed twitches evoked by magnetic cortical stimulation during maximal voluntary contractions., Following a fatiguing contraction of elbow extensors, the voluntary drive to unfatigued flexor muscles was reduced with continued activation of small-diameter muscle afferents produced by a blood pressure cuff inflated to maintain muscle ischaemia., Continued discharge of small-diameter muscle afferents from one muscle can decrease voluntary drive to another muscle in the same limb and can reduce its maximal voluntary torque., Abstract During muscle fatigue, firing of small-diameter muscle afferents can decrease voluntary activation of the fatigued muscle. However, these afferents may have a more widespread effect on other muscles in the exercising limb. We examined if the firing of fatigue-sensitive afferents from elbow extensor muscles in the same arm reduces torque production and voluntary activation of elbow flexors. In nine subjects we examined voluntary activation of elbow flexors by measuring changes in superimposed twitches evoked by transcranial magnetic stimulation of the motor cortex during brief (2-3 s) maximal voluntary contractions (MVC). Inflation of a blood pressure cuff following a 2-min sustained MVC blocked blood flow to the fatigued muscle and maintained firing of small-diameter afferents. After a fatiguing elbow flexion contraction, maximal flexion torque was lower (26.0 ± 4.4% versus 67.9 ± 5.2% of initial maximal torque; means ± s.d.; P < 0.001) and superimposed twitches were larger (4.1 ± 1.1% versus 1.8 ± 0.2% ongoing MVC, P= 0.01) with than without ischaemia. After a fatiguing elbow extensor contraction, maximal flexion torque was also reduced (82.2 ± 4.9% versus 91.4 ± 2.3% of initial maximal torque; P= 0.007), superimposed twitches were larger (2.7 ± 0.7% versus 1.3 ± 0.2% ongoing MVC; P= 0.02) and voluntary activation lower (81.6 ± 8.2% versus 95.5 ± 6.9%; P= 0.04) with than without ischaemia. After a fatiguing contraction, voluntary drive to the fatigued muscles is reduced with continued input from small-diameter muscle afferents. Furthermore, fatigue of the elbow extensor muscles decreases voluntary drive to unfatigued elbow flexors of the same arm. Therefore, firing of small-diameter muscle afferents from one muscle can affect voluntary activation and hence torque generation of another muscle in the same limb. [ABSTRACT FROM AUTHOR]