Fatigue-related firing of distal muscle nociceptors reduces voluntary activation of proximal muscles of the same limb

With fatiguing exercise, firing of group III/IV muscle afferents reduces voluntary activation and force of the exercised muscles. These afferents can also act across agonist/antagonist pairs, reducing voluntary activation and force in nonfatigued muscles. We hypothesized that maintained firing of group III/IV muscle afferents after a fatiguing adductor pollicis (AP) contraction would decrease voluntary activation and force of AP and ipsilateral elbow flexors. In two experiments (n = 10) we examined voluntary activation of AP and elbow flexors by measuring changes in superimposed twitches evoked by ulnar nerve stimulation and transcranial magnetic stimulation of the motor cortex, respectively. Inflation of a sphygmomanometer cuff after a 2-min AP maximal voluntary contraction (MVC) blocked circulation of the hand for 2 min and maintained firing of group III/IV muscle afferents. After a 2-min AP MVC, maximal AP voluntary activation was lower with than without ischemia (56.2 +/- 17.7% vs. 76.3 +/- 14.6%; mean +/- SD; P < 0.05) as was force (40.3 +/- 12.8% vs. 57.1 +/- 13.8% peak MVC; P < 0.05). Likewise, after a 2-min AP MVC, elbow flexion voluntary activation was lower with than without ischemia (88.3 +/- 7.5% vs. 93.6 +/- 3.9%; P < 0.05) as was torque (80.2 +/- 4.6% vs. 86.6 +/- 1.0% peak MVC; P < 0.05). Pain during ischemia was reported as Moderate to Very Strong. Postfatigue firing of group III/IV muscle afferents from the hand decreased voluntary drive and force of AP. Moreover, this effect decreased voluntary drive and torque of proximal unfatigued muscles, the elbow flexors. Fatigue-sensitive group III/IV muscle nociceptors act to limit voluntary drive not only to fatigued muscles but also to unfatigued muscles within the same limb.