title: Fatigue-related firing of distal muscle nociceptors reduces voluntary activation of proximal muscles of the same limb.
abstract: 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.
authors.affiliation: Neuroscience Research Australia, Randwick, New South Wales, Australia;
authors.forename: David S
authors.initials: DS
authors.lastname: Kennedy
authors.forename: Chris J
authors.initials: CJ
authors.lastname: McNeil
authors.forename: Simon C
authors.initials: SC
authors.lastname: Gandevia
authors.forename: Janet L
authors.initials: JL
authors.lastname: Taylor
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D001917:Brachial Plexus / Q000503:physiopathology; D004558:Electric Stimulation; D005260:Female; D006801:Humans; D007511:Ischemia / Q000378:metabolism / Q000503:physiopathology*; D008297:Male; D009044:Motor Cortex / Q000503:physiopathology; D009119:Muscle Contraction; D018763:Muscle Fatigue; D053580:Muscle Strength; D018482:Muscle, Skeletal / Q000098:blood supply / Q000294:innervation*; D009619:Nociceptors / Q000378:metabolism*; D010146:Pain / Q000378:metabolism / Q000503:physiopathology; D012039:Regional Blood Flow; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation; D014459:Ulnar Nerve / Q000503:physiopathology; D034941:Upper Extremity
keywords: group III and IV muscle afferents; muscle fatigue; superimposed twitch; transcranial magnetic stimulation; voluntary activation