Firing of fatigue-sensitive small-diameter afferents of the calf muscles impairs maximal contractions of the knee extensor muscles
Finn H[1, 2], Kennedy D[2, 3], Green S, Taylor J[1, 5]
1. NeuRA, Sydney, Australia
2. University of New South Wales, Sydney, Australia
3. University of Technology Sydney, Australia
4. Western Sydney University, Campbelltown, Australia
5. Edith Cowan University, Perth, Australia
In the arm, maintained firing of fatigue-sensitive group III/IV afferents of hand muscles reduce voluntary activation of the elbow flexors (1). Analogous effects in the leg have not been investigated. Here we test whether maintained group III/IV afferent feedback from the plantarflexor muscles reduces voluntary activation of the knee extensors.
On two days, voluntary activation (VA) of the knee extensors was assessed during brief maximal voluntary contractions (MVCs) before and after a 3-min fatiguing task of the plantarflexors (n=12). On one day immediately post-exercise, a cuff inflated around the calf occluded blood flow for two minutes. Supramaximal stimulation of the femoral nerve during and 2-3 s after MVCs of the knee extensors elicited superimposed and resting twitches. VA was calculated as (1- (superimposed/resting twitch)) x100. Muscle pain was reported on a 0-10 point scale.
In the 2 min after the fatiguing plantarflexor task, VA was 5.4% (SD 6.9) lower with blood flow occlusion than without (P=0.045). MVC force was also reduced by 12% (SD 15) (P=0.022) with the cuff inflated, and pain rated 4.9 points higher (P=0.001).
Maintained firing of group III/IV afferents from the fatigued plantarflexor muscles reduced voluntary activation and maximal force of the unfatigued knee extensors. This suggests that fatigue-related sensory feedback from the calf acts centrally to inhibit neural drive to the knee extensor muscles. When perfusion of working muscles is low, as in peripheral artery disease, this mechanism could contribute to poor performance of the knee extensors.
- Kennedy DS, McNeil CJ, Gandevia SC, Taylor JL. (2014). Fatigue-related firing of distal muscle nociceptors reduces voluntary activation of proximal muscles of the same limb. J Appl Physiol 116(4), 385-94.