Aboodarda, S. J., Power, K., Copithorne, D., Drinkwater, E. J., & Behm, D. G. (2014). Upper body fatiguing protocols decreased voluntary activation and increased motoneurone excitability of non-fatigued knee extensors. Medicine & Science in Sports & Exercise, 46(5), Supplement abstract number 3178.

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"There is evidence of crossover or non-localized fatigue effects characterized by a global impairment in the coordination and performance of non-exercised muscles. Given that central fatigue is a net effect of excitatory and inhibitory responses of supraspinal and spinal motor pathways, studying responses of spinal motoneurones in one extremity (e.g. quadriceps muscle) following a fatiguing exercise performed by another extremity (elbow flexors) may provide insight about the contribution of supraspinal and spinal mechanisms in the crossover effect of fatigue."

This study investigated the effects of exercise-induced upper-limb muscle fatigue on motoneuronal excitability and motor-task performance of non-exercised lower-limb muscles. Ss (N = 9) attended three testing sessions: i) control, ii) fatiguing unilateral elbow-flexion, and iii) fatiguing bilateral elbow-flexion. The non-fatigued knee-extensor muscles were assessed with neuromuscular tests including thoracic stimulation (thoracic motor-evoked potential, quadriceps muscle-stimulation, and knee-extensor maximum voluntary contractions before and at 30 seconds, three minutes, and five minutes post-fatiguing protocol.

Fatiguing unilateral elbow-flexion and fatiguing unilateral elbow-flexion caused a moderate effect size drop in the knee extension maximum voluntary contraction force and normalized EMG compared with Control at three and five minutes . The thoracic motor-evoked potential to M-wave ratio showed moderate and large effect size increases after the fatiguing unilateral elbow-flexion and fatiguing unilateral elbow-flexion exercises compared with Control at 30 seconds post-fatiguing protocol.

Implication. Maximum voluntary contraction-force and voluntary activation of the knee-extensors were decreased following elbow-flexor fatigue. Interestingly, knee extensor spinal motoneurone excitability was transiently increased (30 seconds post-fatiguing protocol only) following elbow-flexor fatigue. Each of the above effects was more pronounced during the fatiguing bilateral elbow-flexion compared to the fatiguing unilateral elbow-flexion condition, suggesting that the observed interlimb/intersegmental effect of fatigue is muscle-volume-dependent. It is likely that the observed effects on voluntary force and EMG were centrally mediated given that there were no changes in the M-wave or evoked contractile properties, both measures of peripheral excitability.

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