Non-local neuromuscular effects and mechanisms

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Our laboratory has undertaken work on non-local (stress on one body part affecting the performance of another body part) effects of fatigue (crossover fatigue), flexibility and pain. Our recently published review (Halperin et al. 2015) revealed that non-local muscle fatigue (NLMF) effects are more predominant when the lower body (primarily the quadriceps) rather than the upper body is tested. This preference may reflect differences in the neural network of the upper and lower body. There is evidence for NLMF effects to be attributed to primarily neurological mechanisms however there are biochemical, biomechanical and psychological factors also influencing the outcome and these are likely to be inter-related. There are many questions still outstanding on this topic. We have recently provided a revision to a submitted paper that shows that NLMF is more prevalent with adolescent males affecting upper and lower body strength, jump height and balance performance. The decreases in the contralateral limb were nearly as substantial as the fatigued limb emphasizing the important role of the nervous system with fatigue in male youth. We also recently published the only study with adolescent girls showing the NLMF effect was less apparent in this group (Reid et al. 2016). There are no investigations comparing contralateral to ipsilateral non-local responses, which would help highlight neural fatigue pathways. Such investigations enhance our understanding of basic muscle fatigue mechanisms. Our non-local changes in flexibility and pain may be important findings for those suffering from localized contraction flexures from strokes or if stimulation of unaffected body parts can minimize pain and the order of exercises used in training programs. Whereas fatigue has non-local or crossover effects, we have just published an article demonstrating that while a muscle group treated with a conditioning exercise can experience post-activation potentiation, the contralateral muscle experiences fatigue. These findings highlight the specific and differing responses of peripheral (muscle) and (central) neural components to a bout of exercise. It may be possible to elicit neuromuscular potentiation in non-local, non-conditioned muscles which would demonstrate the extent of central inhibitory and excitatory effects. Considering the importance and relevance of this topic, more research is needed to clarify the physiological mechanisms, extent of the effects using adequate methods, and with an attempt to account for some of the described confounding variables.