Hypoalgesic Effects of Exercise and Training of Different Modalities

Plain-language summary

Further research is needed to explore the analgesic effects of post-acute and chronic exercise and if effects are partially dependent on lower H-reflex/V-wave inhibition by group III and IV muscle afferents desensitization. In addition, it is important to note that the available research examining the interaction between exercise (acute and chronic) and pain did not compare the effect of different exercise modalities. Moreover, since discharge rate of III and IV afferents is highly dependent on specific task-metabolic requirements, it is relevant to determine if exercise modality differentially modulates the possible analgesic response via metaboreflex desensitization. Therefore, with this project, the investigators aim at exploring the influence of experimentally induced metaboreflex activation (group III and IV afferents) (maintained submaximal isometric contraction followed by blood-flow occlusion - post-exercise circulatory occlusion (PECO)) Vs. a control condition (no isometric contraction nor blood flow occlusion), on H-reflex recruitment curve and V-wave excitability (aim #1). Since the activation of group III/IV afferents can inhibit the corticospinal pathway, the investigators hypothesize that the normalized maximal amplitude of H- and V-wave and the slope of the ascending limb of the H-recruitment curve will decrease with metaboreflex activation (during PECO). Additionally, the investigators intend to examine the acute impact of different lower-limb exercise regimens (aerobic, anaerobic and dynamic resistance) on pain sensitivity (by the application of a foot cold pain tolerance test, by determining pressure pain threshold of exercising and non-exercising muscles and through self-reported leg ischemic pain ratings (0-10 Cook scale) - during PECO) (aim #2). Participants are also asked to mark the intensity of cold pain sensation using a visual analogue scale (VAS) with a horizontal line of 100mm, every 30 s of testing.This will allow the investigators to investigate the analgesic role of different acute exercise paradigms through possible metaboreflex desensitization given by altered neuromuscular responses at post-exercise time point (attenuated inhibition of H-reflex and V-wave amplitude during PECO). Also, once past research has shown that muscle metaboreflex activation reduces the sensitivity of the baroreflex, the investigators will measure arterial pressure (AP) and heart rate variability (HRV) during cold test and AP during ischemic testing. Despite all exercise modalities could enhance reducing pain sensibility after exercise, the investigators hypothesize that resistance and anaerobic exercise stimuli will enhance pain threshold/tolerance and reduced pain ratings via metaboreflex desensitization compared to that seen with endurance exercise (attenuated increase of AP and sympathetic component of HRV during cold pressor test and attenuated increase of AP during ischemic pain test). Finally, the investigators intend to explore the impact of long-term aerobic, anaerobic and resistance training on pain sensibility, using the same tests of aim #2 (aim #3). The investigators hypothesize that chronic EIH will be of greater magnitude after anaerobic and resistance exercise training vs. endurance training. This will be related with attenuated inhibition of H-reflex and V-wave amplitude measured during PECO and attenuated increase of AP and sympathetic component of HRV during metaboreflex activation after those training modalities. Nevertheless, the investigators expect to see an increased motor performance in all training regimens via heightened H-reflex (aerobic and anaerobic groups) and V-wave (resistance group) excitability.

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