Single joint fatiguing exercise decreases long but not short-interval intracortical inhibition in older adults

Relative Neuroadaptive Effect of Resistance Training along the Descending Neuroaxis in Older Adults

Age-related decline in voluntary force production represents one of the main contributors to the onset of physical disability in older adults and is argued to stem from adverse musculoskeletal alterations and changes along the descending neuroaxis. The neural contribution of the above is possibly indicated by disproportionate losses in voluntary activation (VA) compared to muscle mass. For young adults, resistance training (RT) induces muscular and neural adaptations over several levels of the central nervous system, contributing to increased physical performance. However, less is known about the relative neuroadaptive contribution of RT in older adults. The aim of this review was to outline the current state of the literature regarding where and to what extent neural adaptations occur along the descending neuroaxis in response to RT in older adults. We performed a literature search in PubMed, Google Scholar and Scopus. A total of 63 articles met the primary inclusion criteria and following quality analysis (PEDro) 23 articles were included. Overall, neuroadaptations in older adults seemingly favor top-down adaptations, where the preceding changes of neural drive from superior levels affect the neural output of lower levels, following RT. Moreover, older adults appear more predisposed to neural rather than morphological adaptations compared to young adults, a potentially important implication for the improved maintenance of neuromuscular function during aging.

Submaximal isometric fatiguing exercise of the elbow flexors has no age-related effect on GABAB mediated inhibition

Age-related changes in the neuromuscular system can result in differences in fatigability between young and older adults. Previous research has shown that single joint isometric fatiguing exercise of small muscle results in an age-related compensatory decrease in GABA_B mediated inhibition. However, this has yet to be established in a larger muscle group. In 15 young (22 ± 4 years) and 15 older (65 ± 5 years) adults, long interval cortical inhibition (LICI; 100 ms ISI) and corticospinal silent period (SP) were measured in the biceps brachii during a 5% EMG contraction using transcranial magnetic stimulation (TMS) before, during and after a submaximal contraction (30% MVC force) held intermittently to task failure. Both age groups developed similar magnitude of fatigue (~24% decline in MVC; P = 0.001) and ~28% decline in LICI (P = 0.001) post fatiguing exercise. No change in SP duration was observed during and immediately following fatigue (P = 0.909) but ~ 6% decrease was seen at recovery in both age groups (P<0.001)." Contrary to previous work in a small muscle, these findings suggest no age-related differences in GABA_B mediated inhibition following single joint isometric fatiguing exercise of the elbow flexors, indicating that GABA_B modulation with ageing may be muscle group dependent. Furthermore, variations in SP duration and LICI modulation during and post fatigue in both groups suggest that these measures are likely mediated by divergent mechanisms.

Remote muscle priming anodal transcranial direct current stimulation attenuates short interval intracortical inhibition and increases time to task failure of a constant workload cycling exercise

Anodal transcranial direct current stimulation (atDCS), a non-invasive neuromodulatory technique has been shown to increase the excitability of targeted brain area and influence endurance exercise performance. However, the effect of atDCS applied on an unexercised muscle motor cortex (M1) representation on GABA_A-mediated intracortical inhibition and endurance exercise performance remains unknown. In two separate sessions, twelve subjects performed fatigue cycling exercise (80% peak power output) sustained to task failure in a double-blinded design, following either ten minutes of bicephalic anodal tDCS (atDCS) or sham applied on a non-exercised hand muscle M1 representation. Short interval intracortical inhibition (SICI) was measured at baseline, post neuromodulation and post-exercise using paired-pulse transcranial magnetic stimulation (TMS) in a resting hand muscle. There was a greater decrease in SICI (P < 0.05) post fatigue cycling with atDCS priming compared to sham. Time to task failure (TTF) was significantly increased following atDCS compared to sham (P < 0.05). These findings suggest that atDCS applied over the non-exercised muscle M1 representation can augment cycling exercise performance; and although this outcome may be mediated via a multitude of mechanisms, a decrease in the global excitability of GABA_A inhibitory interneurons may be a possible contributing factor.