Submaximal isometric fatiguing exercise of the elbow flexors has no age-related effect on GABA_B-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 γ-aminobutyric acid (GABA_B)-mediated inhibition. However, this has yet to be established in a larger muscle group. In 15 young (22 ± 4 yr) and 15 older (65 ± 5 yr) adults, long interval cortical inhibition [LICI; 100 ms Interstimulus interval (ISI)] and corticospinal silent period (SP) were measured in the biceps brachii during a 5% electromyography (EMG) contraction using transcranial magnetic stimulation (TMS) before, during, and after a submaximal contraction [30% of maximum voluntary contraction (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 aging 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.NEW & NOTEWORTHY Transcranial magnetic stimulation was used to examine GABA_B-mediated inhibition during fatiguing exercise of large muscle group in older adults and young adults. We provide novel evidence to show that when older and young adults are faced with a similar magnitude of elbow flexor muscle fatigue, they have a similar decline in GABA_B-mediated inhibition. This suggests that when measured in a large muscle group, older adults maintain the ability to modulate GABA_B inhibition during fatiguing exercise.