Neurophysiological responses and adaptation following repeated bouts of maximal lengthening contractions in young and older adults

Abstract

A bout of maximal lengthening contractions is known to produce muscle damage, but confers protection against subsequent damaging bouts, with both tending to be lower in older adults. Neural factors contribute to this adaptation, but the role of the corticospinal pathway remains unclear. Twelve young (27 ± 5 yr) and 11 older adults (66 ± 4 yr) performed two bouts of 60 maximal lengthening dorsiflexions 2 weeks apart. Neuromuscular responses were measured preexercise, immediately postexercise, and at 24 and 72 h following both bouts. The initial bout resulted in prolonged reductions in maximal voluntary torque (MVC; immediately postexercise onward, P < 0.001) and increased creatine kinase (from 24 h onward, P = 0.001), with both responses being attenuated following the second bout (P < 0.015), demonstrating adaptation. Smaller reductions in MVC following both bouts occurred in older adults (P = 0.005). Intracortical facilitation showed no changes (P ≥ 0.245). Motor-evoked potentials increased 24 and 72 h postexercise in young (P ≤ 0.038). Torque variability (P ≤ 0.041) and H-reflex size (P = 0.024) increased, while short-interval intracortical inhibition (SICI; P = 0.019) and the silent period duration (SP) decreased (P = 0.001) in both groups immediately postexercise. The SP decrease was smaller following the second bout (P = 0.021), and there was an association between the change in SICI and reduction in MVC 24 h postexercise in young adults (R = -0.47, P = 0.036). Changes in neurophysiological responses were mostly limited to immediately postexercise, suggesting a modest role in adaptation. In young adults, neural inhibitory changes are linked to the extent of MVC reduction, possibly mediated by the muscle damage-related afferent feedback. Older adults incurred less muscle damage, which has implications for exercise prescription.NEW & NOTEWORTHY This is the first study to have collectively assessed the role of corticospinal, spinal, and intracortical activity in muscle damage attenuation following repeated bouts of exercise in young and older adults. Lower levels of muscle damage in older adults are not related to their neurophysiological responses. Neural inhibition transiently changed, which might be related to the extent of muscle damage; however, the role of processes along the corticospinal pathway in the adaptive response is limited.