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

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 yrs) and eleven older adults (66±4 yrs) performed two bouts of 60 maximal lengthening dorsiflexions two weeks apart. Neuromuscular responses were measured pre, immediately-post and at 24- and 72-hours following both bouts. The initial bout resulted in prolonged reductions in maximal voluntary torque (MVC; immediately post-exercise onwards, p<0.001) and increased creatine kinase (from 24-hours onwards, 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-hours post-exercise in young (p≤0.038). Torque variability (p≤0.041) and H-reflex size (p=0.024) increased, whilst intracortical inhibition (SICI; p=0.019) and the silent period duration (SP) decreased (p=0.001) in both groups immediately post-exercise. 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-hours post-exercise in young adults (R=−0.47, p=0.036). Changes in neurophysiological responses were mostly limited to immediately post-exercise, suggesting a modest role in adaptation. In young, 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.