Neurophysiological responses and adaptation to muscle shortening and lengthening in young and older adults

Škarabot, Jakob (2019) Neurophysiological responses and adaptation to muscle shortening and lengthening in young and older adults. Doctoral thesis, Northumbria University.

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Abstract

Healthy aging is characterised by alterations in the nervous system, leading to decrements in neuromuscular performance, particularly during dynamic contractions. Muscle shortening and lengthening differently modulate the corticospinal output, with the possibility of this modulation being altered in aging adults, which might affect the adaptability of an aging neuromuscular system to maximal lengthening contractions. The aim of this thesis was to elucidate the differences in neurophysiological responses and adaptation to muscle shortening and lengthening between young and older adults. It was hypothesised that the age-related alterations in the nervous system will lead to impaired sensorimotor integration with muscle length changes and reduced corticospinal responses during dynamic contractions, impairing the adaptability of older adults to maximal lengthening contractions. In Study 1, a novel method for assessment of subcortical excitability of descending tracts was developed, followed by investigation of corticospinal responses during passive muscle shortening and lengthening. Corticospinal excitability was modulated by muscle length changes in young adults, likely through inhibitory input of muscle spindle afferents on cortical areas. In contrast, older adults showed no modulation, which may be linked to altered sensorimotor integration. In Study 2, a method for normalising torque outputs during submaximal dynamic contractions was developed, followed by assessment of muscle fascicle behaviour. Subsequently, evoked responses were assessed during submaximal contractions of different types in young and older individuals. Despite preserved maximal torque producing capacity, corticospinal responses were reduced in older compared with younger adults across contraction types, along with increased torque variability during dynamic contractions. Study 3 assessed the contribution of spinal and supraspinal properties in adaptation to repeated bouts of maximal lengthening contractions in young and older adults. Less damage was incurred in older individuals, but the rate of adaptation was similar between young and older adults. However, the corticospinal processes played a limited role in the adaptive response. This work extends the understanding of the modulation of corticospinal networks with muscle shortening and lengthening and age-related alterations in corticospinal pathway during dynamic contractions. It also suggests that the adaptability of an aging neuromuscular system to maximal dynamic contractions remains preserved.

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