Muscle fibre denervation in ageing.

Muscle fibre denervation describes the loss of effective neural input from a motor neuron to one or more muscle fibres. In ageing, denervation is increasingly recognised as an important contributor to progressive declines in muscle strength and functional capacity, yet it remains heterogeneous and difficult to define in humans. This ambiguity reflects both biological complexity and current methodological limitations. The purpose of the present review is to synthesise current human evidence for muscle fibre denervation in ageing, clarify key conceptual distinctions, and evaluate methodological approaches used to assess denervation in humans. Muscle fibre denervation can occur through structural disconnection of the motor neuron from the fibre or through functional impairment of neuromuscular transmission. Evidence for denervation in ageing is derived from histological, molecular, electrophysiological, and circulating biomarker approaches, each capturing distinct and only partially overlapping aspects of neuromuscular integrity. Importantly, no single measure provides a comprehensive assessment of denervation. Experimental models of disuse in humans reveal a functional denervation phenotype, characterised by molecular and electrophysiological changes that partially resemble those observed with ageing. Physical activity appears to mitigate against aspects of muscle fibre denervation; however, the mechanisms underlying these effects remain incompletely understood. Collectively, the available evidence indicates that denervation in ageing is a multifaceted and dynamic process that requires multimodal, longitudinal approaches to define, detect, and ultimately target denervation-related mechanisms to preserve neuromuscular function across the human lifespan.