Failure to expand the motor unit size to compensate for declining motor unit numbers distinguishes sarcopenic from non‐sarcopenic older men

Abstract

Key points The age‐related loss of muscle mass is related to the loss of innervating motor neurons and denervation of muscle fibres.Not all denervated muscle fibres are degraded; some may be reinnervated by an adjacent surviving neuron, which expands the innervating motor unit proportional to the numbers of fibres rescued.Enlarged motor units have larger motor unit potentials when measured using electrophysiological techniques.We recorded much larger motor unit potentials in relatively healthy older men compared to young men, but the older men with the smallest muscles (sarcopenia) had smaller motor unit potentials than healthy older men.These findings suggest that healthy older men reinnervate large numbers of muscle fibres to compensate for declining motor neuron numbers, but a failure to do so contributes to muscle loss in sarcopenic men. AbstractSarcopenia results from the progressive loss of skeletal muscle mass and reduced function in older age. It is likely to be associated with the well‐documented reduction of motor unit numbers innervating limb muscles and the increase in size of surviving motor units via reinnervation of denervated fibres. However, no evidence exists to confirm the extent of motor unit remodelling in sarcopenic individuals. The aim of the present study was to compare motor unit size and number between young (n = 48), non‐sarcopenic old (n = 13), pre‐sarcopenic (n = 53) and sarcopenic (n = 29) men. Motor unit potentials (MUPs) were isolated from intramuscular and surface EMG recordings. The motor unit numbers were reduced in all groups of old compared with young men (all P < 0.001). MUPs were higher in non‐sarcopenic and pre‐sarcopenic men compared with young men (P = 0.039 and 0.001 respectively), but not in the vastus lateralis of sarcopenic old (P = 0.485). The results suggest that extensive motor unit remodelling occurs relatively early during ageing, exceeds the loss of muscle mass and precedes sarcopenia. Reinnervation of denervated muscle fibres probably expands the motor unit size in the non‐sarcopenic and pre‐sarcopenic old, but not in the sarcopenic old. These findings suggest that a failure to expand the motor unit size distinguishes sarcopenic from pre‐sarcopenic muscles.

Fidelity of muscle fibre reinnervation modulates ageing muscle impact in elderly women

“…This has been suggested to be indicative of a larger motor unit, and one which retains the capacity of successful reinnervation to promote maintenance of muscle mass (Piasecki et al . ). In the present study, we found that although both FE and MA groups had significantly higher type I fibre grouping compared to YI controls, the MA group showed 18% more type I fibre grouping compared to FE.…”

“…Specifically, Piasecki et al . () observed in both tibialis anterior and vastus lateralis muscles that elderly individuals with low muscle mass had fewer motor units and the remaining motor units were unchanged in size relative to young adults, whereas individuals who maintained muscle mass had a smaller reduction in motor units (vastus lateralis only) and larger remaining motor unit size, consistent with a greater capacity to expand the surviving motor units in the latter group. Further to this point, the reinnervation process can also be detected in muscle biopsies as a grouping of fibres of the same type, and Mosole and colleagues (Mosole et al .…”

Muscle atrophy and regeneration associated with behavioural loss and recovery of function after sciatic nerve crush

“…Denervation is a common course for disrupted function of skeletal muscles and it may origin in a trauma (ie, mechanical lesioning of the nerve trunk), infections, systemic diseases affecting, eg, the microvasculature of the peripheral nerves or diseases like amyotrophic lateral sclerosis . Denervation may also be a driving force in the loss of skeletal muscle strength and mass during normal ageing, ie, sarcopenia …”

Mechanisms of sarcopenia: motor unit remodelling and muscle fibre type shifts with ageing

“…Piasecki et al . () found that muscle mass loss related to ageing is not directly proportional to declining motor unit numbers. As age and atrophy continue to compound, a critical inflection point is reached and existing motor units no longer increase as a compensatory mechanism.…”

“…The investigation by Piasecki et al . () had the advantage of a large subject population that was also stratified by sarcopenic status. Additionally, they used more sensitive measures via intramuscular electromyogram (EMG) in both the TA and the VL compared to previous literature that utilized surface EMG and/or only observed the TA.…”