Motor unit number estimates and neuromuscular transmission in the tibialis anterior of master athletes: evidence that athletic older people are not spared from age-related motor unit remodeling.

Alex Ireland,Mathew Piasecki,Dan W. Stashuk,David A. Jones,Jessica Coulson,Jamie S. McPhee,Agnieszka Swiecicka,Martin K. Rutter,Andrew Hamilton-Wright

doi:10.14814/phy2.12987

Abstract

Muscle motor unit numbers decrease markedly in old age, while remaining motor units are enlarged and can have reduced neuromuscular junction transmission stability. However, it is possible that regular intense physical activity throughout life can attenuate this remodeling. The aim of this study was to compare the number, size, and neuromuscular junction transmission stability of tibialis anterior (TA) motor units in healthy young and older men with those of exceptionally active master runners. The distribution of motor unit potential (MUP) size was determined from intramuscular electromyographic signals recorded in healthy male Young (mean ± SD, 26 ± 5 years), Old (71 ± 4 years) and Master Athletes (69 ± 3 years). Relative differences between groups in numbers of motor units was assessed using two methods, one comparing MUP size and muscle cross‐sectional area (CSA) determined with MRI, the other comparing surface recorded MUPs with maximal compound muscle action potentials and commonly known as a “motor unit number estimate (MUNE)”. Near fiber (NF) jiggle was measured to assess neuromuscular junction transmission stability. TA CSA did not differ between groups. MUNE values for the Old and Master Athletes were 45% and 40%, respectively, of the Young. Intramuscular MUPs of Old and Master Athletes were 43% and 56% larger than Young. NF jiggle was slightly higher in the Master Athletes, with no difference between Young and Old. These results show substantial and similar motor unit loss and remodeling in Master Athletes and Old individuals compared with Young, which suggests that lifelong training does not attenuate the age‐related loss of motor units.

Highlights

Loss of muscle mass and strength are implicated in the general decline in mobility and the propensity to fall that are common features of old age
Muscle fiber loss is associated with death of spinal motor neurons and direct counts of motor neuron cell bodies in postmortem specimens of human lumbar spine show around 30% fewer neurons innervating the leg a 2016 The Authors
The Old were of a similar body mass and body mass index (BMI) to the Young but had a higher total body fat while the Master Athletes were the lightest group with significantly less body fat and

Summary

Introduction

Loss of muscle mass and strength are implicated in the general decline in mobility and the propensity to fall that are common features of old age. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society Motor Unit Remodeling in Masters Athletes muscles of old compared with younger people (Kawamura et al 1977; Tomlinson and Irving 1977). This is supported by motor unit number estimates (MUNE) derived from electromyographic (EMG) measurements of several limb muscles suggesting a 30–50% loss of motor units by the age of 70 years (Piasecki et al 2015a). For muscles of the same size, larger motor units are indicative of a smaller total number of motor units

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