Lifelong exercise is associated with more homogeneous motor unit potential features across deep and superficial areas of vastus lateralis

Eleanor J. Jones,Jessica Piasecki,Daniel W. Stashuk,Philip J. Atherton,Bethan E. Phillips,Jamie S. McPhee,Alex Ireland,Mathew Piasecki

doi:10.1007/s11357-021-00356-8

Abstract

Motor unit (MU) expansion enables rescue of denervated muscle fibres helping to ameliorate age-related muscle atrophy, with evidence to suggest master athletes are more successful at this remodelling. Electrophysiological data has suggested MUs located superficially are larger than those located deeper within young muscle. However, the effects of ageing and exercise on MU heterogeneity across deep and superficial aspects of vastus lateralis (VL) remain unclear. Intramuscular electromyography was used to record individual MU potentials (MUPs) and near fibre MUPs (NFMs) from deep and superficial regions of the VL during 25% maximum voluntary contractions, in 83 males (15 young (Y), 17 young athletes (YA), 22 old (O) and 29 master athletes (MA)). MUP size and complexity were assessed using area and number of turns, respectively. Multilevel mixed effects linear regression models were performed to investigate the effects of depth in each group. MUP area was greater in deep compared with superficial MUs in Y (p<0.001) and O (p=0.012) but not in YA (p=0.071) or MA (p=0.653). MUP amplitude and NF MUP area were greater, and MUPs were more complex in deep MUPs from Y, YA and O (all p<0.05) but did not differ across depth in MA (all p>0.07). These data suggest MU characteristics differ according to depth within the VL which may be influenced by both ageing and exercise. A more homogenous distribution of MUP size and complexity across muscle depths in older athletes may be a result of a greater degree of age-related MU adaptations.

Highlights

Sarcopenia is the loss of muscle mass and function with age [3] and is the result of the combination of the atrophy and loss of individual muscle fibres [48]
There were no depth-related differences in MU potentials (MUPs) and near fibre MUPs (NFMs) features between the power and endurance athletes within each age group, so these were grouped
The ratio between the MUP and surface motor unit potential (sMUP) amplitude was significantly greater for deep motor unit (MU) in all groups (Y: β=−1.06; confidence intervals (CI)=−1.52: −0.602; p

Summary

Introduction

Sarcopenia is the loss of muscle mass and function with age [3] and is the result of the combination of the atrophy and loss of individual muscle fibres [48]. As the proportion of older people in the population increases, with UK estimates of 12.2 million people over the age of 65 in 2018 [33], and with sarcopenia estimated to affect >50 million people over 60 globally [2], interventions to maintain musculoskeletal health in older adults at greater risk are more pressing than ever Lower limb muscles such as the quadriceps are important for mobility and balance [19] but often show the greatest reductions in muscle mass and strength with age and inactivity compared to other muscle groups [20, 24]; declines here have a greater detrimental functional impact. A reduction in MU number has been observed in multiple muscles [12, 40, 41] alongside increases in MU size with advancing age [22, 35, 39]

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