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

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.

Automated summary

Motor unit (MU) expansion plays a crucial role in rescuing denervated muscle fibers to mitigate age-related muscle atrophy, with master athletes demonstrating greater success in this process. Data from intramuscular electromyography studies suggest that MUs located superficially are larger than those located deeper within young muscle. However, the effects of aging and exercise on MU heterogeneity across deep and superficial aspects of vastus lateralis (VL) remain uncertain. The study indicates that MU characteristics may vary with muscle depth within the VL, influenced by both the aging process and exercise levels. Older athletes exhibit a more uniform distribution of MUP size and complexity across muscle depths, potentially due to a higher degree of age-related MU adaptations.