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

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.

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

“…; Piasecki et al . ,b). In the present study, we observed that in non‐sarcopenic and pre‐sarcopenic elderly men, a doubling of motor unit size in TA and a 26–42% higher motor unit size in VL (Fig.…”

“…The motor points of VL (proximal) and TA were identified by low‐intensity percutaneous electrical stimulations as previously described (Piasecki et al . ,b). For VL, the motor point was located along the centre line of the muscle around 220 mm from the lateral femoral condyle.…”

“…Dividing the CSA of a muscle by the mean MUP area (cm 2 /mV/ms) provides a value proportional to the total number of MUs within that cross‐section of muscle (Piasecki et al . ,b, ).…”

“…; Piasecki et al . ,b). Larger motor units are thought to result from branching of nearby motor neurons to reinnervate recently denervated muscle fibres (Luff, ; Deschenes, ; Hepple & Rice, ).…”

Faster, higher, farther: outpacing age‐related motor neuron losses

“…, ; Piasecki et al . ). While denervation changes, consistent with motor neuron losses, were noted in both groups of older women in the current work, biopsies from world‐class masters athletes showed findings of improved reinnervation.…”