Long-Term Endurance and Power Training May Facilitate Motor Unit Size Expansion to Compensate for Declining Motor Unit Numbers in Older Age

Abstract

The evidence concerning the effects of exercise in older age on motor unit (MU) numbers, muscle fiber denervation and reinnervation cycles is inconclusive and it remains unknown whether any effects are dependent on the type of exercise undertaken or are localized to highly used muscles. MU characteristics of the vastus lateralis (VL) were assessed using surface and intramuscular electromyography in eighty-five participants, divided into sub groups based on age (young, old) and athletic discipline (control, endurance, power). In a separate study of the biceps brachii (BB), the same characteristics were compared in the favored and non-favored arms in eleven masters tennis players. Muscle size was assessed using MRI and ultrasound. In the VL, the CSA was greater in young compared to old, and power athletes had the largest CSA within their age groups. Motor unit potential (MUP) size was larger in all old compared to young ( p < 0.001), with interaction contrasts showing this age-related difference was greater for endurance and power athletes than controls, and MUP size was greater in old athletes compared to old controls. In the BB, thickness did not differ between favored and non-favored arms ( p = 0.575), but MUP size was larger in the favored arm ( p < 0.001). Long-term athletic training does not prevent age-related loss of muscle size in the VL or BB, regardless of athletic discipline, but may facilitate more successful axonal sprouting and reinnervation of denervated fibers. These effects may be localized to muscles most involved in the exercise.

Frailty phenotype and frailty index are associated with distinct neuromuscular electrophysiological characteristics in men

“…The maximal CMAP represents the summation of all action potentials from all muscle fibres within the recording range of the surface electrode after being activated at the same time. Age‐related decreases in CMAP size have been reported for a number of muscles, including the VL (Piasecki et al., , ), tibialis anterior (McNeil, Doherty, Stashuk, & Rice, ; Piasecki et al., ), biceps brachii (Power et al., ) and soleus (Dalton, McNeil, Doherty, & Rice, ), based on comparisons of young and old participants. Although CMAP size invariably decreases with advancing older age, the association between CMAP size and frailty is not simply explained by ageing per se , because it remained significant after adjusting for age.…”

Absence of an aging‐related increase in fiber type grouping in athletes and non‐athletes

“…Nevertheless, the aging-related reductions in axons in nerves 8 and loss of motor neurons 50 do suggest that motor neuron loss does indeed occur. Electromyographic studies also suggest almost invariably that motor unit loss occurs during aging as well as an increase in individual motor unit sizes in both athletes and non-athletes, 10,[19][20][21]23,66,67 the latter suggestive of reinnervation. The question thus arises how such an apparent discrepancy between our data and the overwhelming evidence for motor unit remodeling during aging can be reconciled.…”

“…15,17,18 Some studies reported no aging-related loss of motor units in master athletes 19 or an attenuated loss even in octogenarians, 20,21 while others found that master athletes do suffer similar motor unit loss as seen in non-athletes. 22,23 Several studies interpreted increased motor unit size 10,23 and larger fiber type groups 24,25 as evidence for improved reinnervation in master athletes, but others found smaller motor unit size, suggested to be representative of less collateral reinnervation in master athletes. 26 Thus, the evidence is equivocal as to whether regular physical activity protects against aging-related motor neuron loss and facilitates reinnervation.…”

“…In addressing this objective, we considered the extent to which any observed grouping is explained by the fiber type composition of the muscle so that the chance occurrence of groups can be separated from the possible exercise-mediated fiber type grouping. Based on previous observations on the size of fiber groups 24,25 and the loss of motor units 22,23 in master athletes, we hypothesized that both the fiber group size and number of fiber groups increase with increasing age in master athletes beyond that expected from muscle fiber type composition. As it has been suggested that master athletes may have better reinnervation capacity than non-athletes, 23,24 we hypothesized that the group size is larger in master athletes than non-athletes.…”

Effects of Physiotherapy Interventions on the Function of the Locomotor System in Elder Age: View of Theory and Practice

“…The number of motor units has been larger in young power training group compared to young controls but in elder age neither the power nor the endurance training have impacted this parameter The size of motor units has increased by age without significant effect of power or endurance training. In conclusion, the benefit of regular training may be the improvement in motor unit remodeling [49].…”