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.
Highlights
Voluntary movements are controlled through precise activation of motor unit (MU) populations
Whilst the agerelated reductions in neuromuscular control are multifactorial including alterations in MU firing rate (Dideriksen et al, 2012), proprioception and afferent feedback (Baudry, 2016), the lower MU numbers observed in older age directly coincides with loss of muscle fibers (Lexell et al, 1988; McPhee et al, 2018) and contributes to sarcopenia defined by low muscle mass and functional impairments
We have shown that the loss of MUs occurs relatively early into older age and precedes sarcopenia (Piasecki et al, 2016c) because not all of the muscle fibers of affected MUs are lost; some may be “rescued” by axonal sprouting of adjacent surviving neurons, thereby helping to preserve total muscle mass
Summary
Voluntary movements are controlled through precise activation of motor unit (MU) populations. A single MU is composed of an alpha motor neuron with its cell body in the ventral horn of the spinal cord, an axon, and all of the muscle fibers innervated by that single motor neuron. Estimates made from cadaveric specimens showed around 30% fewer motor neuron cell bodies in the lumbo-sacral spinal cord of older compared with younger adults (Kawamura et al, 1977; Tomlinson and Irving, 1977; Mittal and Logmani, 1987). Whilst the agerelated reductions in neuromuscular control are multifactorial including alterations in MU firing rate (Dideriksen et al, 2012), proprioception and afferent feedback (Baudry, 2016), the lower MU numbers observed in older age directly coincides with loss of muscle fibers (Lexell et al, 1988; McPhee et al, 2018) and contributes to sarcopenia defined by low muscle mass and functional impairments
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