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

Abstract

The ageing‐related loss of muscle mass is thought to be partly attributable to motor neuron loss and motor unit remodelling that result in fibre type grouping. We examined fibre type grouping in 19‐ to 85‐year‐old athletes and non‐athletes and evaluated to which extent any observed grouping is explained by the fibre type composition of the muscle. Since regular physical activity may stimulate reinnervation, we hypothesised that fibre groups are larger in master athletes than in age‐matched non‐athletes. Fibre type grouping was assessed in m. vastus lateralis biopsies from 22 young (19‐27 years) and 35 healthy older (66‐82 years) non‐athletes, and 14 young (20‐29 years), 51 middle‐aged (38‐65 years) and 31 older (66‐85 years) athletes. An ‘enclosed fibre’ was any muscle fibre of a particular type surrounded by fibres of the same type only. A fibre type group was defined as a group of fibres with at least one enclosed fibre. Only type II fibre cross‐sectional area (FCSA) showed an age‐related decline that was greater in athletes (p < 0.001) than in non‐athletes (p = 0.012). There was no significant age‐related effect on fibre group size or fibre group number in athletes or non‐athletes, and the observed grouping was similar to that expected from the fibre type composition. At face value these observations do 1) neither show evidence for an age‐related loss and remodelling of motor units nor 2) improved reinnervation with regular physical activity, but 3) histological examination may not reveal the full extent of ageing‐related motor unit remodelling.