Myonuclear content and domain size in small versus larger muscle fibres in response to 12 weeks of resistance exercise training in older adults

Aim: To assess the relation between muscle fibre hypertrophy and myonuclear accretion in relatively small and large muscle fibre size clusters following prolonged resistance exercise training in older adults. Methods: Muscle biopsies were collected before and after 12 weeks of resistance exercise training in 40 healthy, older men (70 ± 3 years). All muscle fibres were ordered by size and categorized in four muscle fibre size clusters: ‘Small’: 2000-3999 µm ², ‘Moderate’: 4000-5999 µm ², ‘Large’: 6000-7999 µm ² and ‘Largest’: 8000-9999 µm ². Changes in muscle fibre size cluster distribution were related to changes in muscle fibre size, myonuclear content and myonuclear domain size. Results: With training, the percentage of muscle fibres decreased in the Small (from 23 ± 12 to 17 ± 14%, P <.01) and increased in the Largest (from 11 ± 8 to 15 ± 10%, P <.01) muscle fibre size clusters. The decline in the percentage of Small muscle fibres was accompanied by an increase in overall myonuclear domain size (r = −.466, P =.002) and myonuclear content (r = −.390, P =.013). In contrast, the increase in the percentage of the Largest muscle fibres was accompanied by an overall increase in myonuclear content (r =.616, P <.001), but not in domain size. Conclusion: Prolonged resistance-type exercise training induces a decline in the percentage of small as well as an increase in the percentage of the largest muscle fibres in older adults. Whereas the change in the percentage of small fibres is best predicted by an increase in overall myonuclear domain size, the change in the percentage of the largest fibres is associated with an overall increase in myonuclear content.