Single muscle fibre contractile characteristics with lifelong endurance exercise

A hallmark trait of ageing skeletal muscle health is a reduction in size and function, which is most pronounced in the fast muscle fibres. We studied older men (74 ± 4 years) with a history of lifelong (>50 years) endurance exercise to examine potential benefits for slow and fast muscle fibre size and contractile function. Lifelong endurance exercisers had slow muscle fibres that were larger, stronger, faster and more powerful than young exercisers (25 ± 1 years) and age-matched non-exercisers (75 ± 2 years). Limited benefits with lifelong endurance exercise were noted in the fast muscle fibres. These findings suggest that additional exercise modalities (e.g. resistance exercise) or other therapeutic interventions are needed to target fast muscle fibres with age.

We investigated single muscle fibre size and contractile function among three groups of men: lifelong exercisers (LLE) (n = 21, 74 ± 4 years), old healthy non-exercisers (OH) (n = 10, 75 ± 2 years) and young exercisers (YE) (n = 10, 25 ± 1 years). On average, LLE had exercised ∼5 days week^-1 for ∼7 h week^-1 over the past 53 ± 6 years. LLE were subdivided based on lifelong exercise intensity into performance (LLE-P) (n = 14) and fitness (LLE-F) (n = 7). Muscle biopsies (vastus lateralis) were examined for myosin heavy chain (MHC) slow (MHC I) and fast (MHC IIa) fibre size and function (strength, speed, power). LLE MHC I size (7624 ± 2765 μm² ) was 25-40% larger (P < 0.001) than YE (6106 ± 1710 μm² ) and OH (5476 ± 2467 μm² ). LLE MHC I fibres were ∼20% stronger, ∼10% faster and ∼30% more powerful than YE and OH (P < 0.05). By contrast, LLE MHC IIa size (6466 ± 2659 μm² ) was similar to OH (6237 ± 2525 μm² ; P = 0.854), with both groups ∼20% smaller (P < 0.001) than YE (7860 ± 1930 μm² ). MHC IIa contractile function was variable across groups, with a hierarchical pattern (OH > LLE > YE; P < 0.05) in normalized power among OH (16.7 ± 6.4 W L^-1 ), LLE (13.9 ± 4.5 W L^-1 ) and YE (12.4 ± 3.5 W L^-1 ). The LLE-P and LLE-F had similar single fibre profiles with MHC I power driven by speed (LLE-P) or force (LLE-F), suggesting exercise intensity impacted slow muscle fibre mechanics. These data suggest that lifelong endurance exercise benefited slow muscle fibre size and function. Comparable fast fibre characteristics between LLE and OH, regardless of training intensity, suggest other exercise modes (e.g. resistance training) or myotherapeutics may be necessary to preserve fast muscle fibre size and performance with age.