Effects of old age on human skeletal muscle energetics during fatiguing contractions with and without blood flow

We recently reported lower glycolytic flux (ATP^GLY) and increased reliance on oxidative ATP synthesis (ATP^OX) in contracting muscle of older compared to young humans. To further investigate this age-related difference in the pathways of ATP synthesis, we used magnetic resonance spectroscopy to determine the rates of ATP^OX, ATP^GLY and net phosphocreatine hydrolysis in vivo during maximal muscle contractions under free-flow (FF) and ischaemic (ISC) conditions in the ankle dorsiflexors of 20 young (27 ± 3 years; 10 male, 10 female) and 18 older (70 ± 5 years; 10 male, 8 female) adults. We hypothesized that ATP GLY would be higher in young compared to old during FF contractions, but that old would be unable to increase ATP^GLY during ISC to match that of the young, which would suggest impaired glycolytic ATP synthesis with old age. Peak glycolytic flux during FF was lower in older (0.8 ± 0.1 mM ATP s^-1) compared to young (1.4 ± 0.1 mM ATP s^-1, P < 0.001) subjects. During ISC, peak ATP^GLY increased in old to a level similar to that of young (1.4 ± 0.2 mM ATP s^-1, 1.3 ± 0.2 mM ATP s^-1, respectively; P = 0.86), suggesting that glycolytic function remains intact in aged muscle in vivo. Notably, older adults fatigued less than young during both FF and ISC (P ≤ 0.004). These results provide novel evidence of unimpaired in vivo glycolytic function in the skeletal muscle of older adults during maximal isometric dorsiflexion, and suggest a potential role for differences in metabolic economy and as a result, metabolite accumulation, in the fatigue resistance of the old.