Age and sex differences in muscle ATP production in vivo during maximal and submaximal contractions

The purpose of this study was to examine age and sex differences in flux through the ATP‐producing pathways during contractions at various force levels. Twelve young (6 men, 23±1 yr, mean±SE) and 13 older (8 men, 72±2) adults performed 12‐s ankle dorsiflexion contractions at 20, 50 and 100% of maximal voluntary contraction (MVC) torque. Phosphorous magnetic resonance spectroscopy was used to quantify muscle oxidative capacity and ATP produced by the creatine kinase reaction (CK), glycolysis (GLY) and oxidative phosphorylation (OxPhos) in vivo . ATP flux is expressed relative to total ATP cost. Oxidative capacity was similar in young (Y) and older (O) men (M) and women (W) (YM=1.3±0.12 mM ATP·s −1 ; YW=1.4±0.10; p=0.32; OM=1.4±0.15; OW=1.6±0.11). At 20% MVC, relative ATP production was higher by CK and lower by OxPhos in O than Y (p<0.04). Also at 20% MVC, flux through GLY was higher in M than in W (p=0.03). At 50% MVC, flux through the 3 pathways was similar across groups (p>0.2). The relative proportion of ATP produced by GLY at 100% MVC tended to be higher in YM than any other group (p<0.07), while ATP produced through OxPhos was higher in W than in M (p=0.05). Despite similar in vivo oxidative capacities, the relative contribution of each energetic pathway to ATP production varied by age and sex across the contraction intensities. NIH R01 AG21094