In vivo oxidative capacity varies with muscle and training status in young adults

It is well established that exercise training results in increased muscle oxidative capacity. Less is known about how oxidative capacities in distinct muscles, in the same individual, are affected by different levels of physical activity. We hypothesized that 1) trained individuals would have higher oxidative capacity than untrained individuals in both tibialis anterior (TA) and vastus lateralis (VL) and 2) oxidative capacity would be higher in TA than VL in untrained, but not in trained, individuals. Phosphorus magnetic resonance spectroscopy was used to measure the rate of phosphocreatine recovery (k _PCr), which reflects the rate of oxidative phosphorylation, following a maximal voluntary isometric contraction of the TA and VL in healthy untrained (7 women, 7 men, 25.7 ± 3.6 yr; mean ± SD) and trained (5 women, 7 men, 27.5 ± 3.4 yr) adults. Daily physical activity levels were measured using accelerometry. The trained group spent threefold more time (∼90 vs. ∼30 min/day; P < 0.001) in moderate to vigorous physical activity (MVPA). Overall, k_PCr was higher in VL than in TA (P = 0.01) and higher in trained than in untrained participants (P < 0.001). The relationship between k_PCr and MVPA was more robust in VL (r = 0.64, P = 0.001, n = 25) than in TA (r = 0.38, P = 0.06, n = 25). These results indicate greater oxidative capacity in vivo in trained compared with untrained individuals in two distinct muscles of the lower limb and provide novel evidence of higher oxidative capacity in VL compared with TA in young humans, irrespective of training status. The basis for this difference is not known at this time but likely reflects a difference in usage patterns between the muscles.