Skeletal muscle energetics during submaximal contractions are linked to metabolic performance during ambulatory...

Skeletal muscle's capacity for oxidative energy production can be measured in vivo by phosphocreatine (PCr) recovery following maximal contractions inside a magnetic resonance scanner. However, muscle energetic characteristics during submaximal contractions of similar intensity as used in free-living activities may be more relevant to the energetic support of ambulatory tasks during daily life. We measured vastus lateralis muscle oxidative capacity, acidification, submaximal oxidative energy production, and acetylcarnitine accumulation in response to an incremental contraction protocol (6%-15% maximal torque). We then evaluated the relationships between these metrics and whole-body metabolic responses to a 30-min treadmill walk (30MTW) and a peak oxygen consumption (VO_2peak) test using Spearman rank correlations (r_s, n = 7 males, 28 ± 4 years). Muscle oxidative capacity was not related to any metric of whole-body metabolism, but submaximal PCr recovery was associated with slower VO₂ off-kinetics following the 30MTW (r_s = 0.96, p = 0.003). Muscle [acetylcarnitine] was associated with respiratory exchange ratio (RER) during the 30MTW and VO_2peak test (r_s = 0.81, p = 0.035 and r_s = 0.86, p = 0.024). Muscle acidification was associated with RER only at VO_2peak (and r_s = ^-0.89, p = 0.012). These preliminary results provide novel connections between muscle and whole-body energetics and suggest that submaximal muscle energetics may be more relevant to free-living tasks than oxidative capacity.