In Vivo Muscle Bioenergetics Predict Whole-body Metabolic Responses During A 30-minute Treadmill Walk

Abstract

While the bioenergetics of working skeletal muscle have been well-studied in humans, the impact of muscle energetic characteristics on the performance of everyday mobility tasks is less clear. PURPOSE: To evaluate the relationships between muscle oxidative and non-oxidative bioenergetic variables, an indicator of muscle substrate use (acetylcarnitine), and whole-body energy metabolism during a standardized mobility activity. METHODS: After obtaining written consent, intramyocellular energetics were measured in vivo in the vastus lateralis of 7 males (mean 28.4 yr, range 25-45) using proton and phosphorus magnetic resonance spectroscopy in response to 2 knee extension protocols: 1) 24 s of maximal isokinetic contractions (0.5 Hz), and 2) 8 min of incremental isotonic contractions (0.5 Hz, 2-min stages at 6, 9, 12, and 15% maximal torque). Continuous measures of ATP production via oxidative phosphorylation, glycolysis and the creatine kinase reaction (CK), and changes in [acetylcarnitine] from pre- to post-contractions, were made. Whole-body metabolic responses to a 30-min treadmill walk (30MTW) at 1.3 m∙s-1 were measured using indirect calorimetry by metabolic cart, from which the rate constant for oxygen (O2) deficit and excess post-exercise oxygen consumption (EPOC), and mean steady state respiratory exchange ratio (RER) were calculated. Spearman rank correlation coefficients (rs) were calculated to evaluate associations between muscle and whole-body metrics. RESULTS: The O2 deficit from the 30MTW was correlated with muscle non-oxidative ATP production by CK (rs = 0.82, p = 0.03) and glycolysis (rs = 0.79, p = 0.05) during the first 2 min of the incremental protocol, as well as with the decrease in [ATP] at the end of the protocol (rs = 0.90, p = 0.01). Muscle [acetylcarnitine] following the incremental protocol was correlated with RER during the 30MTW (rs = 0.81, p = 0.03), and EPOC was related to both non-oxidative ATP production during the incremental protocol (rs = 0.86, p = 0.02), and the rate of phosphocreatine recovery (reflecting oxidative flux) following this protocol (rs = -0.96, p = 0.003). CONCLUSIONS: The results provide novel evidence linking in vivo muscle oxidative and non-oxidative energetics to whole-body metabolic responses during a mobility task relevant to everyday activities.