Validity and accuracy of calculating oxidative ATP synthesis in vivo during high‐intensity skeletal muscle contractions

Several methods have been developed for using 31P‐MRS to calculate rates of oxidative ATP synthesis (ATPOX) during muscular contractions based on assumptions that (1) the ATP cost of force generation (ATPCOST) remains constant or (2) Michaelis‐Menten coupling between cytosolic ADP and ATPOX does not change. However, growing evidence suggests that one, or both, of these assumptions are invalid during high‐intensity fatigue protocols. Consequently, there is a need to examine the validity and accuracy of traditional ATPOX calculation methods under these conditions. To address this gap, we measured phosphate concentrations and pH in the vastus lateralis muscle of nine young adults during four rest‐contraction‐recovery trials lasting 24, 60, 120, and 240 s. The initial velocity of phosphocreatine resynthesis (ViPCr) following each trial served as the criterion measure of ATPOX because this method makes no assumptions of constant ATPCOST or Michaelis‐Menten coupling between changes in cytosolic ADP and ATPOX. Subsequently, we calculated ATPOX throughout the 240 s trial using several traditional calculation methods and compared estimations of ATPOX from each method with time‐matched measurements of ViPCr. Method 1, which assumes that ATPCOST does not change, was able to model changes in ViPCr over time, but showed poor accuracy for predicting ViPCr across a wide range of ATPOX values. In contrast, Michaelis‐Menten methods, which assume that the relationship between changes in cytosolic ADP and ATPOX remains constant, were invalid because they could not model the decline in ViPCr. However, adjusting these Michaelis‐Menten methods for observed changes in maximal ATPOX capacity (i.e., Vmax) permitted modeling of the decline in ViPCr and markedly improved accuracy. The results of these comprehensive analyses demonstrate that valid, accurate measurements of ATPOX can be obtained during high‐intensity contractions by adjusting Michaelis‐Menten ATPOX calculations for changes in Vmax observed from baseline to post‐fatigue. Rates of oxidative ATP synthesis (ATPOX) were calculated continuously throughout 4 min of high‐intensity knee extensions using several traditional calculation methods, and validated at multiple timepoints against a gold standard measure of ATPOX, the initial velocity of phosphocreatine resynthesis (ViPCr). Notably, none of the traditional calculation methods (open circles) accurately estimated ViPCr (red bars) throughout the protocol. However, when the