Magnetic Resonance Quantification of Muscle Phosphocreatine Resynthesis Kinetics During Exercise Recovery: An In Vivo Measure of Mitochondrial Function in Humans

Quantifying muscle phosphocreatine (PCr) resynthesis kinetics during recovery from exercise can be employed as a sensitive and noninvasive in vivo measure of mitochondrial function, or more specifically mitochondrial oxidative capacity, in humans. Here we describe a protocol employing nonlocalized 31P MRS acquisition in the medial gastrocnemius muscle, which, when combined with mDIXON 1H MRI scanning of muscle volume and fat fractions, ensures anatomical localization for 31P MRS acquisition. The protocol involves the performance of in-bore plantar flexor exercise at a predetermined and controlled exercise intensity (50% maximal isometric force) with limb blood flow occluded, thereby ensuring that end-exercise muscle PCr, ADP, and cytosolic pH are well matched across volunteers at the point of end-exercise reinstatement of limb blood flow. This will ensure maximal stimulation of mitochondrial respiration and PCr resynthesis during subsequent resting, nonischemic recovery. Consideration is given to the view that contraction-induced muscle acidosis complicates the interpretation of 31P-derived PCr resynthesis data during exercise recovery and how this can be circumvented.