Balance between O2 availability and VO2 in rest-work transition as measured by myoglobin saturation in subcellular volumes.

Flow and capillary density increased most rapidly in the first 5 s of phasic twitch contraction in dog gracilis. VO2 increased later and more slowly. Consequently, no anoxic loci whatsoever could be found by Mb cryospectroscopy at any time during the transition from rest to heavy work, or during steady twitch contraction at 70% VO2 max. Spatial resolution was on the order of 2-3 mitochondria. Though no O2 debt was incurred, cytosolic redox increased, creatine charge fell, and the [ADP] doubled between 5 and 30 s. These energy and redox signals initiate the increase in VO2 in exercise, and are necessary to achieve appropriately high rates of respiration. Since they follow rather than precede circulatory change they cannot initiate exercise hyperemia in skeletal muscle.