Relationships between mechanical power, O2 consumption, O2 deficit and high-energy phosphates during calf exercise in humans

Whole-body O-2 uptake (VO2), O-2 deficit and the concentration of high-energy phosphates (determined by P-31 spectroscopy) in human calf muscle were measured during moderate aerobic square-wave exercise of increasing intensity in ten volunteers. Net VO2 (above resting) increased linearly with mechanical power, yielding a delta efficiency of 13.1%. Gross O-2 deficit increased linearly with net VO2. The fraction of phosphocreatine (PC) split at steady state increased linearly with the mechanical power and with the O-2 deficit. If the [PC] in resting muscle is known, the slope of the regression between PC split and O-2 deficit (in millimoles) yields the P/O-2 ratio. To calculate this, the O-2 deficit was corrected for the amount of 02 derived from the body stores, as obtained from literature data. The value so obtained, for a resting [PC] of 30 mM was 5.9, consistent with canonical textbook values. Furthermore, the ratio of true O-2 deficit to steady-state VO2 is a measure of the time constant of VO2 kinetics at work onset at the muscle level: assuming a monoexponential time course without time delays it amounted to about 17 s, close to the value that can be expected in mammalian muscle at 37 degreesC.