Simultaneous in vivo measurements of HbO₂ saturation and PCr kinetics after exercise in normal humans

Simultaneous measurements of phosphocreatine (PCr) and oxyhemoglobin (HbO₂) saturation were made during recovery from exercise in calf muscles of five male subjects. PCr was measured using magnetic resonance spectroscopy in a 2.0-T 78-cm-bore magnet with a 9-cm-diam surface coil. Relative HbO₂ saturation was measured as the difference in absorption of 750- and 850-nm light with use of near-infrared spectroscopy. The light source and detectors were 3 cm apart. Exercise consisted of isokinetic plantar flexion in a supine position. Two 5-min submaximal protocols were performed with PCr depletion to 60% of resting values and with pH values of >7.0. Then two 1-min protocols of rapid plantar flexion were performed to deplete PCr values to 5-20% of resting values with pH values of <6.8. Areas of PCr peaks (every 8 s) and HbO₂ saturation (every 1 s) were fit to a monoexponential function, and a time constant was calculated. The PCr time constant was larger after maximal exercise (68.3 ± 10.5 s) than after submaximal exercise (36.0 ± 6.5 s), which is consistent with the effects of low pH on PCr recovery. HbO₂ resaturation approximated submaximal PCr recovery and was not different between maximal (29.4 ± 5.5 s) and submaximal (27.6 ± 6.0 s) exercise. We conclude that magnetic resonance spectroscopy measurements of PCr recovery and near-infrared spectroscopy measurements of recovery of HbO₂ saturation provide similar information as long as muscle pH remains near 7.0.