The assumption that cellular oxygen pressure (Po2) is close to zero in maximally exercising muscle is essential for the hypothesis that 02 transport between blood and mitochondria has a finite conductance that determines maximum 02 consumption.

Near-infrared (NIR) spectroscopy is a noninvasive technique that uses the differential absorption properties of hemoglobin to evaluate skeletal muscle oxygenation. Oxygenated and deoxygenated hemoglobin absorb light equally at 800 nm, whereas at 760 nm absorption is primarily from deoxygenated hemoglobin. Therefore, monitoring these two wavelengths provides an index of deoxygenation. To investigate whether venous oxygen saturation and absorption between 760 and 800 nm (760-800 nm absorption) are correlated, both were measured during forearm exercise. Significant correlations were observed in all subjects (r = 0.92 +/- 0.07; P < 0.05). The contribution of skin flow to the changes in 760-800 nm absorption was investigated by simultaneous measurement of skin flow by laser flow Doppler and NIR recordings during hot water immersion. Changes in skin flow but not 760-800 nm absorption were noted. Intra-arterial infusions of nitroprusside and norepinephrine were performed to study the effect of alteration of muscle perfusion on 760-800 nm absorption. Limb flow was measured with venous plethysmography. Percent oxygenation increased with nitroprusside and decreased with norepinephrine. Finally, the contribution of myoglobin to the 760-800 nm absorption was assessed by using 1H-magnetic resonance spectroscopy. At peak exercise, percent NIR deoxygenation during exercise was 80 +/- 7%, but only one subject exhibited a small deoxygenated myoglobin signal. In conclusion, 760-800 nm absorption is 1) closely correlated with venous oxygen saturation, 2) minimally affected by skin blood flow, 3) altered by changes in limb perfusion, and 4) primarily derived from deoxygenated hemoglobin and not myoglobin.

During conventional cycle ergometry, as work rate (WR) is increased toward maximum, O2 extraction increases hyperbolically, typically achieving values of 80-90% at peak O2 uptake (VO2). In contrast, studies using isolated knee-extensor exercise report much higher mass-specific blood flows (Q) and lower maximal O2 extractions (approximately 70%), which have been interpreted as transit time limitation to O2 movement out of the muscle capillary. However, maximal achievable WR levels during conventional cycle ergometry are generally reached (over 10-15 min) after rapid increases in WR, whereas the reported knee-extensor studies have used only more lengthy protocols (45 min). The duration of these protocols may have prevented the attainment of high WR levels and thus high O2 extraction ratios. Accordingly, this investigation examined leg Q and O2 extraction responses during single-leg knee-extensor exercise incremented rapidly (steps of 15-25 W per 2- to 3-min interval), which produced fatigue in 13-15 min. Q and muscle VO2 increased linearly with WR to fatigue with Q-WR and VO2-WR slopes similar to those reported in previous knee-extensor studies. However, with the use of this protocol, very high maximal achievable WR [99 +/- 6 (SE) W] and muscle Q (385 +/- 26 ml.min-1 x 100 g-1) levels were attained, some 80% greater than previously reported. An O2 extraction of 84.6 +/- 2.1% was reached, giving a maximal VO2 of 60.2 +/- 5.8 ml.min-1 x 100 g-1. We conclude that, even under the high Q conditions of single-leg knee-extensor exercise, O2 extraction does not reach a plateau on the basis of short transit times and that previous conclusions to the contrary reflect failure to attain sufficiently high WR levels. Maximal VO2, Q, and O2 extraction in this model have yet to be defined.

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.

Abnormal skeletal muscle metabolism in patients with heart failure usually occurs in the absence of myoglobin deoxygenation, suggesting that the abnormalities are not a result of cellular hypoxia during exercise with minimal cardiovascular stress.

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A non-invasive, near-infrared tissue spectrophotometer has been previously described.3 The device utilizes the differential absorption characteristics of oxy-hemoglobin (Hb02) and deoxy-hemoglobin (Hb) at 760 and 850 nm to indicate localized changes in tissue oxygenation. The present study was undertaken to examine the relative magnitude of vastus lateralis deoxygenation in highly-trained cyclists at maximal effort. A ramp exercise to exhaustion was immediately followed by a cuff ischemia to elicit a maximal Hb signal. All of the subjects exhibited further deoxygenation upon cuff ischemia; determined to be 63%, 69%, 72%, and 56% of maximal Hb signal. This suggests that there was incomplete depletion of oxygen supply in the vastus lateralis muscles of these subjects at exhaustion.