Evidence Supporting the Existence of an Exercise Anaerobic Threshold
Since 5 liters of blood with normal arterial oxygen (O2) concentration contains only 1 liter of O2, the increase in muscle blood flow to support an O2 requirement of 1 L/min, aerobically, must be greater than 5 L/min. The slope for the increase in cardiac output for an increase in oxygen uptake (V̇O2) is normally approximately 6 (Faulkner et al., 1977; Yamaguchi et al., 1986), suggesting that approximately 18% of the O2 delivered to the exercising muscles remains at the venous end of the muscle capillary. This would result in an O2 tension of approximately 15 mmHg. This is consistent with the 8 mmHg O2 tension in exercising muscle below which the muscle lactate/pyruvate ratio was observed to abruptly increase by Bylund-Fellenius et al (1981). To maintain total aerobic metabolism, there must be little inhomogeneity in the muscle blood flow/O2 consumption ratio (Qm/V̇O2m) since a ratio of 5.5 or less would obligate anaerobic metabolism (Figure 1). After maximal hyperemia, the Qm/V̇O2m ratio must increase to obtain an increased capillary to mitochondrial PO2 difference (P(c-m)O2) necessary to achieve the mass flow of O2 to avoid anaerobiosis for increasing work rate. An additional mechanism regulating the decrease in capillary PO2 is the shift to the right of the oxyhemoglobin dissociation curve accompanying the local acidosis of anaerobic metabolism.
KeywordsLactic Acidosis Work Rate Anaerobic Threshold Incremental Exercise Test COHb Level
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