Evidence Supporting the Existence of an Exercise Anaerobic Threshold

Abstract

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.