The anaerobic threshold (AT) remains a widely recognized, and contentious, concept in exercise physiology and medicine. As conceived by Karlman Wasserman, the AT coalesced the increase of blood lactate concentration ([La- ]), during a progressive exercise test, with an excess pulmonary carbon dioxide output ( ). Its principal tenets were: limiting oxygen (O2 ) delivery to exercising muscle→increased glycolysis, La- and H+ production→decreased muscle and blood pH→with increased H+ buffered by blood [HCO3 - ]→increased CO2 release from blood→increased and pulmonary ventilation. This schema stimulated scientific scrutiny which challenged the fundamental premise that muscle anoxia was requisite for increased muscle and blood [La- ]. It is now recognized that insufficient O2 is not the primary basis for lactataemia. Increased production and utilization of La- represent the response to increased glycolytic flux elicited by increasing work rate, and determine the oxygen uptake ( ) at which La- accumulates in the arterial blood (the lactate threshold; LT). However, the threshold for a sustained non-oxidative contribution to exercise energetics is the critical power, which occurs at a metabolic rate often far above the LT and separates heavy from very heavy/severe-intensity exercise. Lactate is now appreciated as a crucial energy source, major gluconeogenic precursor and signalling molecule but there is no ipso facto evidence for muscle dysoxia or anoxia. Non-invasive estimation of LT using the gas exchange threshold (non-linear increase of versus ) remains important in exercise training and in the clinic, but its conceptual basis should now be understood in light of lactate shuttle biology.
Keywords: CPET; cardiopulmonary exercise test; critical power; critical speed; dysoxia; exercise; gas exchange; gas exchange threshold; gluconeogenesis; glycolysis; hypoxia; isocapnic buffering; lactate; lactate appearance; lactate clearance; lactate disposal; lactate oxidation; lactate signalling; lactate threshold; lactic acid; maximal lactate steady state; oxygen; ventilatory threshold.
© 2020 The Authors. The Journal of Physiology © 2020 The Physiological Society.