In support of the continued use of the term anaerobic threshold

'Anaerobic threshold' is a commonly used term, but its meaning seems to depend as much on the user as on any convention. The recent review by Poole and colleagues (2021) in The Journal of Physiology provides an interesting and timely discourse on the fascinating history of lactate and the anaerobic threshold concept, including some very important ideas regarding the lactate shuttle. However, the reader is left confused about what is actually the anaerobic threshold. In fact, these authors propose that we dispose of the term altogether! We believe the term is useful, but there needs to be consensus on the meaning. We would like to suggest that the appropriate meaning of the term 'anaerobic threshold' is the highest intensity that can be sustained exclusively by aerobic metabolism, as suggested previously (Svedahl & MacIntosh, 2003). Above this intensity there is a sustained contribution of energy from glycolysis, leading to accumulation of lactate for as long as the exercise is continued. This intensity is clearly a threshold, according to the definition found in the Oxford English Dictionary and endorsed by Poole and colleagues (2021): 'the magnitude or intensity that must be exceeded for a certain reaction, phenomenon, result, or condition to be manifested'. The primary concern seems to be how we use the term 'anaerobic'. Historically, it has been thought that lactate production is evidence for an inadequate supply of oxygen. However, the review under consideration (Poole et al. 2021) makes it clear that lactate production commonly proceeds in muscle while adequate oxygen is available. Can we then continue to use 'anaerobic' to refer to metabolic production of ATP without the use of oxygen? We conclude that this is necessary if we want to be able to interpret the volumes of literature referring to anaerobic glycolysis in living muscle. Hence, we believe the anaerobic threshold is an important concept and should be held onto. In the review (Poole et al. 2021), it is pointed out that D. B. Dill thought it was possible that Wasserman and McIlroy (1964) had identified the '… threshold of anaerobic metabolism'. If we consider the continuous accumulation of lactate during sustained exercise at a constant intensity to be the phenomenon or condition that manifests, then the anaerobic threshold should be the highest intensity of exercise which can be sustained by aerobic metabolism. Above this intensity, exercise can only be sustained with continued accumulation of lactate. This was apparently the intent when the term was first introduced, as uncovered by Poole and colleagues. Wasserman and McIlroy (1964) identified an intensity of exercise above which ventilation increased disproportionately to oxygen uptake while CO2 excretion was still in proportion to the ventilation. They surmised that metabolic acidosis due to increasing lactate production was stimulating the ventilation. However, as pointed out by Poole and colleagues (2021), increased blood lactate does not necessarily mean increased anaerobic metabolism. As long as the rate of formation of lactate from active muscle is balanced by oxidation of lactate in the same or other tissues, then lactate will not accumulate and there is no net anaerobic metabolism. Having identified this discrepancy, would it not seem logical to find a threshold above which there is sustained anaerobic contribution to the energy requirement of the exercise and recommend using the term 'anaerobic threshold' to refer to that intensity? There are apparently at least two important thresholds. The lower threshold, corresponding with what Wasserman and McIlroy (1964) called the anaerobic threshold, is now recognized as ventilatory threshold 1 or the gas exchange threshold. Sometimes, this threshold is identified as the intensity during an incremental test when blood lactate is first measurably above resting values, called the lactate threshold. Detection of this intensity of exercise may still have clinical relevance (Myers & Ashley, 1997), but there is no justification for calling this the anaerobic threshold. We recognize that it is difficult to identify the precise exercise intensity that is the highest that can be sustained with steady state oxygen uptake and no accumulation of lactate. However, there are several techniques that are recognized to identify an intensity of exercise which is not much different from this threshold. This includes the ventilatory threshold 2 or respiratory compensation point. This intensity is also sometimes identified during an incremental test by a substantial increase in blood lactate, another lactate threshold. Furthermore, constant intensity tests have been used to identify a threshold that may be synonymous with the anaerobic threshold as we have defined it. For example, critical speed or power (Dekerle et al. 2003) and the maximal lactate steady state (Haverty et al. 1988; MacIntosh et al. 2002) have considerable support for being valued proxies for the threshold of anaerobic metabolism. It seems far more appropriate to use the term 'anaerobic threshold' in the way it was intended – to refer to the threshold for sustained anaerobic contribution to exercise – than to continue using the term for an intensity that is well below that critical threshold. None. All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. None.

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