A ‘fatigue threshold’ during incremental exercise was identified (and then forgotten) 100 years ago

A work rate corresponding to the maximal O2 uptake () can be sustained for only ∼6–8 minutes. To continue the exercise for longer periods of time, the work rate must be submaximal, corresponding to a fraction of . This fraction may vary depending on training, health status, age and environmental conditions, etc. The identification of this submaximal ‘threshold’, a key factor in the evaluation of exercise tolerance, has been the object of thousands of papers over the last 50 years, and has led to the development of concepts such as the ‘anaerobic’, ‘lactate’, ‘ventilatory’, ‘gas exchange threshold’ (Wasserman & Whipp, 1975) and ‘critical power’ (Monod & Scherrer, 1965); for reviews, see Jones et al. (2010) and Craig et al. (2019). Within this scenario, it is striking that a paper describing a method to discriminate between fatiguing and non-fatiguing work rates during an incremental test was first published 100 years ago in the Journal of Physiology (Briggs, 1920). The author of that paper was Henry Briggs (1883–1935), a Professor of Mining at the Heriot-Watt College of Edinburgh. Briggs’ paper reports beautifully presented data and heralds an elegant and intriguing discussion which introduced, several decades before others, concepts that can be considered fully valid also today. Surprisingly, the study has been cited (Web of Science) only 45 times throughout the 100-year period subsequent to its publication. Moreover, the majority of these citations mainly related to other aspects of the paper, such as the impact of breathing O2-enriched air on physical capacity, or the economy of walking. Briggs’ paper clearly documented a phenomenon which the author defined the ‘crest load’, corresponding to what is known today as the ‘respiratory compensation point’ (Wasserman & Whipp, 1975) detected during an incremental exercise test. In his paper, Briggs concluded: ‘When exertion of steadily increasing magnitude is undertaken, then expired-CO2-percentage first rises and then falls. The load at which the percentage is a maximum is called the ‘crest load’. It is shown that the crest load demarcates between normal loads and overloads.’ According to Briggs’ definitions ‘normal loads’ can be carried out for prolonged periods of time, whereas ‘overloads’ would be associated with fatigue and lead to exhaustion. A typical example of detection of the crest load is presented in Figure 1 (taken from the original paper by Briggs), which shows the CO2 fraction in expired air (, Douglas bags method) during cycling exercise at increasing power outputs; in the example reached a maximum at a submaximal work rate of ∼8000 foot-pounds per minute (∼180 W). maximum was immediately followed by a disproportionate increase of minute ventilation (), a phenomenon which now demarcates the respiratory compensation point (Wasserman & Whipp, 1975). In other words, Briggs’ crest load was identified as the power output at which reached a maximum, followed by a decrease, as today's respiratory compensation point is indicated by the first decrease in the end-tidal CO2 partial pressure () determined by breath-by-breath measurements (Wasserman & Whipp, 1975). Both ‘thresholds’ share a common cause, that is the increased , aimed at correcting the metabolic acidosis deriving from the accumulation or lactate associated protons (another concept elegantly discussed in Briggs’ paper), by decreasing the denominator of the Henderson–Hasselbalch equation. Thus, Briggs’ crest load would not correspond to the ‘gas exchange threshold’, in which the ‘extra CO2’ eliminated by the lungs mainly derives from the buffering of H+ by bicarbonate, in the presence of a ‘non-falling’ (Wasserman & Whipp, 1975). ‘This makes Briggs' (1920) discovery even more modern because it identifies the point of respiratory compensation, and likens it to what we would now call critical power (Craig et al. 2019): Brigg's ‘crest point’ is proposed as a measure of ‘stamina’, below which ‘… the subject can support such rates of exertion for a considerable time’. Furthermore, Briggs (1920) demonstrated for the first time that breathing a high fraction of inspired oxygen moved the ’crest load" to higher work rates (and presumably also increased the critical power), as well as enhanced the power output at fatigue. In his paper Briggs wrote: ‘In every case but one (Subject VIII) the crest load was higher (i.e. the crest was further to the right) when breathing oxygen than when breathing air … . The lower the person's fitness the greater the improvement brought about.’ Interestingly, the impact of varied fractions of O2 in the inspired air (inspiratory hypoxia / hyperoxia) on the critical power in humans is still a matter of investigation today (see Craig et al. 2019)’. Measurements of gas exchange variables in the study by Briggs were conducted in collaboration with Professor John S. Haldane, the inventor of the Haldane gas-exchange apparatus. Analysis of the raw data of and power output during submaximal cycling (see Table II of the paper) shows that the mean value of the gain of /power output for the three work rates was 10.4 ml O2 W−1, with a slope of the vs. work rate power output relationship of 8.9 ml O2 W−1, which would also be considered as reference values today (Wasserman & Whipp 1975). Although the concept of (Hill & Lupton 1923) was still to come, the paper by Briggs (1920) identified, 100 years, ago an elegant method for discriminating, during an incremental exercise, a threshold between non-fatiguing and fatiguing submaximal work rates, a topic that is at the core of research in exercise tolerance also today, in healthy subjects, sport activities and diseased populations (Jones et al. 2010; Craig et al. 2019). Although truly ‘revolutionary’, Briggs's discovery went substantially un-noticed for 100 years, and we hope that our brief note will bring to the paper some of the attention and recognition it clearly deserves. No competing interests declared. JAZ was responsible for the conception and design of the work, the analysis and interpretation of data for the manuscript, and the drafting of the manuscript. BG was responsible for interpretation of the data for the work and for revising the manuscript critically for important intellectual content. Both authors approved the final version of manuscript submitted for publication and agree to be accountable for all aspects of the work. No funding was received.