the article linked to from TFA, https://link.springer.com/article/10.1007/s40279-017-0811-2?app=true https://link.springer.com/article/10.1007/s40279-017-0811-2

the article linked to from TFA, https://link.springer.com/article/10.1007/s40279-017-0811-2?app=true https://link.springer.com/article/10.1007/s40279-017-0811-2?... (2017), says: > We measured submaximal oxygen uptake and carbon dioxide production during minutes 3–5 and averaged energetic cost (W/kg) for the two trials in each shoe model. normally respiration gives very good results, better than other proxies such as accelerometry or heart rate measurement, but TIL that there's an even better method: doubly labeled water (DLW) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5426207/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5426207/ (2017) > ...after the subject ingests a dose of ²H2¹⁸O, there is an equilibration of the 2 isotopes with total body water (TBW) followed by their elimination from the body, which occurs at different rates. Deuterium (²H) is lost from the body via only water (H2O) while ¹⁸O is lost both via water and carbon dioxide (CO2). The rate of CO2 production (rCO2) is calculated as the difference between the elimination rates of ²H and ¹⁸O... the advantage is not needing to be masked during activity; the disadvantage is dealing with isotopes