Same training, one hot bath added after sessions, and VO2max climbed about 4.4% in 10 well-trained runners. Jenkins et al. (2025) ran a within-subject crossover, N=10 well-trained runners (9 men, 1 woman). Training volume and intensity stayed exactly the same in both conditions. The only addition was a passive hot bath: water at or above 40C, 45 min, 5x per week, for 5 weeks. Over the block, VO2max rose about 4.4%, roughly 2.7 mL/kg/min. Two engines moved together. Haemoglobin mass, the oxygen-carrying capacity of the blood, rose +33 g (3.9%), and blood volume climbed +284 mL by week 5. On the cardiac side, the volume the heart chamber holds before each beat (LVEDV) rose +10 mL and stroke volume rose +7 mL. In a regression inside the trial, haemoglobin mass and chamber fill were the two strongest predictors, together tracking about 81% of the VO2max variation. That is an association inside the trial, not proof one adaptation alone caused the gain. One thing did not move: running economy, the oxygen cost at a fixed pace, was unchanged (P=0.473). Heat lifted the ceiling, not the efficiency. Speed at VO2max went up +0.8 km/h, but the fuel-per-mile stayed the same. The authors note the blood gain here looks comparable to estimates from live-high-train-low altitude work in prior studies (around 3.3% haemoglobin mass from one meta-analysis). There was no altitude group here, so read it as comparable on paper, not a head-to-head test. The practical edge is that a bath is cheap and local. Scope it honestly: N=10, well-trained, predominantly male, 5 weeks. Worth testing as an add-on to training, not yet a prescription for everyone. Thanks to Jenkins and colleagues for this work. Jenkins et al. (2025), The Journal of Physiology. doi.org/10.1113/JP289874 Follow for more research breakdowns.