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5 min readrunning / endurance

8% more work in the heat: a 10-day acclimation protocol

Twelve trained cyclists rode 90 minutes a day at 40°C for 10 days straight. Every session was easy, around 50% of VO2max. Their hour time-trial work rose 8% in the heat and 6% in a cool 13°C lab, and their plasma volume expanded 6.5% 1. I run about 5,000 km a year and race up to 100 km, and heat costs me more pace than any climb on the course.

What the research shows.

Lorenzo and colleagues tested their cyclists in both environments, before and after the block. VO2max climbed 5% in the cool, from 66.8 to 70.2 ml/kg/min, and 8% in the hot condition, from 55.1 to 59.6. Work completed in a 1-hour time trial went from 879.8 to 934.7 kJ at 13°C and from 718.7 to 776.2 kJ at 38°C. Power at lactate threshold gained 5% in both conditions, and plasma volume rose 6.5 ± 1.5% 1.

The pooled numbers line up. Tyler and colleagues meta-analyzed the heat adaptation literature and found exercising heart rate down 16 ± 6 bpm, exercising core temperature down 0.32 ± 0.24°C, whole-body sweat rate up 19 ± 21%, plasma volume up 4.3 ± 4.7%, and sweating starting at a core temperature 0.28 ± 0.21°C lower. Exercise capacity in the heat improved 23% on average, and fixed performance tests improved 7% 2.

Adaptation inside the block arrives on two different clocks. Up to 80% of it lands in the first 4 to 7 days, led by heart rate and plasma volume. Sweat rate trails behind: short protocols moved it 5 ± 11%, while protocols of 7 to 14 days moved it 29 ± 29% 2. A 5-day block buys most of the cardiovascular benefit and little of the sweating benefit.

Benjamin and colleagues pooled 35 studies and ranked outcomes by effect size: time to exhaustion 0.86 (95% CI 0.71 to 1.01), time trial 0.49 (0.26 to 0.71), VO2max 0.30 (0.07 to 0.53), peak power 0.29 (0.09 to 0.48) 3. Hold-on-as-long-as-you-can tests flatter heat training. Your race sits closer to the time-trial figure, so plan around half an effect size.

Two low-cost routes have their own data. Six competitive runners sat in a 89.9°C sauna for 31 minutes after training, about 13 times across 3 weeks. Run time to exhaustion at 5 km pace rose 32% (90% confidence limits 21 to 43%), worth roughly 1.9% in a time trial, and plasma volume rose 7.1%. Individual performance change tracked plasma volume change at r = 0.96 4. In the second study, 17 men ran 40 minutes at 65% VO2max in 18°C and then sat in a 40°C bath for 40 minutes, 6 days running. Time-trial performance at 33°C improved 4.9%, with no change at 18°C 5.

The cool-weather bonus is the contested part.

Lorenzo's 6% gain in the cool lab is the number that gets quoted in training forums, and it rests on one study of 12 cyclists. Nybo and Lundby published the opposing case in a Journal of Physiology CrossTalk, arguing that transfer of heat acclimatization to performance in cool conditions has not been established 6. The bath study found its 4.9% gain at 33°C and nothing at 18°C 5. Benjamin's pooled effect sizes come mostly from testing done in the heat 3.

Build the block for a hot race. If cool-weather speed also shows up, you got it for free.

The protocol.

  1. Count back 12 days from a hot race. Run 10 to 14 consecutive heat days and finish 3 days out. The consensus recommendation is 10 to 14 consecutive days of exposure holding core temperature at or above 38.5°C with profuse sweating for at least 60 minutes per session 7.
  2. Keep every heat session easy. 60 to 90 minutes at about 50% of VO2max, near 65 to 70% of maximum heart rate. Lorenzo's cyclists gained 8% without a single hard session in the chamber 1. Interval work in 40°C stacks two stresses on the same days.
  3. Pick your heat source and stop shopping. No chamber: overdress on the treadmill with the fan off; or sauna for 30 minutes at 80 to 90°C right after an easy run 4; or soak 40 minutes in a 40°C bath within minutes of finishing 5. Delay after training weakens the stimulus in both passive protocols.
  4. Measure two things. Heart rate at one fixed easy pace, logged every session, and body mass before and after one session per week. Expect the fixed-pace heart rate to fall by something like 16 bpm across the block 2. Each kilogram lost in a session equals about 1 litre of sweat; divide by session hours to get your hourly rate, then drink to that number on race day.
  5. Top up after the block. Adaptations fade about 2.5% per decay day, and 2 days away from heat costs roughly 1 day of acclimation 8. One heat session every 3 to 5 days holds the line between the end of the block and the start line.
  6. Set stop rules before you start. Goosebumps, chills, a headache or confusion during a heat session means out of the heat now, hydrate, and skip the rest. Never take a 90°C sauna alone, and drop the block entirely if you are ill.

Open your calendar, find your next race above 25°C, and put session 1 on the day 12 days before it. Weigh yourself before and after that session and write both numbers down.

Sources.

  1. Lorenzo, S., Halliwill, J. R., Sawka, M. N., & Minson, C. T. (2010). Heat acclimation improves exercise performance. Journal of Applied Physiology, 109(4), 1140-1147. doi.org/10.1152/japplphysiol.00495.2010
  2. Tyler, C. J., Reeve, T., Hodges, G. J., & Cheung, S. S. (2016). The effects of heat adaptation on physiology, perception and exercise performance in the heat: a meta-analysis. Sports Medicine, 46(11), 1699-1724. doi.org/10.1007/s40279-016-0538-5
  3. Benjamin, C. L., Sekiguchi, Y., Fry, L. A., & Casa, D. J. (2019). Performance changes following heat acclimation and the factors that influence these changes: meta-analysis and meta-regression. Frontiers in Physiology, 10, 1448. doi.org/10.3389/fphys.2019.01448
  4. Scoon, G. S. M., Hopkins, W. G., Mayhew, S., & Cotter, J. D. (2007). Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. Journal of Science and Medicine in Sport, 10(4), 259-262. doi.org/10.1016/j.jsams.2006.06.009
  5. Zurawlew, M. J., Walsh, N. P., Fortes, M. B., & Potter, C. (2016). Post-exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat. Scandinavian Journal of Medicine & Science in Sports, 26(7), 745-754. doi.org/10.1111/sms.12638
  6. Nybo, L., & Lundby, C. (2016). CrossTalk opposing view: heat acclimatization does not improve exercise performance in a cool condition. The Journal of Physiology, 594(2), 245-247. doi.org/10.1113/JP270880
  7. Racinais, S., Alonso, J. M., Coutts, A. J., Flouris, A. D., Girard, O., González-Alonso, J., et al. (2015). Consensus recommendations on training and competing in the heat. Scandinavian Journal of Medicine & Science in Sports, 25(S1), 6-19. doi.org/10.1111/sms.12467
  8. Daanen, H. A. M., Racinais, S., & Périard, J. D. (2018). Heat acclimation decay and re-induction: a systematic review and meta-analysis. Sports Medicine, 48(2), 409-430. doi.org/10.1007/s40279-017-0808-x

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