Issue #7: Heat Training and Acclimation

Turning Summer Heat into Performance Gains

✍️ Author’s Note

Welcome to this edition of the Threshold Lab newsletter! I’m Stephen Pelkofer, an experienced HYROX athlete with a men’s pro personal best of 59:41 and men’s pro doubles best of 51:39. If you want to learn about my story and how I got here, check out this Instagram post here.

The goal of this newsletter each week is to pick a training topic related to running or HYROX, do deep research into it, and provide actionable protocols that the reader can take away and apply to their training immediately. Let me read the research, listen to the experts on podcasts and youtube, and then give you the tools to make it work for you.

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👨‍💻 Introduction

Topic: Heat Training and Acclimation

Many athletes are currently slogging through extreme summer heat in their training, and it’s tempting to view the sweltering temperatures as a hurdle. But what if those hot, humid sessions are actually building your fitness? Training in high heat can feel brutal, but research suggests it’s actually a performance booster. This issue, we explore why embracing the summer heat could pay off come race day, drawing on insights from the study Heat Acclimation Improves Exercise Performance (Lorenzo et al., 2010).

⚡ Fast Finds for the busy reader:

  • Heat training boosts your engine: A 10-day heat acclimation protocol increased athletes’ VO₂max and power at lactate threshold.

  • Performance gains even in cool weather: Heat training isn’t just for hot races – it can make you faster even on race days with mild weather.

  • Cardio and cooling adaptations: Heat-acclimated athletes expand their plasma volume and bump up max cardiac output. These changes let you deliver more oxygen to muscles and dissipate heat more effectively.

🔬 Deep Dive

Study Background & Setup: The 2010 study by Lorenzo and colleagues set out to examine how systematic heat acclimation affects performance. They recruited 12 highly trained cyclists and split them into two groups. One group underwent a 10-day heat acclimation program: about 90 minutes of cycling each day at ~50% VO₂max intensity in a climate chamber heated to ~40°C (104°F). The other 8 cyclists served as a control group, doing the identical training protocol for 10 days but in a cool 13°C (55°F) environment. Before and after this period, all athletes did benchmark tests in both a cool room (13°C, 30% humidity) and a hot room (38°C, 30% humidity): a VO₂max test, a lactate threshold test, and a 1-hour cycling time trial. This design lets us see not only how training in heat helps you in the heat, but also whether those adaptations carry over to cooler conditions.

Key Findings: The heat-acclimation group saw significant improvements across the board, even when tested in cool conditions. After 10 days of heat training, their maximal aerobic capacity (VO₂max) increased by about 5% in the cool environment and by about 8% when tested in the hot environment. In practical terms, a higher VO₂max means a bigger “engine” – the ability to consume and use more oxygen during maximal exercise, which is a major predictor of endurance performance.

Perhaps even more impressive (and relevant to the Threshold Lab community), lactate threshold, the effort level at which lactate begins to rapidly accumulate, shifted higher after heat acclimation. The power output they could sustain at lactate threshold went up by around 5% in both cool and hot conditions.

Finally, the cyclists’ time trial performance (the total work completed in a 1-hour all-out effort) improved substantially. After heat acclimation, they managed about 6% more work in the 1-hour trial in the cool condition, and about 8% more in the hot condition. Remember that these are already highly trained cyclists, so 6-8% gains for a 60-min effort are pretty insane. The control group (who trained in cool conditions) showed no improvements in any of these metrics. Heat training might be the new altitude training.

Why Does Heat Make You Fitter? Heat is a form of stress, and the body responds to heat stress with a suite of adaptations. One major change observed was an increase in blood plasma volume. The heat-acclimated athletes expanded their plasma volume by about 6.5%. More blood volume means the heart can pump more blood per beat (increasing stroke volume) and deliver more oxygen during exercise. Essentially, heat training can strengthen your cardiovascular system similarly to how living at high altitude or doing lots of volume can → by increasing the blood your heart pushes to muscles.

Heat acclimation also improves your thermoregulatory efficiency. Past research and common experience show that after adapting to heat, your body starts sweating earlier and more profusely, and your resting heart rate in the heat is lower. In this study, those effects likely contributed to better performance especially in the hot trial: a lower core temperature and heart rate for a given workload mean you can sustain a hard effort longer before overheating. Even for athletes who race in cooler conditions, these heat adaptations can translate to improved efficiency, essentially giving you more headroom before you hit your max.

Time for the lab notes ⬇️

🧪 Lab Notes

In every issue of this newsletter, the “Lab Notes” are going to be the protocols that you can apply to your training and routine right away. The goal of this section is to translate the science into actionable steps for the reader, whether you’re a recreational runner/HYROX competitor, or someone pushing the limits of their peak potential.

  • Embrace the Heat Slowdown: Expect and allow yourself to run slower in hot weather (it’s both normal and smart). When it’s 90°F with high humidity, your body diverts energy to cool you down, so don’t try to force your usual pace. A slow, sweaty run in heat is still building your fitness.

  • Plan a Heat Acclimation Block: Schedule 1-2 weeks of deliberate heat-focused training to get the benefits. Research shows many adaptations occur within the first 7-10 days of heat exposure. This could mean a 10-day period of training in the hottest part of the day, or using an indoor bike/treadmill in a heated room. The key is consistent daily (or near-daily) exposure. For example, do your easy runs in the afternoon heat or add a 20-30 minute easy spin in a hot garage each day.

  • Play the Long Game: Remind yourself that feeling sluggish in the summer is okay if you’re playing the long game. Heat training is an investment; once cooler weather arrives, you’ll likely find a performance rebound, and you might be able to hit a massive PR in a September-November race due to the work you put in all summer.

Remember: Heat is a powerful training tool, but it’s one that you must also respect. Always prioritize safety: stay hydrated, listen to your body, and avoid the hottest hours for very hard efforts.

That’s it for this edition of the Threshold Lab. If you enjoyed this and found it useful, please share it with a friend!

What I Read and Researched for this Issue