Heat Adaptation for Cool Weather Performance?

Heat acclimation (HA) has long been recognized for its benefits in hot environments, but what about its effects in cooler conditions?

Introduction

Welcome summer! As we hit the official start of summer in June, it also signals the season of heat and humidity. For cyclists, this means adapting to new training conditions and finding ways to optimize performance despite the rising temperatures. 

In today’s Toolbox article, we’ll look into the findings of a recent study to see if prolonged heat acclimation from riding in the mid-summer heat can translate into additional fitness gains when the temperatures cool back down. So, can heat training in the sweltering months help you stay ahead of the pack when the weather turns mild? Let’s find out!

Background on Heat Acclimation

Heat acclimation involves physiological processes that are known to benefit aerobic exercise, such as improved sweat response, lower core body temperature, & reduced resting heart rate. These adaptations can be brought about actively – by simply riding your bike in hot & humid conditions, or passively, like sitting in a sauna. Benefits from HA can occur in as little as 4-5 days, but longer protocols (2+ weeks) have greater performance-enhancing effects.

These adaptations in response to heat stress are known to improve performance in hot conditions, detailed in a comprehensive 2024 meta-analysis by our own Toolbox Editor Stephen Cheung (Tyler et al. 2024). But there is ongoing discussion about whether these beneficial adaptations are able to improve aerobic capacity and athletic performance in cooler conditions. In today’s article, we’ll take a look at a 2019 study (Mikkelsen et al. 2019) and see if prolonged head acclimation from riding in the mid-summer heat can translate into some additional fitness later this year when the temperatures cool back down!

Also check out our archive of articles on exercising and adapting to the heat:

Dr. Cheung has also had many different podcast interviews about exercise and adapting to the heat through the years. Check out some recent ones in Canadian Cycling Magazine Podcast and the Consummate Athlete Podcast.

Study Context

​​This is not the first study to investigate the effects of HA on athletic performance in cooler conditions. Previous research has shown mixed results, with some studies showing positive benefits and others showing no effect. The differences in results likely reflect variations in study designs and participants. 

To help conclude if HA translates into improved aerobic performance in cool conditions, the authors of this study aimed to:

1. Show improved performance in well-trained individuals, as improvements in recreationally active or inactive adults could be due to general fitness gains rather than heat acclimation.
2. Demonstrate that the HA protocol should outperform a control group that also performs high-intensity intervals.

The Study Design

The authors made use of a randomized control trial (RCT) with 24 well-trained male cyclists, of which 21 completed the entire study. For reference, participant characteristics are listed below:

• Age: 38 ± 9 years
• Height: 184 ± 4 cm
• Weight: 80.4 ± 8.0 kg
• Relative VO2max: 58.1 ± 5.3 mL/kg/min

Following familiarization and a two-week lead-in, the cyclists were split into a heat training (HEAT) or control (CON) group. Both groups performed a lead-in phase to reduce the potential ‘training effect’ of the intervention and ensure the results reflected the intended heat adaptations. All participants completed pre-intervention performance testing, including a 15k Time Trial (TT) and a VO2max test. The intervention period lasted 5.5 weeks, with the HEAT group undergoing training sessions in 40ºC and the CON group training in cooler conditions. At the conclusion of the training, the athletes again completed performance testing.

Training and Testing Protocols

The HEAT group completed training sessions in a climate chamber five times per week. The chamber was set to 35ºC (95ºF) the first week and increased 1ºC each week, ending at 40ºC (104ºF). The CON group completed one session per week in the environmental chamber at 15ºC (59ºF) and performed the rest of their training in cool settings. Both groups performed this training in the lab at a moderate intensity (60% VO2max), which averaged 204 ± 3 W for all participants. All other training and habitual intervals for both groups were performed in cool settings outside the lab.

A 15km TT was used as a measure of endurance performance. VO2max was tested via an incremental ramp to exhaustion, starting with 5 min at 100 W, 5 min at 175 W, and then increasing by 25 W/min until volitional exhaustion. 

Key Findings

• Acclimation Verification: The HEAT group showed expected signs of HA, including lower sweat sodium concentration, reduced heart rate, and improved endurance in hot conditions.
• Performance in Cool Conditions: Both HEAT and CON groups improved TT performance similarly, with no significant differences.
• VO2max and Peak Power Output: No significant changes in VO2max or peak power output were observed in either group.
• Training Volume: Weekly training volume and intensity increased for both groups, but improvements were comparable.

Figure 1. A) Mean power output during 15 km TT performance and B) time trial performance in seconds. Taken from Mikkelsen et al. 2019.

Analysis and Interpretation

Training in heat did not lead to superior improvements in aerobic power or TT performance in cool conditions. The authors mention three potential reasons:

1. Similar overall training loads between the groups—perhaps a larger difference would be seen in elite athletes.
2. The physiological strain of heat training—some athletes mentioned the strain from heat impaired their ability to complete their habitual high-intensity training.
3. Individual variations in adaptation—each person responds differently to the same stimulus.

The authors also mentioned differences in study designs, participant training status, and intervention durations, which might explain the varied findings in the literature.

Practical Implications

• If you’re considering incorporating HA, focus on the context of your races, since HA benefits hot environments more directly.
• The authors suggest that athletes might consider incorporating HA strategically, perhaps during pre-season or when specifically preparing for hot-weather events.

Conclusion

Heat acclimation is not superior for improving performance in cool conditions but doesn’t harm it either. While HA can be beneficial for performance in hot environments, it may not provide additional benefits for performance in cooler conditions. If you’re training for a hot summer event, incorporating some training in the heat might be beneficial. However, you might not expect to gain extra benefits for a cooler race via heat stress training. Further research is needed to clarify the nuanced effects of HA and further optimize training strategies.

That’s all for this month… stay safe, ride fast, and I’ll see you next time!

References

Tyler CJ, Reeve T, Sieh N, Cheung SS. Effects of Heat Adaptation on Physiology, Perception, and Exercise Performance in the Heat: An Updated Meta-Analysis. J Science Sport Exercise. 2024 Feb 14. https://doi.org/10.1007/s42978-023-00263-8

Mikkelsen CJ, Junge N, Piil JF, Morris NB, Oberholzer L, Siebenmann C, Lundby C, Nybo L. Prolonged Heat Acclimation and Aerobic Performance in Endurance Trained Athletes. Front Physiol. 2019 Nov 1;10:1372. doi: 10.3389/fphys.2019.01372. PMID: 31749712; PMCID: PMC6843002.

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