Can Passive Heat Exposure Boost Your Performance?

William Horkoff
Sep 4
3 min read

When most triathletes think about heat training, the image that comes to mind is riding indoors with extra layers, sweating buckets, and grinding through uncomfortable sessions. It works but let’s be honest it’s mentally draining and often hard to stick with, especially when your normal training load is already high.

That’s where passive heat exposure comes in. Instead of turning your easy ride into a sauna, you can simply sit in a hot tub or sauna post-training and still tap into many of the same physiological benefits.

A new study from the University of Innsbruck (Rodrigues et al., 2025) gives us some fresh insight into how effective this strategy can be.

The Study: Hot Water Immersion vs. Control

Participants: 20 recreationally active men and women (~28 years old, VO₂peak ~47 ml/kg/min).
Training Protocol: Everyone did 3x per week of high-intensity intervals (4×4 min at ~90% max HR).
The Intervention Included the following:
- Hot Water Immersion (HWI): 5 sessions per week of sitting in 42°C water (up to the sternum) for 40–50 minutes.
- Control Group: Same schedule, but immersion in thermoneutral water (~34.5°C).

What They Found

After 6 weeks, the hot water immersion group saw:

Hemoglobin mass ↑5.3% vs. 2.0% in control. (This simply means more red blood cells = more oxygen delivery to working muscles)
VO₂max ↑6.1 ml/kg/min vs. 4.5 ml/kg/min in control.
Peak Power Output ↑22 W vs. 15 W in control.
Core temperature stress ↓ post-training in HWI group (better thermoregulation).

Why This Matters for Triathletes

Endurance performance is built on your ability to deliver and use oxygen efficiently. Expanding hemoglobin mass and improving heat tolerance are big wins.

Traditionally, this has been done through active heat training e.g., riding indoors in extra layers. But there are drawbacks:

It adds mental fatigue (already high in a big triathlon build).
It makes easy sessions harder than they should be.
It’s tough to stay consistent week after week.

Hot water immersion or sauna, on the other hand, is far easier to wrap your head around. You’re sitting, relaxing, and letting the heat do its work. From a psychological standpoint, this makes it much easier to stick with and consistency is where the real gains are made.

Active vs. Passive Heat: Which Is Better?

The research (and decades of field practice) still leans toward active heat training being the gold standard. When you combine exercise with heat stress, the adaptations tend to be greater.

But here’s the key:

Passive methods still work and they’re easier to sustain.
For athletes already pushing big volumes, the added mental load of heat training on the bike or treadmill can tip things toward burnout.
A hybrid approach often works best: e.g., 2-3 active heat sessions per week, plus 1-2 passive exposures (sauna or hot tub).

Practical Takeaways

Protocol to try: 40-50 minutes of hot water immersion (or sauna) ~4-5 times per week, especially after hard training sessions.
Temperature: Aim for ~42°C water or a sauna that pushes core temperature meaningfully higher.
Duration matters: Short dips won’t cut it - the study used sustained exposures.
Think consistency: It’s better to commit to a realistic plan you can stick with for 4-6 weeks than to overreach and quit.

The Bottom Line

Heat exposure is a powerful tool to improve hemoglobin mass, aerobic capacity, and performance in the heat all critical for triathletes racing in warm climates or pushing long-course events.

While active heat training may deliver slightly larger gains, the psychological cost is high. Passive methods like hot water immersion or sauna provide a practical, consistent alternative that can slot into your training week without crushing you mentally.

In endurance sport, the “best” method is the one you can actually stick with. For many triathletes, that might mean swapping some sweaty indoor heat sessions for a hot tub.