Infrared heat warmed the muscle nearest the skin most and left core temperature unchanged
To see how deep infrared heat goes, this 2025 Journal of Applied Physiology study put temperature probes into the muscle of 10 adults during a 45-minute far-infrared session.
The paper
Muscle temperature increases during a single far infrared sauna session without changes in intestinal temperature
Reed EL, Uzoekwe CC, Atencio JK, Minson CT, et al.. Journal of Applied Physiology, 2025;138(6):1628-1637.
See the study at a glance
- Design
- Single-group experimental study
- People
- 10 adults (6 women)
- Who they were
- Healthy adults.
- What they did
- One 45-minute session in a commercial far-infrared cabin. The thermostat started at 45°C and was then set to 65°C to keep the emitters on for 93% of the session. Probes read muscle temperature at three depths, and a swallowed pill tracked core temperature.
- How long
- One 45-minute session
These numbers came out of it
+3.0°C
Muscle 1.4 cm below the skin warmed 3.0°C.
+1.1°C
Muscle 3.4 cm down warmed 1.1°C.
2.4 cm
At 2.4 cm the warming had fallen by 63%, and the authors called this the heat's effective penetration depth.
0.0°C
Core temperature didn't change over the session.
The researchers did this
Ten adults sat for 45 minutes in a commercial far-infrared cabin made by Finnleo while probes inside their muscle read temperature at three depths.Source 1 · Clinical trialMuscle temperature increases during a single far infrared sauna session without changes in intestinal temperatureReed EL, Uzoekwe CC, Atencio JK, et al.. Journal of Applied Physiology, 2025.Read the paper ↗PubMed ↗ A swallowed pill tracked core temperature. The researchers set the thermostat to 45°C at entry and then raised it to 65°C so the emitters stayed on for 93% of the session.
Muscle 1.4 cm below the skin warmed 3.0°C. Deeper down the warming shrank to 1.9°C at 2.4 cm and 1.1°C at 3.4 cm.Source 1 · Clinical trialMuscle temperature increases during a single far infrared sauna session without changes in intestinal temperatureReed EL, Uzoekwe CC, Atencio JK, et al.. Journal of Applied Physiology, 2025.Read the paper ↗PubMed ↗ The authors defined effective thermal penetration as the depth where heating had fallen by 63%, and they put it at 2.4 cm.
Core temperature did not change. The shift came to 0.0°C, and the plausible range (95% confidence interval) ran from −0.1 to 0.1. Average skin temperature rose 6.2°C. Heart rate went from 67 to 101 beats a minute, and blood pressure held steady.Source 1 · Clinical trialMuscle temperature increases during a single far infrared sauna session without changes in intestinal temperatureReed EL, Uzoekwe CC, Atencio JK, et al.. Journal of Applied Physiology, 2025.Read the paper ↗PubMed ↗ People lost 0.48% of body weight, and the sweat rate worked out to 0.46 L an hour.
The authors noted that the popular 3–4 cm figure comes mostly from one review.Source 1 · Clinical trialMuscle temperature increases during a single far infrared sauna session without changes in intestinal temperatureReed EL, Uzoekwe CC, Atencio JK, et al.. Journal of Applied Physiology, 2025.Read the paper ↗PubMed ↗ A 2015 study of infrared sauna after training repeats the figure as background without measuring it.Source 2 · Crossover trialEffects of far-infrared sauna bathing on recovery from strength and endurance training sessions in menMero A, Tornberg J, Mäntykoski M, et al.. SpringerPlus, 2015.Read the paper ↗PubMed ↗
The study can't tell you these things
- Ten people and one cabin make a small sample.
- It covered one session and can't show what repeated use does.
- The researchers kept the emitters on for 93% of the session, so the study didn't test home cabins that switch their emitters on and off.
- It had no traditional-sauna comparison, so it can't say whether infrared warms muscle more than other heat.
People quote it like this
Infrared brands describe heat that reaches 3–4 cm into the body. In this study muscle warming fell off steeply with depth and core temperature stayed flat. The authors traced the 3–4 cm figure to one review.Source 1 · Clinical trialMuscle temperature increases during a single far infrared sauna session without changes in intestinal temperatureReed EL, Uzoekwe CC, Atencio JK, et al.. Journal of Applied Physiology, 2025.Read the paper ↗PubMed ↗
From our sister magazine
Sauna Monthly explains what the heater specs on infrared cabins mean
We cited these sources
- 01Reed EL, Uzoekwe CC, Atencio JK, Minson CT, Halliwill JR. Muscle temperature increases during a single far infrared sauna session without changes in intestinal temperature. Journal of Applied Physiology. 2025;138(6):1628-1637. doi:10.1152/japplphysiol.00067.2025 PMID 40331900
- 02Mero A, Tornberg J, Mäntykoski M, Puurtinen R. Effects of far-infrared sauna bathing on recovery from strength and endurance training sessions in men. SpringerPlus. 2015;4:321. doi:10.1186/s40064-015-1093-5 PMID 26180741