Muscle temperature increases during a far infrared sauna session in healthy adults

Increased muscle temperature during exercise and passive heating is associated with beneficial outcomes. Far infrared (FIR) saunas are a radiant heating stimulus. It has been claimed that far infrared waves penetrate 3 to 4 cm deep into the peripheral tissues but muscle temperature during FIR sauna bathing has not been reported. The purpose was to quantify muscle temperature at three different depths below the skin surface during a FIR sauna session. Ten adults had a multi-sensor intramuscular temperature probe inserted into the quadriceps muscles (vastus lateralis and intermedius) prior to sitting in a FIR sauna for 45 min. Thermocouples were 3.4 cm (deep), 2.4 cm (middle), and 1.4 cm (superficial) below the skin surface. Muscle, core, and skin temperatures, perceptions, and nude body weight (to calculate whole body sweat rate) were collected before and at the end of heating. Muscle temperatures were analyzed with a two-way mixed effects model for time x depth. Core, skin, and mean body temperatures and perceptions were analyzed before and at the end of heating with a paired t -test. Data reported as mean [95% confidence intervals] with α set at 0.05. Before heating, the muscle temperature at superficial (34.2 [33.3, 35.1]°C) was lower than middle (34.8 [33.9, 35.7]°C) and deep (35.4 [34.6, 36.3]°C) and middle was lower than deep (all p<0.01). Muscle temperature increased at superficial (+3.0 [1.8, 4.1]°C; p<0.01), middle (+1.9 [1.0, 2.9]°C; p<0.01), and deep (+1.1 [0.3, 1.9]°C; P=0.04) during heating. At the end of heating, the muscle temperature at superficial (37.2 [36.5, 37.8]°C) was greater than middle (36.6 [36.2, 37.3]°C; P=0.01) and deep (36.5 [36.1, 37.0]°C ; p<0.01) but there was no difference between middle and deep depths. From before heating to end of heating, there was no change in core temperature (0.0 [-0.1, 0.1]°C) but an increase in mean body temperature (+1.3 [1.1, 4.]°C ; p<0.01) driven by increases in mean skin temperature (+6.2 [5.8, 6.8]°C ; p<0.01). Participants reported feeling warmer, more uncomfortable, and sweatier at the end of heating. Participants lost 0.48 [-0.60, -0.37] % of body weight and had a whole body sweat rate of 0.46 [0.31, 0.61] L/h. These data imply that skeletal muscle temperature increased during a far infrared sauna session, but the magnitude of the increase was dependent on the depth of the skeletal muscle relative to the skin surface. NIH HL144128, NIH AG072805, and the Eugene and Clarissa Evonuk Memorial Graduate Fellowship. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.