Muscle temperature increases during a single far infrared sauna session without changes in intestinal temperature

Increases in muscle temperature during exercise and passive heating are associated with beneficial outcomes. Far infrared (FIR) saunas are a radiant heating stimulus. It has been claimed that FIR waves penetrate 3 to 4 cm deep into the peripheral tissues, but muscle temperature during FIR sauna bathing is unknown. The purpose was to quantify muscle temperature at three different depths during an FIR sauna session. Ten adults had a multisensor intramuscular temperature probe inserted into the quadriceps muscles before 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, and nude body weight (to calculate whole body sweat rate) were collected before and at the end of heating. Data are reported as (mean [95% confidence intervals]). Muscle temperature increased at the deep (+1.1 [0.3, 1.9]°C), middle (+1.9 [1.0, 2.9]°C), and superficial (+3.0 [1.8, 4.1]°C) depths (all <i>P</i> < 0.04). There was no change in core temperature (0.0 [-0.1, 0.1]°C) (<i>P</i> = 0.94), but there was an increase in mean body temperature (+1.3 [1.1, 4.1]°C) (<i>P</i> < 0.01) driven by increases in mean skin temperature (+6.2 [5.8, 6.8]°C) (<i>P</i> < 0.01). 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. The magnitude of the increase in muscle temperature was dependent on depth relative to the skin surface. These data imply that commercially available FIR saunas provide only superficial heating of peripheral tissues.<b>NEW & NOTEWORTHY</b> To our knowledge, this is the first investigation on far infrared sauna bathing with a commercially available sauna and muscle temperature. Muscle temperature increased in the absence of changes in core temperature. The increases in muscle temperature were lessened with increasing depth and were negligible beyond 3.8 cm below the skin surface. More practically, the thermic effect had lessened by 63% at a depth of 2.4 cm, which can be considered the effective thermal penetration.