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

Abstract

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 a FIR sauna session. Ten adults had a multi-sensor intramuscular temperature probe inserted into the quadriceps muscles 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, 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 P<0.04). There was no change in core temperature (0.0 [−0.1, 0.1]°C) (P=0.94) but there was an increase in mean body temperature (+1.3 [1.1, 4.1]°C) (P<0.01) driven by increases in mean skin temperature (+6.2 [5.8, 6.8]°C) (P<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 increase in muscle temperature was dependent on depth relative to the skin surface. These data imply commercially available FIR saunas provide only superficial heating of peripheral tissues.

NEW & NOTEWORTHY

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 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.

Graphical Abstract

INTRODUCTION

Skeletal muscle contributes to multiple functions including postural support, physical movement ranging from activities of daily living to elite performance, and metabolism. The maintenance of skeletal muscle mass and function is important for overall health as a reduced skeletal muscle mass is associated with a greater mortality (1–3). While regular physical activity is protective of muscle mass and function, only about 30% of the adult population meets the recommended guidelines for both aerobic and strength physical activities (4). The use of passive heating, for example with hot tubs and saunas, has been proposed as another lifestyle means to reduce the risk of cardiovascular and metabolic disease, improve performance or recovery, and maintain skeletal muscle function (5–8). This may be in part due to the similar increases in skeletal muscle temperature during passive heating as observed during exercise.

Skeletal muscle temperature increases during exercise as a byproduct of skeletal muscle contractions, is directly related to the workload, and can be further altered by the environmental conditions (9, 10). The skeletal muscle temperatures during different modalities, intensities, and durations of exercise range between 37.8–40.2°C (9, 11–16). Changes in skeletal muscle temperatures during passive heating are due to the heat exchange with the external environment, adjacent tissues, and blood flow (17–19). The muscle temperatures during passive heating with localized short-wave diathermy (~39–40°C)(16, 20–22), hot water immersion (39°C)(23), and traditional sauna (37.5°C)(24) are similar to exercise. The increase in skeletal muscle temperature is associated with increased endothelial nitric oxide synthase (25, 26), angiogenesis and capillary growth (15, 25, 27, 28), mitochondrial function (21, 22), insulin signaling (29, 30), and heat shock proteins (21, 22, 26, 31). These responses are associated with improvements in cardiovascular, metabolic, and musculoskeletal outcomes with repeated exposures and offer support for the use of passive heating as a lifestyle intervention.

Far infrared saunas have panels on the interior of the sauna that emit far infrared waves, a radiant heat stimulus, and are claimed to penetrate 3 to 4 cm into the peripheral tissues (6, 32). The full far infrared range within the electromagnetic spectrum is between 3 to 1000 μm (33) but commercially available sources are within a more narrow range of 5 to 14 μm (34). These saunas are continuing to increase in popularity and accessibility by the general public. A typical far infrared sauna session is between 15 to 30 min at temperatures between 45–60°C (35). The sauna session can then be followed up by a 30 min period wrapped in blankets, known as Waon therapy (36). Some cardiovascular improvements have been observed with repeated use of Waon therapy with a custom made sauna in cardiac patients (37, 38). These improvements were likely due to increases (1.0–1.5°C) in pulmonary artery temperature. Others have reported tympanic temperature increased during a single far infrared session (40 min) in adults with type 2 diabetes (39). In addition, repeated far infrared sauna bathing (45 min) in older adults increased skeletal muscle capillarization (40). However, Atencio et al. (41) reported minimal or no change to core temperature (rectal and intestinal) during a 45 min far infrared sauna session in young, healthy adults. These participants felt warmer and sweat appreciably (as marked by changes in body mass of about −0.3%). However, the skeletal muscle temperature responses during far infrared sauna bathing are currently unknown. Quantifying the skeletal muscle temperatures may offer further insight for the thermoregulatory responses, mechanistic understanding, and application of these saunas as a passive heating modality used by recreational, athletic, and clinical populations to optimize targeted outcomes. Therefore, the purpose of this exploratory study was to quantify the skeletal muscle temperature response during a single far infrared sauna visit.

MATERIALS AND METHODS

The protocol was approved by the Institutional Review Board at the University of Oregon and conformed to the principles of the Declaration of Helsinki, except for registration in a database.

Participants

Ten young, healthy individuals (6 women, 4 men) volunteered to participate in the protocol and the characteristics of the participants are shown in Table 1. Inclusion criteria included being between the ages of 18–40 years, a body mass index between 18–28 kg/m², and classification of Tier 0 or Tier 1 for physical activity levels according to McKay et al. (42). Exclusion criteria included self-reported medical conditions impacting cardiovascular, respiratory, metabolic, neurologic, gastrointestinal systems, or blood clotting or anemia disorders, use of prescription medications other than hormonal contraceptives, and any hypersensitivities or allergies to drugs, local anesthetics, skin disinfectants, adhesives, or latex. Participants were excluded if systolic blood pressure was > 120 mmHg or diastolic blood pressure > 80 mmHg (Tango M2, Suntech Medical, North Carolina, USA). Ultrasonography (iE33, Philips, Amsterdam, Netherlands) was used to quantify tissue thickness of the thigh (subcutaneous adipose and muscle) at half the distance between the anterior superior iliac spine and superior aspect of the patella, with the target tissue being the vastus lateralis quadriceps muscle. This was to ensure sufficient tissue for the placement of the intramuscular temperature probe while avoiding large veins, arteries, and the iliotibial band. Participants were excluded if the tissue thickness was < 4.0 cm.

Table 1.

Participant characteristics.

Variable
Age (y)	24 ± 4
Height (cm)	168 ± 10
Weight (kg)	68.3 ± 11.7
Body mass index (kg m-2)	23.8 ± 1.9
Body fat (%)	19.8 ± 6.8
Thigh volume (cm3)	9612 ± 2009
Subcutaneous fat thickness (cm)	0.72 ± 0.33
Vastus lateralis muscle thickness (cm)	2.10 ± 0.58
Vastus intermedius muscle thickness (cm)	1.44 ± 0.33
n	10 (6 female, 4 male)

Values are means ± standard deviations.

Study design

Participants were instructed to refrain from eating food for 2 h, caffeine for 10 h, and nutritional supplements, alcohol, drugs, and moderate to vigorous exercise for 24 h prior to the start of the study visit. Participants were provided with the ingestible telemetry pill (e-Celsius Performance, BodyCap Medical, Hérouville Saint-Clair, France) for the measurement of core temperature and instructed to ingest the telemetry pill at least 4 h prior to the start of the study visit (mean time of ingestion was 7.5 [5.6, 9.4] h). After instrumentation (described below), participants were seated for 60 min in an ambient temperature room (23 [23, 24]°C; 49 [46, 52]% relative humidity) then the “before heating” data were recorded. Next, participants entered the far infrared sauna and remained seated in the sauna for 45 min at which the “end of heating” data were recorded. Lastly, after exiting the sauna, participants remained seated for 30 min in the ambient temperature room then de-instrumented.

Far infrared sauna

The sauna thermostat was set to 45°C until the participant entered, then was immediately increased to 65°C to maximize the “on” cycle of the far infrared emitters throughout the duration of heating (IS 565, Finnleo, Sauna360 Inc., Minnesota, USA). According to vendor websites, the emitters for this sauna emit with the 8.4 to 9.4 μm range. We directly tracked the on/off cycle by linking the signal to the emitters to a data acquisition system (WinDaq, DATAQ Instruments, Akron, Ohio, USA). Far infrared emitters were on for 93% (42 [40, 44] min) of the 45 min that participants were in the sauna. While we used the factory-installed thermostat and recommended temperature settings (45–65°C) of the sauna, we also recorded temperature and humidity with a probe (Vaisala HUMICAP^®, Vantaa, Finland) located at the same height as the thermostat but at the opposing upper corner of the sauna. We note that this additional probe recorded a lower average temperature of 43 [42, 44]°C) compared to the factory-installed thermostat sensor which averaged 57 [52, 62]°C.

Instrumentation and measurements

Upon arrival to the laboratory, a urine sample was obtained to quantify hydration status via urine specific gravity (<1.020) and a urine pregnancy test for those participants of childbearing potential to confirm they were not pregnant. A nude body weight was measured in a private room to the nearest 10 g (Sartorius Midrics 2, Goettingen, Germany). Participants changed into the self-selected clothes for the sauna – shorts and a t-shirt, tank top, or sports bra.

A telemetry heart rate monitor (Polar Team Pro, Polar Electro, Kempele, Finland) was placed at the level of the heart. A blood pressure cuff was placed on the upper arm. Mean arterial blood pressure was calculated as: 0.3(systolic blood pressure-diastolic blood pressure) + diastolic blood pressure. Skin temperature was measured with temperature thermocrons (iButtons, iButton Link Technology, Wisconsin, USA) secured on the skin via adhesive tape to the chest, upper arm (triceps), thigh, and calf. Mean skin temperature was calculated as: 0.3(chest + upper arm) + 0.2(thigh + calf) (43). Mean body temperature was calculated as: 0.8(core temperature) + 0.2(mean skin temperature) (44). To quantify body fat percentage, skin fold thickness was measured via skinfold thickness at three locations: for women - the triceps, hip, and thigh; for men - the chest, abdomen, and thigh (45, 46). The measurement of thigh volume was calculated by measuring the total thigh length (L), the circumferences at midway (C₁) and 10 cm above (C₂) and 10 cm below (C₃) the midline, and accounting for skinfold thickness at the thigh (S) (47):

$$Thighvolume\left(c{m}^3\right)=L\times {\left(12\pi \right)}^{-1}\times \left(C_1^2+C_2^2+C_3^2\right)$$

$$-(S-0.4)\times 2^{-1}\times L\times (C_1+C_2+C_3)\times 3^{-1}$$

A multi-sensor intramuscular temperature probe (Physitemp Instruments, LLC, New Jersey, USA) was inserted into the lateral thigh with the participant in the supine position. The probe had three thermocouples including one at the tip (deep), 10 mm from the tip (middle), and 20 mm from the tip (superficial). The location for the placement was scouted via ultrasonography half the distance between the anterior superior iliac spine and the superior border of the patella. Images were obtained with the ultrasound probe both parallel and perpendicular to the vastus lateralis while avoiding compression of the tissues (48, 49). The images were analyzed for the thickness of subcutaneous fat and quadricep muscles (vastus lateralis and vastus intermedius). This allowed us to define the tissue localization for the three thermocouples following the protocol. The surface of the thigh was sterilized with 2% chlorhexidine gluconate/70% isopropyl alcohol followed by the placement of a fenestrated drape. The skin was anesthetized with prilocaine hydrochloride (4%) followed by lidocaine (2%)/epinephrine. A 17-gauge introducer needle was inserted perpendicular to the muscle tissue and the intramuscular probe inserted so the tip of the probe would be at ~4 cm deep. This depth was selected as it has been reported that far infrared penetrates 3 to 4 cm into peripheral tissues (6, 32). We opted for an absolute depth across participants in order to determine the depth of heating of peripheral tissues, rather than attempting to control the placement of probes relative to subcutaneous fat and/or muscle thickness as suggested by others (50). This was to increase the ecological validity of our results for users of commercially available saunas. We do recognize that tissue types, thickness, and depths can alter temperature profiles and should be considered for future research (15, 23, 50–53). The probe was secured with steri-strips and covered with a Tegaderm dressing.

After the probe was placed, participants rested in the seated position for 60 min for muscle temperature to stabilize post-insertion. At the end of 60 min, “before heating” data were obtained in the seated position. Then, participants were carefully moved into the sauna while research team members offered physical support to avoid the participants putting weight on the leg with the temperature probe secured in position with the Tegaderm dressing.

Heart rate, muscle temperature, core temperature, skin temperature, and sauna conditions (temperature and humidity) were recorded continuously during the sauna session. Perceptions of thermal sensation (54), thermal comfort (54), and perceived sweating (55) were reported by participants every 10 mins. At the end of 45 min, “end of heating” data were obtained prior to participants exiting the sauna.

At the end of the protocol, the probe was marked where it exited the skin so that when it was removed, the length that had resided in tissue could be directly measured. A second nude body weight was measured. Whole body sweat loss was calculated as the change in nude body weight corrected for ad libitum fluid intake and urine loss, if applicable.

Statistical analyses

Data were analyzed in GraphPad Prism (Version 10.4.0) with α set at 0.05 for all statistical inferences including familywise error rates. Inferences regarding the change in absolute muscle temperatures were drawn from a two-way mixed-effects model (tissue depth and before heating vs. end of heating) with pre-planned comparisons (Šidák’s multiple comparisons test, restricted to comparisons at the same timepoint between depths or same depth but between timepoints). Inferences regarding the change in muscle temperatures at each tissue depth (from before heating to end of heating) were drawn from a one-way mixed effects model (Šidák’s test between each tissue depth). A simple linear regression between the tissue depth and change in temperatures (from before heating to end of heating) was completed with thigh skin temperature representing 0 cm and all other depths were from the multi-sensor intramuscular temperature probe. From this regression, we calculated the effective thermal penetration as the depth at which the thermic effect was reduced by 63%, a concept derived from analysis standards for radiant energy (56). Inferences regarding the changes in core temperature, mean body temperature, skin temperatures, perceptions, and heart rate were drawn from paired two-tailed t-tests (before heating vs. end of heating). Values represent means [95% confidence intervals], unless otherwise noted.

RESULTS

Skeletal muscle temperature

The skeletal muscle temperatures at each depth are shown in Figure 1 and Supplemental Table 2. The depths of the thermocouples within the single intramuscular temperature probe were 1.4 cm (superficial), 2.4 cm (middle), and 3.4 cm (deep) below the surface of the skin. Further, the superficial thermocouple was in the vastus lateralis muscle (n=8) and subcutaneous fat (n=2), the middle thermocouple was in the vastus lateralis (n=8) and vastus intermedius (n=2) muscles, and the deep thermocouple was in the vastus lateralis (n=1) and vastus intermedius (n=9) muscles. All three depths increased in temperature from before heating to end of heating (all P<0.04). At before heating, 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). At end of heating, superficial (37.2 [36.5, 37.8] °C) was greater than middle (36.7 [36.2, 37.3] °C) and deep (36.5 [36.1, 37.0] °C) (both P<0.01) but middle was not different from deep (P=0.35). The change in temperature from before heating to end of heating was greater in superficial (+3.0 [1.8, 4.1] °C) compared to middle (+1.9 [1.0, 2.9] °C) and deep (+1.1 [0.3, 1.9] °C) (both P<0.01) and in middle compared to deep (P=0.01). The results from a simple linear regression analysis between depth and change in temperature are shown in Figure 2. The overall regression was significant (R²=0.720, F(1, 38)=[97.92], P<0.01). The fitted regression model was y= −1.57x + 5.97, which has an x-intercept of 3.79 cm (indicating no change in temperature at that depth). The effective thermal penetration (the depth at which the thermic effect was reduced by 63%) was 2.39 cm.

Figure 1.

Individual muscle temperatures at before heating and end of heating (A). The means and 95% confidence intervals are represented by the black lines. Data were analyzed with a two-way mixed effects model for depth x time with Sidak’s multiple comparisons test. The change in muscle temperatures from before heating and end of heating (B). The data show means (gray bars) and 95% confidence intervals (black lines) with individual data (white symbols). Data were analyzed with a one-way mixed effects model with Sidak’s multiple comparisons test. The depths of the thermocouples were superficial (1.4 cm), middle (2.4 cm), and deep (3.4 cm).

Figure 2.

Simple linear regression analysis between the depth and change in temperature (from before heating to end of heating) where thigh skin temperature is represented by 0 cm and all other depths are from the multi-sensor intramuscular temperature probe.

Core temperature, skin temperatures, heart rate, and blood pressures

The core and skin temperature responses are shown in Figure 3. There was no change in intestinal core temperature from before heating to end of heating (P=0.94). There was an increase in mean body temperature from before heating to end of heating (P<0.01). This was driven by increases in mean skin temperature (P<0.01). There was an increase in skin temperature for all the skin temperature locations (chest, upper arm, thigh, and calf) (all P<0.01). There was an increase in heart rate from before heating (67 [64, 71] bpm) to end of heating (101 [89, 112] bpm) (P<0.01). There were no changes from before heating to end of heating for systolic, diastolic, or mean arterial blood pressures (Supplemental Table 3).

Figure 3.

Individual core and skin temperatures at before heating and end of heating (A). The means and 95% confidence intervals are represented by the black lines. Data were analyzed with paired two-tailed t-test between pre and end of heating. The change of core and skin temperatures from before heating to end of heating (B) are shown as means (gray bars) and 95% confidence intervals (black bars) with individual data (white symbols).

Perceptions and whole body sweat rate

The perceptual responses are shown in Figure 4 and Supplemental Table 4. Thermal sensation, thermal comfort, and sweating perception all increased from before heating to min 40 of the sauna (all P<0.01). Participants lost 0.48 [0.37, 0.60] % of their body mass during the study visit and whole body sweat rate was calculated to be 0.46 [0.31, 0.61] L/h.

Figure 4.

Individual perceptual responses at before heating and end of heating for thermal sensation (A), thermal comfort (B), and sweating (C). The means and 95% confidence intervals are represented by the black lines. Data were analyzed with paired two-tailed t-test between before heating and end of heating.

DISCUSSION

The overall purpose of the present study was to quantify the skeletal muscle temperature responses during a single far infrared sauna session with a commercially available sauna in young, healthy adults. The main finding was that skeletal muscle temperature increased during the sauna session with the magnitude of the increase being dependent on the depth of the skeletal muscle relative to the skin surface. The increase in skeletal muscle temperature coincided with increases in skin and mean body temperatures, heart rate, perceptions of warmth, thermal discomfort, and sweating. Interestingly, these all occurred without changes to core temperature measured via ingestible pill due to the increase in whole-body sweating. Together, this would imply that the commercially available far infrared sauna used in this current protocol increased the temperature of superficial tissues to a greater extent than deep muscles, heating is negligible by 3.8 cm below the skin surface, and the effective thermal penetration is about 2.4 cm (or a little less than an inch) in young, healthy adults.

Pattern of muscle heating

Before heating, the deep muscle (3.4 cm) had the highest before-heating temperature (35.4°C) and the superficial muscle (1.4 cm) had the lowest before-heating temperature (34.2°C). This temperature gradient before heating was expected as the superficial thermocouple was the furthest away from the core and closest to the external environment. The deep muscle had the smallest change in temperature during heating (+1.1°C). The superficial muscle had the greatest temperature response (+3.0°C), even when excluding the two individuals with the superficial probe in the subcutaneous fat (+2.4°C). The pattern was a depth dependent temperature gradient before heating and a more homogenous temperature profile at the end of heating, as shown in Figure 5. This is similar to previous findings of multi-depth muscle temperatures with hot water immersion, localized heating, and exercise (12, 23, 57, 58). While the patterns are similar, the muscle temperatures at the end of the far infrared sauna session (36.5–37.2°C) were lower compared to previously observed muscle temperatures with exercise (37.8–40.2°C)(9, 11–16) and passive heating (37.5–40°C)(16, 20–24).

Figure 5.

The muscle temperatures at each depth at 5 min intervals during the 45 min sauna session and the 30 min post-heating period. The means and 95% confidence intervals are shown for deep (white), middle (gray), and superficial (black) muscle temperatures.

Effective thermal penetration

The claim of far infrared waves penetrating up to 3 to 4 cm (1.5 inches) below the surface is cited across scientific and media platforms (6, 32). To our knowledge, there is a lack of data supporting this statement and it appears that there is heavy reliance on its mention in one review (32). Penetration depth is defined as the depth at which the radiant energy is at 37% of the original intensity (56) and absorption is the increase in energy within a target matter originating from a source. The penetration depth and absorption of electromagnetic waves are dependent on the wavelength and the composition (type and thickness) of the target tissue. Water has a high absorption capacity within the far infrared range due to the oxygen-hydrogen bonds and is essential for the use of far infrared waves as a heating stimulus. Far infrared waves penetrate the tissue and are absorbed by the molecular bonds of water, resulting in an increase in vibration, stretching, rotation, and bending of the molecular bonds with heat as the byproduct. It has been discussed that far infrared energy is absorbed more by the superficial tissues due to high water content, thus, penetration depth is more shallow than other bands of electromagnetic radiation (59). There is also reflection and scattering of the infrared waves which may further reduce penetration depth. To adequately quantify penetration depth and absorption, ex vivo work with specialized equipment such as a far infrared emitter source, a target tissue of known depth, and a detector (i.e., Fourier Transform Infrared spectroscopy) is needed. While this would be the ideal way to quantify the depth to which far infrared waves penetrate human tissue, we employed an alternative and more practical or applied approach, one that intends to capture the penetration of not only the primary infrared waves but also the potentially conductive conveyance of thermal energy (that is absorbed by more superficial tissue) toward deeper tissue. Hence, we determined the effective thermal penetration of far infrared as it is deployed in our commercially available sauna to be 2.4 cm, or a little less than an inch.

Our findings are from only one anatomical region (lateral thigh) from a small cohort of individuals (young, healthy). Regions with less subcutaneous fat and/or more superficial structures may have greater increases in regional muscle temperatures. On the contrary, individuals with more subcutaneous fat may have lesser increases in muscle temperatures due to more of the far infrared energy being absorbed in a thicker superficial layer and less reaching the underlying muscle. In our cohort, subcutaneous fat thickness (0.72 cm) comprised ~30% of the effective thermal penetration whereas the subcutaneous fat thickness in a cohort of individuals with obesity (2.3 cm) (60) would have comprised ~96% of the effective thermal penetration. Furthermore, subcutaneous fat is a thermal insulator (low thermal conductivity) (53, 61), so we speculate that heating of subcutaneous fat may lead to less conductive heating of underlying muscle, but this assumption is untested. We do not know if heating subcutaneous fat will have health benefits, but perhaps fat inflammation or metabolic function would be improved, if the tissue temperature remains elevated (62, 63). It is also possible that heated subcutaneous fat would help maintain muscle temperature for longer following heating. Rodrigues et al. reported longer temperature recovery of the vastus lateralis temperature following hot water immersion (42°C for 120 min) with greater subcutaneous fat thickness (23). Therefore, anatomical regions and/or populations with differences in subcutaneous fat thicknesses should be considered in the application of our findings and future research, especially in the context of utilizing far infrared saunas to target cardiovascular, metabolic, and musculoskeletal function and health.

Infrared penetration versus thermal conduction

It is important to note that a temperature increase at a depth may not necessarily equate to the far infrared waves penetrating to that depth, as introduced above. Early work comparing localized heating of muscle with a near infrared lamp (1 μm) versus a far infrared carborundum rod (4–5 μm) showed similar increases in muscle temperatures despite different wavelengths (57, 58). These early reports concluded that conductive heat exchange between adjacent tissues cannot be ignored and may explain the similar temperature profiles. The contribution of conductive heat exchange with a commercially available far infrared sauna may be overlooked as marketing emphasizes the radiant heat thermal stimulus, often characterized as more “directly” or “efficiently” heating the body, at lower sauna temperatures (64). However, comparing other passive heating modalities suggest that ambient temperature may be more important than electromagnetic wavelength. Specifically, muscle temperature increased more (by ~2°C at 3.5 cm depth) with a traditional “Finnish” sauna at 80°C (24) compared to the current far infrared investigation (by ~1°C at 3.4 cm depth).

Whole body thermic effects of far infrared sauna

Similar to the prior report by Atencio et al. (41), there was no change in core temperature via an ingestible telemetry pill after 45 min in the far infrared sauna. One prior report, which relied on pulmonary artery temperature, found a 1.0–1.2°C increase core temperature in cardiac patients with far infrared sauna (65). Pulmonary artery and intestinal temperatures are thought to compare favorably during whole-body passive heating (66). Another report, using tympanic temperature, found a ~1.8°C increase in adults with type 2 diabetes during far infrared sauna (39). However, tympanic temperature is considered a limited estimate of core temperature in a warm environment (67). Thus, far infrared sauna does not typically elevate core temperature when measured by best methods in healthy individuals, but results have varied based on methodology and population. Other sources of variation across studies include variation in temperature throughout the sauna, the size of the sauna, distance from the far infrared panels, and duration of exposure. We opted for 45 min of sauna time to match the overall heating time with Waon therapy (15 min sauna, 30 min blankets). However, there was ~50% more exposure to far infrared radiation in our protocol compared to the typical Waon therapy sessions and yet no changes to core temperature. During Waon therapy, the post-sauna 30-min period that is spent wrapped in blankets may prevent effective convective and evaporative heat loss, thus generating a more sustained elevation in body temperature during this phase. In our protocol, the rate of evaporative heat loss via sweating was sufficient to achieve thermal balance throughout the passive heating.

Even though we found no change in core temperature, our participants reported increased thermal sensation (feeling warmer), thermal comfort (more uncomfortable), and perceived sweating (sweatier). The perception of sweating was corroborated by measured sweat losses. These observations are consistent with Atencio et al. (41). Clearly, far infrared saunas elicit whole body thermal responses. In this case, thermal sensation and responses may be dictated more by skin and peripheral tissue temperatures (possibly including the spinal cord) whereas with other passive heating modalities they may be dictated by skin, peripheral tissue, and core temperatures (54, 68–72).

Thermoregulation pathways

Body temperature of humans is regulated at neural circuits in the preoptic anterior hypothalamus that receive inputs from peripheral (skin) and central (brain, spinal cord, visceral organs, and great veins) thermoreceptors (Romanovsky, 2007). Additionally, the skeletal muscle may also have thermosensitive afferent signals or non-thermosensitive afferent signals that elicit thermoregulatory responses with small increases in temperature, but this is not well documented in humans (Todd et al., 2014). Early work determined the importance of peripheral afferents for the onset threshold and sensitivity of thermoeffector output (73, 74) which supports the use of a calculated mean body temperature, weighted for both skin and core temperature, for quantifying thermoeffector output (75, 76). In the present study, the rise in calculated mean body temperature was driven solely by the elevation in skin temperature, which would evoke sweating (and although not measured, cutaneous vasodilation) to offset increases in core temperature (74, 77).

It is unknown if central thermoreceptors within the spinal cord or brain would be sensitive to far infrared heating. While the preoptic anterior hypothalamus is likely too deep to be impacted, it remains plausible that far infrared radiation could warm the spinal cord and some thermosensitive brain regions at depths comparable to what was observed for muscle. Overall, it would appear that peripheral thermoreceptors in the skin are the primary drivers of thermoeffector responses, and additional contributions from deeper tissues would likely be less due to smaller changes in tissue temperature.

What do we expect from routine far infrared sauna use?

It is tempting to believe that far infrared sauna may offer a more tolerable form of passive heating due to the minimal changes to core temperature and the sensation of being warm but not overly so. However, it is unclear if the passive heat stress pattern of elevated skin and superficial peripheral tissue (e.g., muscle) temperatures in the absence of core temperature elevation is a sufficient stimulus for adaptations to the cardiovascular, metabolic, and muscular systems that are often touted for heat therapy and heat acclimation. Passive heating, including with far infrared saunas, has also been proposed as a means to reduce cardiovascular strain during heat waves (78) but this has not been directly investigated. Additionally, far infrared emitters have been used locally and shown improvements in lymphedema and neuropathy (79–81) but the parameters (wavelength, distance) appear to be more regulated and controlled than with commercially available saunas.

Local elevations in skeletal muscle temperatures between 39–40°C have previously shown to elicit muscle mitochondrial adaptations (i.e., content and function) and heat shock proteins (21, 22, 82) whereas lower skeletal muscle temperatures of ~37°C were still sufficient to result in vascular adaptations (i.e., endothelial nitric oxide synthase, capillarization) within the skeletal muscle (25–27, 40). The local vascular adaptations could be due to the increased blood flow and shear stress patterns in the femoral arteries (common, superficial, and profunda) with isolated thigh heating that were similar to moderate whole body heat stress (83). While in vitro and in vivo glucose uptake increased with increasing muscle temperature in rats (38–42°C) (29), prolonged passive heating without an increase in core temperature, and likely lower skeletal muscle temperatures, did not change glucose transporter type 4 (GLUT4) expression in human skeletal muscle (25). Therefore, one would predict that the modest increases in muscle temperature may elicit local adaptations (i.e., vascular) whereas cellular adaptations (i.e., heat shock protein, mitochondrial function or content, glucose transporters) may be absent, limited, or more superficially located and not broadly expressed in skeletal muscle with the commercially available far infrared sauna used in this protocol.

In the present study and recent report by Atencio et al. (41), the whole body response to far infrared sauna, evidenced by increases in heart rate and sweating, was modest when compared to hot water immersion (i.e., conventional hot tubs) and traditional sauna. The heart rate response to far infrared sauna was similar to what was reported with Waon therapy (39, 65). As passive heating can be promoted as an exercise mimetic (8, 84), a commercially available far infrared sauna may offer a starting point for individuals with a lower thermal tolerance and/or cardiovascular fitness before progressing to other heating modalities or exercise. While the modest thermal and cardiovascular stimuli may hold merit over a sedentary lifestyle, some commercially available saunas may not offer the most robust adaptations given the large differences in the heart rate, cardiac output, and core temperature responses between passive heating modalities reported recently (41). It is unknown whether there is an increase in shear stress with our temperature patterns, or if the increase is sufficient for subsequent endothelial adaptations (85, 86), as shown with other passive heating modalities (87). Recent findings reported lower systolic blood pressure after repeated far infrared sauna bathing in older adults (40). However, there were no measures of core or muscle temperatures to compare temperature patterns with their model of a far infrared sauna (40). Lastly, mild increases in muscle temperature are associated with increased contractile function (88). Far infrared saunas may be appealing as a pre-exercise warm-up modality as the peripheral tissues are warm but do not hinder performance due to thermoregulatory and cardiovascular adjustments that can come with higher core temperatures (89, 90).

Conclusion

We conclude that skeletal muscle temperature increases during a single far infrared sauna session and these increases are depth dependent up until 3.8 cm below the skin surface. While this would align with the claim that far infrared penetrates 3 to 4 cm below the skin surface, the increase in temperature is likely due to a combination of both radiant and conductive heating and the effective thermal penetration is much less (2.4 cm, or less than an inch). Despite these muscle temperature increases and due to the lack of core temperature changes, far infrared saunas may offer a more tolerable form of passive heating for individuals naïve to or with a lower thermal tolerance for heat while targeting the more superficial levels of skeletal muscle. While more tolerable, we speculate that the cardiovascular, metabolic, and musculoskeletal adaptations or heat acclimation adaptations may be limited with repeated use in healthy and clinical populations compared to other forms of passive heating. We intend for the results from this exploratory protocol to be considered to further the understanding and application of commercially available far infrared saunas to optimize targeted outcomes.

Supplementary Material

Supplemental Tables S1-S4: DOI:10.6084/m9.figshare.27849996.

DATA AVAILABILITY

Source data for this study are not publicly available due to privacy or ethical restrictions. The source data are available to verified researchers upon request by contacting the corresponding author.