Crossover trial of the effects of a far-infrared heater that heats the feet with ceramic balls on autonomic nervous activity and mood states

Abstract

Introduction:

The accumulation of fatigue and stress creates problems, including reductions in quality of life and productivity.

Objectives:

To investigate the effects of a far-infrared heater that heats the feet with ceramic balls on autonomic nervous activity and mood states.

Methods:

This study was performed as a crossover trial. Participants comprised 20 women. On different days, each participant underwent 15 min of foot warming with the far-infrared heater (far-infrared group) or remained seated for 15 min (control group). Autonomic nervous activity (low-frequency component/high-frequency component, high-frequency) and mood states scales (Profile of Mood States Second Edition and Two-Dimensional Mood Scale for Self-monitoring and Self-regulation of Momentary Mood States) during the study intervention were measured and compared between groups.

Results:

Low-frequency/high-frequency was significantly higher in the control group 10 min after the start of intervention than at baseline (P = .033). Low-frequency/high-frequency was significantly lower in the far-infrared group than in the control group at 5 min (P = .027), 10 min (P = .011), and 15 min (P = .015). High-frequency was significantly higher in the far-infrared group at 5 min (P = .008), 10 min (P = .004), and 15 min (P = .015) than at baseline. High-frequency 5 min after the start of intervention was significantly higher in the far-infrared group than in the control group (P = .033). POMS2 scores improved significantly more in the far-infrared group than in the control group, including in fatigue-inertia (P = .019), tension-anxiety (P = .025), and total mood disturbance (P = .019). Finally, the far-infrared group showed greater improvements in Two-Dimensional Mood Scale-Short Term scores such as stability (P = .002) and pleasure (P = .013).

Conclusion:

Using the far-infrared heater to heat the feet with ceramic balls stabilized and improved mood, reduced Fatigue-Inertia and Tension-Anxiety, and alleviated total mood disturbance. Parasympathetic nervous system activation was observed from 5 min after the start of heating, suggesting that short-duration heat stimulation of the feet is effective.

Introduction

Stress adversely impacts modern society in a variety of forms. The accumulation of fatigue and stress creates problems, including reductions in quality of life and productivity. Society would thus benefit from the development of methods to conveniently and quickly achieve relaxation that can be practiced even by individuals with hectic lifestyles.

The objective of this study was to investigate the effects on autonomic nervous activity and mood states of a far-infrared (FIR) heater that heats the feet with ceramic balls. Users of this dry infrared heater immerse their feet into a container of electrically warmed 8-mm ceramic balls to warm the lower extremities with FIR radiation. The heater is modeled after footbaths and is used by immersing the feet into the container filled with ceramic balls to a depth of about 10 cm. FIR provides heat as the spherical ceramic balls surround the feet and make contact with the skin.

Different FIR heaters employ a variety of heating systems. Classified according to the location of the heater relative to the body, these include heaters that direct FIR rays toward the skin surface from a certain distance,^1,2 dry saunas and other heaters that use convection currents,^3,4 and heaters that surround the surface of the skin with heated ceramic balls or other materials.^5,6 Classified according to the range of heating, heaters can heat the entire body or only parts of the body, such as the back or lower extremities. The FIR heater investigated in this study surrounded the foot with ceramic balls, locally heating the skin of the feet. This small, portable device can be plugged into a standard electrical outlet, allowing individuals to easily use the device in various locations. Another advantage is that the fine movement of the ceramic balls in contact with the skin can be expected to have a massaging effect on the lower extremities.

Physiological effects of exposure to FIR include increased cardiac output, ⁷ improved flow-mediated vasodilatations, ⁷ reduced ankle-brachial index in patients with peripheral artery disease, ⁸ effects on brain activity, ² and better sleep quality. ⁹

Several papers have discussed the relaxation provided by FIR heaters. One study used subjective scales of mood states to evaluate outcomes and found that 40 min of whole-body exposure reduced Depression-Dejection, Anger-Hostility, and Fatigue. ¹⁰ Other studies have examined effects on the autonomic nervous activity as objective endpoints. Effects included activation of the sympathetic nervous system with 40 min of exposing the upper arms to a heater located 30 cm from the skin ² and improved parasympathetic nervous system activity from 25 min during 40 min of exposing the legs to FIR from a distance of 20 cm from the skin. ¹

Other studies that used mood state scales for subjective evaluation found that 15 min in an FIR dry sauna reduced anxiety, depression, and fatigue, ³ increased parasympathetic nervous system activity, and decreased sympathetic nervous system activity in patients with chronic heart failure. ¹¹ As these findings show, various relaxing effects of convection currents in dry saunas and FIR exposure directed from a source a certain distance from the body have been demonstrated.

The relaxation provided by FIR heaters that envelop body parts to warm the skin, however, has been poorly characterized. A sandbath-like FIR heater that warms the entire body using ceramic balls heated with hot water has been developed. Participants exposed to whole-body heat at 50 °C for 15 min using this heater were evaluated for safety in terms of physiological and biochemical parameters and immune effects such as elevations in peripheral white blood cell counts.^5,6 No previous studies, however, have evaluated the relaxation provided by local heat stimulation using such heaters to cover the body or a part thereof.

We decided to conduct this study to investigate the effects on autonomic nervous activity and mood states of locally heating the feet with an FIR heater that covers the feet with ceramic balls.

Methods

Study design

This study was performed as a crossover trial. The sequence of FIR and control exposures was randomly decided by simple randomization using a lottery method.

Participants

Women between 20 and 64 years old were recruited for the period from April 2019 to March 2020. Women are more likely to develop edema in the lower extremities because of the weaker muscle-pumping action compared to men. As a result, women experience lower extremity fatigue more often than men.^12,13 For this reason, this study focused on women. Individuals on cardiovascular medications, receiving outpatient care, with a sensory disorder or injury of the legs, with a pulmonary or neurological condition, or with a previous diagnosis of depression or on medications for depression or anxiety were excluded.

Each participant was evaluated on two separate days (FIR and control days). On the day of FIR exposure, each participant underwent foot warming in a seated position. On the control day, each participant remained seated. Participants were allowed to choose the dates of participation. The sequence of FIR and control exposures, however, was randomly decided by a lottery held on the day of participation. This lottery was conducted by an investigator. The FIR and control days were separated by a washout period of 1 week.

Procedures

Study duration

The study was conducted in March 2020 and from September to October 2020.

Study device

The foot heater used to provide the study intervention was the Omotenashi Ashiyu (Yumeron Kurokawa Co., Fukui, Japan).

The heater measures 470 mm in width, 470 mm in length, and 315 mm in height. An electrical heating wire beneath a metamorphic rock slab with a high heat capacity heats the slab, which releases FIR radiation that warms the ceramic balls that in turn warm the feet (Figure 1). When warmed, the rock slab radiates predominantly FIR. The ceramic balls measure 8 mm in diameter, comprising 70% silica, 20% alumina, and 10% iron, manganese, and other substances. The balls are coated with silver ions to prevent microbial growth. The ceramic balls are held in a wooden container filled to a depth of about 10 cm. The participant immerses the feet in the ceramic balls up to the ankles.

Figure 1.

“Omotenashi Ashiyu” (Yumeron Kurokawa Co.).

Study environment and establishment of conditions

Each participant performed the study task after work. The study was conducted in a private room free of noise, sunlight, and other stimulation. A cloth screen was placed between each investigator–participant pair to prevent the movements of the investigator from affecting the autonomic nervous activity of the participant. Data collection was performed for all participants during the follicular phase of the menstrual cycle to standardize for the effects of this cycle on the autonomic nervous system.

Each participant was asked to sleep sufficiently and avoid alcohol intake from the day before participation, and to avoid eating, intake of caffeinated beverages, smoking, or strenuous exercise during the 2-h period before participation. Before participation, the participant voided and changed into a loose-fitting T-shirt, shorts, and socks provided by the investigator. The FIR and control groups wore the same clothing. The participant covered their feet with nylon bags before placing the feet in the heater to prevent transmission of infections.

Participants in the FIR and control groups remained seated at rest for 10 min before participating to stabilize the autonomic nervous system. Participants underwent measurements in a seated position, with the FIR group seated for 15 min using the heater and the control group seated for 15 min.

Participant safety

Participants were asked to wear socks to prevent the skin from coming into direct contact with the underlying rock slab while using the heater to prevent low-temperature burns. The incidence of low-temperature burns was much higher at temperatures of ≥ 45 °C in a previous study. ¹⁴ The heater was therefore adjusted to remain below 45 °C. The study was configured to stop immediately if the temperature of the skin on the soles of the feet reached ≥ 45 °C. The skin temperature of the soles was measured using an LT-8A skin temperature probe (Gram Co., Saitama, Japan) attached to the skin above the sesamoid bone of the left foot. The temperature was adjusted or the session was suspended if the participant reported discomfort or excessive sensations of hotness in their feet.

Primary outcome

Autonomic nervous system activities.

Secondary outcome

Mood states.

Instruments and surveillance methods

Study environment

The temperature and humidity of the room were monitored during the study.

Autonomic nervous system activities

A real-time autonomic nervous system monitor (Reflex Meijin, Crosswell, Kanagawa, Japan) ¹⁵ was used to measure autonomic nervous activity. In this technique, autonomic nervous activity is evaluated by measuring R–R intervals on electrocardiography and analyzing their frequency domains. These frequency domains are divided into two components: low frequency (LF; 0.05–0.15 Hz) and high frequency (HF; > 0.15 Hz). The HF component is regarded as an indicator of parasympathetic nervous activity. ¹⁶ The LF component is believed to reflect both sympathetic and parasympathetic nervous activity. The ratio of these two components (LF/HF) thus reflects the balance of autonomic nervous activity ¹⁶ and is regarded as a relative index of sympathetic nervous activity. ¹⁷ Arrhythmias or artifacts were detected and deleted automatically by the software.

Mood states

The Japanese version of the Profile of Mood States Second Edition (POMS2) ¹⁸ and Two-Dimensional Mood Scale-Short Term (TDMS-ST) ¹⁹ were administered to each participant before and after intervention for the subjective evaluation of mood.

Statistical analysis

Mean room temperature and humidity from the start to end of each intervention were compared between the FIR group and control group. The mean minimum and mean maximum temperatures at the surface of the soles were calculated together with the mean change in skin temperature.

Mean autonomic nervous activity was calculated every 5 min for each participant. Mean values were compared for 5–10 min seated at rest before the intervention, and for 0–5, 5–10, and 10–15 min after starting the intervention. Changes in activity from when participants were at rest before the intervention in the FIR group were compared with values in the control group. Mood state scale results at the start of the intervention were compared with those at the end of the intervention. Differences from before to after the study session were also compared between FIR and control groups.

Statistical analysis was performed using a paired t-test for environment and skin surface temperatures, and the Wilcoxon signed-rank test was performed for autonomic nervous system activities and mood state scales (POMS2 and TDMS-ST). These analyses were performed using SPSS version 23 statistical software (IBM Japan, Tokyo, Japan). Values of P < .05 were considered statistically significant. A sample size of 19 participants was calculated as necessary to provide 80% power and a 0.615 effect size (G*Power 3.1; Heinrich-Heine-Universität Düsseldorf). ²⁰

Results

Subject characteristics

Twenty-four women consented to participate, of whom three were excluded because of measurement errors in autonomic nervous system activity. Two participants were excluded because of arrhythmia, and one participant was excluded after an electrode detached from the skin during the experiment. Another woman was excluded from the analysis after withdrawing consent. The remaining 20 women were included in all study analyses (Figure 2). None of the participants reported discomfort or excessive sensations of hotness in their feet. The mean age was 38.7 ± 16.7 years (Table 1).

Figure 2.

Enrollment.

Table 1.

Demographic data.

		n = 20
	N	%
Sex
Female	20	100.0
Male	0	0.0
Age
20–29	9	45.0
30–39	2	10.0
40–49	1	5.0
50–59	6	30.0
60–69	2	10.0
BMI (body mass index)
15.1–20.0	6	30.0
20.1–25.0	9	45.0
25.1–30.0	1	5.0
30.1–35.0	4	20.0

Study environment during an intervention

Mean (±standard deviation) room temperature was 23.6 ± 1.5 °C during the FIR intervention and 23.8 ± 1.2 °C during the control intervention, showing no significant difference (P = .751). Mean humidity was 50.4 ± 11.3% during the FIR intervention and 49.3 ± 11.0% during the control intervention, again showing no significant difference (P = .681).

Changes in sole temperature

The mean minimum sole temperature was 33.4 ± 3.2 °C in the FIR group and 31.1 ± 3.4 °C in the control group. The mean maximum temperature was 37.0 ± 2.2 °C in the FIR group and 32.3 ± 3.7 °C in the control group. The mean change in sole temperature was 3.6 ± 1.5 °C during the FIR intervention and 0.5 ± 1.1 °C during the control intervention.

Autonomic nervous system activities

LF/HF: low frequency/high frequency

In the FIR group, the median difference from baseline in LF/HF was −0.25 (interquartile range [IQR] −1.55 to 0.31) at 5 min after starting the intervention, −0.23 (IQR −1.29 to 0.20) after 10 min, and 0.04 (IQR, −0.71 to 0.41) after 15 min. These values in the control group were 0.31 (IQR, −0.23 to 2.14) after 5 min, 0.65 (IQR, −0.33 to 1.58) after 10 min, and 0.48 (IQR, −0.66 to 1.56) after 15 min. LF/HF was significantly higher in the control 10 min after starting the intervention compared to baseline (P = .033, z = –2.128). LF/HF in the FIR group was significantly lower than in the control group at 5 min (P = .027, z = 2.213), 10 min (P = .011, z = 2.539), and 15 min (P = .015, z = 2.427) (Figure 3).

Figure 3.

Difference between before and after low-frequency/high-frequency (LF/HF) intervention.

HF: high frequency

In the FIR group, the median difference from pre-intervention rest in HF was 36.28 (IQR, −1.44 to 124.75) at 5 min after starting the intervention, 41.68 (IQR, 1.96–190.84) after 10 min, and 20.54 (IQR, −0.73 to 185.92) after 15 min. HF in the FIR group was significantly higher at 5 min (P = .008, z = 2.651), 10 min (P = .004, z = 2.857), and 15 min (P = .015, z = 2.427) than at baseline. These values in the control group were −10.12 (IQR, −92.88 to 19.69) after 5 min, 6.69 (IQR, −19.85 to 56.55) after 10 min, and 7.10 (IQR, −25.26 to 120.54) after 15 min. The median difference in HF was significantly higher in the FIR group than in the control group at 5 min (P = .003, z = −2.949), but no significant differences were seen at 10 min (P = .550, z = −.597) or 15 min (P = .709, z = −.373) (Figure 4).

Figure 4.

Difference between before and after high-frequency (HF) intervention.

Mood states

POMS2

In the FIR group, changes in POMS2 from baseline to post-intervention appeared as reductions in confusion-bewilderment (P = .004, z = −2.889), fatigue-inertia (P = .004, z = −2.847), and tension-anxiety (P = .001, z = −3.310). Moreover, the FIR group achieved significant alleviation of total mood disturbance (P = .001, z = −3.401). The control group showed reductions in confusion-bewilderment (P = .004, z = −2.859) and tension-anxiety (P = .026, z = −2.224), along with alleviation of total mood disturbance (P = .003, z = −2.994).

Compared with the control group, the FIR group experienced significantly better improvements in fatigue-inertia (P = .019, z = 2.347), tension-anxiety (P = .025, z = 2.246), and total mood disturbance (P = .019, z = 2.339) (Table 2).

Table 2.

Difference between before and after POMS2 intervention.

	FIR group			Control group				n = 20
	Median	IQR		Median	IQR		z
		25%	75%		25%	75%		P
Anger-hostility	0	−1	0	0	−1	0	−.719	.472
Confusion-bewilderment	−1	−2	0	−1	−2	0	1.386	.166
Depression-dejection	0	0	0	0	0	0	.514	.607
Fatigue-inertia	−1	−3	0	0	−1	0	2.347	.019*
Tension-anxiety	−2	−4	0	−1	−2	0	2.246	.025*
Vigor-activity	2	0	4	0	0	0	−1.596	.111
Total mood disturbance	−7	−13	−2	−3	−6	0	2.339	.019*

POMS2: Profile of Mood States, second edition; FIR: far-infrared; IQR: interquartile range.

Wilcoxon signed-ranks test.

* P < .05.

TDMS-ST

In TDMS-ST evaluations, the FIR group showed significantly better stability (P = .001, z = 3.224) and pleasure (P = .002, z = 3.107). No significant changes were seen in the control group.

Compared with the control group, the FIR group experienced significantly better improvements in stability (P = .002, z = −3.030) and pleasure (P = .013, z = −2.485) (Table 3).

Table 3.

Difference between before and after TDMS-ST intervention.

	FIR group			Control group				n = 20
	Median	IQR		Median	IQR
		25%	75%		25%	75%	z	P
Vitality	1	−1	4	1	0	2	–.868	.385
Stability	2	1	3	0	−1	1	−3.030	.002*
Pleasure	4	1	7	1	−2	3	−2.485	.013*
Arousal	−1	−2	2	0	−2	3	.785	.432

TDMS-ST: Two-Dimensional Mood Scale-Short Term; FIR: far-infrared; IQR: interquartile range.

Wilcoxon signed-ranks test.

* P < .05.

Discussion

The present findings show that using a heater to locally heat the feet for short periods with FIR radiation from ceramic balls reduced the sympathetic predominance, activated the parasympathetic nervous system, reduced fatigue-inertia and tension-anxiety scores, and alleviated total mood disturbance.

HRV is accepted as a noninvasive method for evaluating autonomic nervous system activity in humans. While HF (0.15–0.4 Hz) represents vagal (parasympathetic) control of heart rate, the LF/HF ratio represents sympathetic modulation and the balance between sympathetic and vagal nerves.^16,17,21 HRV has been used in previous studies to evaluate a range of interventions.^22–24

In the FIR group, autonomic nervous activity in terms of LF/HF was lower after 5 and 10 min of the intervention and had returned to baseline after 15 min. In contrast, LF/HF (as a measure of sympathetic predominance) was dramatically increased in the Control group after 5 and 10 min, before decreasing after 15 min. LF/HF at 15 min was higher than at baseline. Throughout the 15-min study intervention, the FIR group showed lower sympathetic predominance than the Control group. HF, a measure of parasympathetic nervous system activation, rose dramatically after 5 min and gradually after 10 min of FIR intervention. HF gradually decreased after 15 min but remained higher than at baseline. HF for the control intervention decreased after 5 min and increased after 10 min, but remained comparable to that at baseline. These findings show that heating the feet with FIR from the ceramic balls of the heater reduced sympathetic predominance and activated the parasympathetic nervous system more than the control intervention.

The FIR and control groups had comparable measurement conditions and measurement environments. As the study had a crossover design, the same subjects were entered in the FIR and control groups in randomly decided order using a lottery, meaning that conditions in the FIR and control groups were the same. The effects on autonomic nervous activity and improvements in mood states noted in the study were thus attributed to the use of the study intervention, that is, the FIR foot heater.

This study showed that heating the feet with ceramic balls produced relaxation throughout the body, rather than just locally. In a previous study, systemic physiological effects including increased cardiac output decreased pulmonary capillary wedge pressure, and improved flow-mediated vasodilatation were observed in participants who underwent local FIR heat stimulation of the lower extremities. ⁷ Other studies of the local effects of exposing the lower extremities to FIR noted improved sleep ⁹ and parasympathetic nervous system activation. ¹ Those studies similarly show that exposure of the lower extremities to FIR provided systemic rather than just local relaxation. Heating the lower extremities with FIR emitted from the ceramic balls of the Omotenashi Ashiyu device used in this study presumably affected autonomic nervous system activity via afferent vagal fibers. Relaxation effects observed in previous studies that investigated the effects of heating the feet with a warm-water footbath include decreased LF/HF and increased salivary immunoglobulin A ²⁵ and increased α1 brain waves. ²⁶ Local heating of the feet with ceramic balls in our study produced relaxing effects similar to those achieved with the footbath.

The maximum skin temperature of 37 °C at the soles of the feet achieved in this study partially explained the reduced sympathetic predominance and increased parasympathetic nervous system activity provided by the foot heater. The surface temperature of the soles had increased by 3.0 °C after 5 min of FIR heater use, increasing by another 0.9 °C from 5 to 10 min and by 0.6 °C from 10 to 15 min. Skin surface temperature increased from 33.5 °C to 37.0 °C over 15 min. In a previous study, participants using a 40 °C mist sauna achieved lower blood levels of adrenaline and cortisol, and heat stimulation reduced sympathetic predominance. ²⁷ Changes in autonomic nervous system activity observed in association with local heat stimulation with a footbath included significantly higher LF/HF at water temperatures of 40 °C and 42 °C and significantly lower HF at 42 °C. Participants subjectively reported a water temperature of 38 °C as the most pleasant. ²⁸ Brain wave changes during footbath use included increased β1 waves and decreased α1 waves at 42 °C and increased α1 waves at 38 °C. ²⁶ In our study, a temperature of 37 °C proved effective for activating the parasympathetic nervous system.

Parasympathetic nervous system activation in our study was achieved after just 5 min of heating, showing that the study intervention provides relaxation with only a short duration of heating. In other studies of FIR exposure from distances of 20–30 cm, autonomic nervous system changes appeared from 15 min to 40-minute heating sessions.^1,2 In a study that used a warm-water footbath, brain wave changes occurred after 5 min of use, and HF changed after less than 10 min of use. ²⁶ Footbath use quickly provided relaxation as in our study. Immersing the feet in a substance heated to the relatively low temperature of 38 °C thus appears to rapidly provide relaxation similar to that provided by a footbath.

The FIR group experienced significantly better reductions in POMS2 fatigue-inertia and tension-anxiety and significantly better alleviation of total mood disturbance compared with the control group. The FIR group showed greater improvement in TDMS-ST stability and pleasure than the control group. These findings show that heating the feet with the study heater reduced tension and anxiety. Given that vigor-activity and arousal were not significantly different in the FIR and Control groups, the heater did not have activating effects on mood. These subjective findings agreed with the objective findings of autonomic nervous system activities, and are thus credible.

Previous studies of the effects of whole-body FIR exposure on mood states revealed improvements in POMS anxiety, depression, and fatigue ³ and reductions in depression-dejection, anger-hostility, and confusion. ¹⁰ Local FIR exposure in our study likewise produced improvements in mood. Morishita et al. ²⁹ in an evaluation of a footbath found reduced tension-anxiety, anger-hostility, and fatigue-inertia, and our subjective findings are consistent with the effects on the autonomic nervous activity they observed in association with footbath use.

Even short-duration use of the ceramic ball heater reduced sympathetic predominance and activated the parasympathetic nervous system, making such intervention recommendable for providing relaxation in hospitals, care facilities, and work settings.

This study has two main limitations. The first was that all participants were healthy women. The present results are thus of limited applicability. In addition, participants in this study included three women over 45 years and under 55 years. Because they were in menopause, these women were at risk of effects on autonomic outcomes. The second limitation of the study was the unblinded design. Blinding was not possible because the study intervention required participants to place their feet in the heater. This limitation may have impacted the results of self-reported mood state scales. The duration of relaxation provided by the ceramic ball foot heater and the effects of repeated use merit further investigation.

Conclusion

This study compared the effects of heating the feet for 15 min with a ceramic-ball heater with the control intervention, that is, remaining seated for 15 min.

The study showed that heating the feet with ceramic balls stabilized and improved mood, reduced fatigue-inertia and tension-anxiety, and alleviated total mood disturbance. Sympathetic and parasympathetic nervous system changes were observed beginning 5 min after the start of heating, suggesting that short-duration heating of the feet was effective.

Author biographies

Yuko Matsui is a professor at Faculty of Health Sciences Department of Nursing, Komatsu University, Japan. She is registered nurse. She received her PhD in Health Sciences from Kanazawa University in 2011.

Terumi Ueda is an assistant professor at Faculty of Health Sciences Department of Nursing, Komatsu University, Japan.

Yumi Koizumi is a professor at Faculty of Health Sciences Department of Nursing, Komatsu University, Japan.

Chinatsu Kato is an assistant professor at Faculty of Health Sciences Department of Nursing, Komatsu University, Japan.

Yuiko Suzuki is an assistant professor at Faculty of Health Sciences Department of Nursing, Komatsu University, Japan.