The Stress Relief Effects of Foot Warming during Mental Workload

Masahiro Inazawa, Yuki Ban, Rui Fukui and Shin’ichi Warisawa

Development of Human and Engineered Environmental Studies, Trontier Sciences,

The University of Tokyo, Kashiwa, Chiba, Japan

Keywords: Temperature Stimuli, Stress Relief, Biomarker, Comfort Presentation.

Abstract: Stress has become a social problem in recent years, and stress control plays a key role in daily life. Researchers

have studied methods of stress detection and spontaneous stress relief such as listening to soothing music and

walking in a forest. However, some people are unable to take spontaneous breaks; therefore, the development

of a means of taking “nonexplicit breaks,” to relieve stress unconsciously while working, is required. In this

study, we proposed a stress-relief method that did not disturb working. To relieve stress, individuals warm

their feet while working, because their hands and feet often become cold when they are under stress. We

examined the effect of stress relief via foot warming, and the results revealed that warming the feet caused an

increase in RR intervals related to relaxation levels.

1 INTRODUCTION

In recent years, suicide has become a serious problem

among workers, particularly in Japan. In total, 28.2%

of Japanese people who died by suicide were

employees, and 42.1% of the causes of suicide

involved mental diseases such as depression

(National Police Agency, 2017). Depression has been

found to result from chronic stress

(Yuanyuan, 2013),

and it is necessary to detect and relieve stress in the

work environment to prevent this. Studies have

examined some methods of stress detection and

measurement. These methods include questionnaires,

such as the Perceived Stress Scale 14, which uses

physiological signals (e.g., electrocardiograms and

respiration waveforms), and behavioral reactions

involving a keyboard or mouse movement (Alberdi,

2016). Stress can be relieved using luxury goods,

such as tobacco and coffee breaks, and activities such

as meditation

(Alberto, 2009), walking, listening to

music (Taelman, 2009), and looking at natural images

(Kotake, 2004; Ikei, 2013). The stress-relieving

effects of these methods have been demonstrated in

both actual experiences and research.

However, only workers who are able to interrupt

their work and take breaks voluntarily can use these

methods to relieve stress. Workers who accumulate

chronic stress tend to be poor at taking voluntary

breaks; therefore, spontaneous stress-relief methods

could be ineffective for this group. Moreover,

because it is common for workers to be unaware of

their own stress levels, they often fail to suspend their

work spontaneously and take stress-relieving action.

Therefore, there is a high demand for methods that

relieve stress effectively while allowing individuals

to continue working.

We proposed a stress-relief method that did not

disturb work. To relieve stress during work, we

presented participants with tactile stimuli to their feet,

because this conforms to the workplace environment

and does not interrupt tasks. Visual and auditory

stimuli can interfere with work, and olfactory stimuli

are difficult to present selectively because of

diffusibility; therefore, these stimuli were unsuitable

for our proposed approach. Of the tactile stimuli, we

chose to expose participants’ feet to heat during work.

The reason for this choice was that the skin

temperature of the foot decreases with mental stress

(Elam, 1987), and we hypothesized that the opposite

would be true.

2 RELATED WORK

Research regarding stress is largely divided into that

involving stress detection and that examining stress

relief.

2.1 Stress Detection

Research examining stress detection is roughly divid-

Inazawa, M., Ban, Y., Fukui, R. and Warisawa, S.

The Stress Relief Effects of Foot Warming during Mental Workload.

DOI: 10.5220/0007368901470154

In Proceedings of the 12th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2019), pages 147-154

ISBN: 978-989-758-353-7

147

ed into research to calculate stress using biological

information with electrocardiography and pulse

waves as indicators, and that exploring the extraction

of feature quantity using behavior such as keyboard

typing and mouse and body movement. In addition,

some studies have confirmed the usefulness of

psychological questionnaires in stress detection

(Alberdi, 2016).

With respect to biological information, it is clear

that the RR interval (RRI), which is the R wave

interval in the electrocardiogram, and its variance

value decrease in stress tasks

(Taelman, 2009). The

Cardiac Sympathetic Index (CSI) and Cardiac Vagal

Index (CVI), which are calculated from the RRI

Lorentz plot and were developed by Toichi

(Toichi,

1997), are stress indices. Moreover, Hayashi

confirmed an increase in CSI scores and decline in the

CVI following a stress task, while the opposite

occurred after a relaxing presentation

(Hayashi, 2018).

The skin temperature in the body’s extremities is also

an indicator of stress. Zenju reported that the

difference between the skin temperature on the nose

and the forehead increased with pleasant stimulation

via classical music and decreased with deep

stimulation via scratching sounds

(Zenju, 2004).

In addition, behavioral reactions involve using

input feature quantities of keyboards and mice (Vizer,

2009; Salmeron, 2014), and psychological

questionnaires include the Perceived Stress Scale

developed by Cohen (Cohen, 1983) and the Stress

Response Scale 18 developed by Suzuki

(Suzuki,

1997).

In this study, we selected biological information

that could be obtained under experimental conditions

while participants were in a sitting position, even

during cognitive tasks. Because the RRI and CVI

scores increased during relaxation following the

cognitive task in our preliminary experiments, we

selected them as indicators of stress. We paid

particular attention to change rates in these indicators

before and after stress relief.

2.2 Stress Relief

As this study aimed to reduce stress in the workplace,

we examined related research examining stress relief

from the perspective of methods that can and cannot

be performed in the work environment. Those that

cannot be performed in the workplace but exert

stress-relieving effects include forest bathing

(Hansmann, 2007; Stigsdotter, 2017) and exercise

(Guszkowska, 2004).

Stress relief in work environments is based on

active behavior or passive presentation. Mindfulness

therapy is an example of an actively performed stress-

relief method, and numerous studies have

demonstrated that this method exerted stress-

relieving effects (Grossman, 2004). However,

Bohlmeijer et al. showed that mindfulness did not

alleviate depression caused by chronic stress

(Bohlmeijer, 2010).

Examples of the stress mitigation method

involving passive presentation include viewing

natural images (Kotake, 2004; Ikei, 2013), listening

to classical music (Yuko, 2011), and inducing

respiration (Zhu, 2017). However, these methods

stimulate visual and auditory senses, which are

important in the workplace, and could affect task

performance.

Therefore, we selected foot warming as a stress-

relief method that did not interrupt tasks and

conformed to the work environment. The reason why

we chose this approach was that exposing the feet to

stimuli should not interrupt desk work. Moreover,

because it is clear that foot temperature decreases

with stress (Elam, 1987), we hypothesized that the

opposite would be true. In fact, some studies have

succeeded in making the impression of drink better

by warming user’s nose, which is based on the

opposite fact that a nose temperature decreases in

stress condition (Chie Suzuki, 2014).

2.3 Stress Relief via Warming

Previous studies examining thermal and relaxing

effects have verified the effect of wrist warming on

stress reduction following cognitive tasks (Ayami,

2017). Although a significant difference in

psychological questionnaire scores was observed

according to exposure to warming, there was no

significant difference in biological information.

Furthermore, presenting thermal stimuli to the wrist

could inhibit task execution in work environments

that involve mainly personal computer use. In

addition, this previous research did not explain how

thermal stimuli affected users’ task performance.

Therefore, in the current study, we exposed

participants’ feet to heat to relieve stress, as we aimed

to relieve stress in the workplace without hindering

task performance. Moreover, we verified that our

method was compatible with the work environment

by examining the influence of foot warming on task

performance.

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3 STUDY 1: THE

STRESS-RELIEVING EFFECT OF

FOOT WARMING

We conducted an experiment to determine the extent

of stress reduction resulting from foot warming

during a task, using biological information. Nine

participants (seven men and two women, aged 22–25

years) were included in the experiment. To examine

the stress-reducing effect of warming, we analyzed

change rates for the RRI and CVI. In previous studies,

RRI compared between breaks and a Mensa test

changed by 0.968, from 0.816 ± 0.13 to 0.790 ± 0.13

(Taelman, 2009). In the transition from a stress state

to a relaxed state, we expected the RRI to be 1.033

times the reciprocal number. Therefore, we

hypothesized that RRI would change by 1.033

following foot warming.

3.1 Foot-Warming Method

Prior to Study 1, we conducted a preliminary

experiment to examine the effect of warming. In this

experiment, we exposed participants’ toes to a 35° C

thermal stimulus, using the Peltier element. However,

there was no trend in biological information. We

concluded that the reason for this result was that the

temperature was low, and the size of the stimulated

area was narrow. Thus, in Study 1, we used a Hottie

(Figure 1) to warm the entire foot at a high

temperature. By injecting boiling water into the water

injection port, the contact temperature with the foot

increased to 50° C at maximum, as shown in Figure

2, and it is possible to provide a high temperature and

thermal stimulus to the large area of the foot, which

was not possible with the Peltier element in the

preliminary experiment. Regarding the distribution of

the presentation temperature, the foot temperature

was highest, and the heel temperature was lowest.

Figure 1: Experimental environment and Hottie.

Figure 2: Contact temperature with the foot.

3.2 Procedure

We performed experiments according to the

flowchart shown in Figure 3. The experimental

environment is shown in Figure 1.

Figure 3: Experimental flow.

3.2.1 5-Min Rest

Participants rested for 5 min before performing a

cognitive task. The aim of this stage was to collect

biological information while they were a calm state.

3.2.2 30-Min Cognitive Task

Participants performed an n-back cognitive task for

30 min (Kirchner, 1958). Continuous numerical

values were displayed on the computer, and

participants provided n numbers before the displayed

numerical values (Figure 4). In this study, n = 2, and

a number between 1 and 9 was presented every

second.

There were two reasons for choosing the n-

back task. The first was that it was necessary to set a

problem with a high degree of difficulty. We had

conducted experiments using another task, but it was

too easy for participants, and task performance did

not change; therefore, we could not verify the effect

of foot warming on task performance. The second

reason was that it was necessary to impose fatigue on

participants, given the work environment. In the n-

back task, participants use working memory, which is

needed for desk work. Participants provide numbers

in the task, using the keyboard, and have not been

informed about the task duration.

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Figure 4: The n-back task. The upper image shows the

numbers displayed, and the lower image shows the numbers

provided by participants.

3.2.3 10-Min Foot Warming (Temperature

Presentation Time)

Warming was performed 20 min after the cognitive

task, using a device that connected a silicone tube to

a Hottie via a funnel (Figure 1). During the

presentation of the temperature stimulus, 1 liter of

water was poured manually into the funnel. The

temperature of the water differed between the

experimental conditions described in Section 3.3.

Participants were not informed as to when the

temperature presentation would begin.

3.2.4 Questionnaire

After completing the cognitive task, the participants

completed a questionnaire concerning the

temperature stimulus. The questionnaire items

pertained to temperature sensation,

pleasure/discomfort, and a free response.

Temperature sensation was measured in seven grades

ranging from cold to hot, and comfort was measured

in seven grades ranging from discomfort to pleasure.

3.3 Experimental Condition Design

To compare the effects of the thermal stimulus, each

participant completed two types of task. One task

involved a treatment condition in which boiled water

was poured at “temperature presentation time,” at a

maximum of 50° C. The other task involved a control

condition in which water was poured at room

temperature (approximately 30° C ) at “temperature

presentation time.”

Each participant completed the two tasks in 1

day. Participants took a break of at least an hour

between tasks. The order of the tasks was

counterbalanced across participants. The participants

received no information about the experimental

conditions.

3.4 Biological Information

It is necessary to measure increases and decreases in

stress to verify the effect of stress relief resulting from

foot warming. We referred to previous studies and

preliminary experiments and used the RRI and CVI

scores as indicators of stress.

3.4.1 Electrocardiogram

Electrocardiogram waveforms were acquired using

an electrocardiogram sensor. The RRI is an indicator

of stress. If the variance value of RRI is large, the

individual is in a relaxed state, and when it is small,

the individual is in a state of concentration or stress

(Taelman, 2009). SCI and CVI scores, which are

calculated via Lorentz plot, are also stress indices

(Toichi, 1997; Hayashi, 2018). Of the information

obtained from these indices, RRI and CVI were used

to indicate stress in this experiment.

3.5 Results

RRI is the average value for 1 min, and CVI is

calculated using the Lorentz plot of the RRI for 1 min.

In addition, the number of correct answers per minute

of the n-back task was defined and calculated as task

performance. Consequently, the RRI increased after

foot warming (Figure 5). We calculated the number

of times each index increased 5 min before

(approximately 20 min from the start of the cognitive

task) and after temperature presentation. The average

change rates for the RRI, CVI, task performance, and

standard errors under in the warming condition were

as follows: RRI: 1.045 ± 0.0106, CVI: 0.966 ± 0.0141,

and task performance: 0.912 ± 0.0291. The results

observed for the control condition were as follows:

RRI: 1.019 ± 0.00679, CVI: 0.976 ± 0.0111, and task

performance: 0.980 ± 0.0368 (Table 1, Figure 6). In

addition, the rates of change in the warming and

control conditions for each indicator were subjected

to a two-tailed t test with correspondence (assuming

that the population variance was equal). The p values

were as follows: RRI: .07, CVI: .60, task

performance: .19. The questionnaire results showed

that the score for discomfort/pleasure resulting from

warming was 3.8 ± 0.8 out of 7,0, and that for the

control condition was 3.4 ± 0.4 out of 7.0. Further, the

temperature sensation scores were 6.8 ± 0.2 for

warming and 3.8 ± 0.37 in the control condition.

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Figure 5: One participant’s result for the RR interval.

Table 1: Change rates observed after stimulus presentation.

Figure 6: Change rate after stimulus presentation.

3.6 Discussion

We sought to determine whether the two-back task

induced stress, by examining task performance

reduction. Previous studies showed that stress

induced working memory loss (schoofs, 2008).

Therefore, two-back task performance reduction

could be considered to reflect stress. In the current

study, means and standard errors for the first and last

5 min of performance were 52.322 ± 0.853 and

41.753 ± 1.112, respectively. The t-test (one sided, no

correspondence, with equal dispersion) p value was

<.01, suggesting that the two-back task induced stress.

Comparison of the warming and control

conditions showed that the increase in the RRI was

greater in the warming condition, relative to that

observed in the control condition (p = .074). As the

RRI increased by 1.045 ± 0.0106 following foot

warming, the change in the RRI resulting from

warming was similar to that resulting from rest.

Regarding CVI, although the p value was

nonsignificant (p = .52), CVI scores in the warming

condition decreased to a level lower than those

observed in the control condition. Because CVI

scores represent the function of the parasympathetic

nerve, they indicated that hyperthermia weakened the

action of this nerve. The reason for this finding could

have been that the participants were awakened by foot

warming.

Regarding performance, although it declined in

both conditions, the reduction in performance was

greater in the warming condition, relative to that

observed in the control condition (p = .19). This

decline could have occurred because working

memory decreased in the two-back task. In addition,

the reason for the greater decline in performance in

the warming condition, relative to that in the control

condition, could have been that participants in the

experimental group became distracted. In the

warming condition, the temperature of the foot

contact position reached a maximum of 50° C, and

many participants reported that the stimulus

temperature was high.

Participants’ questionnaire responses regarding

temperature sensation were as expected, in that they

felt warmth upon exposure to heat and did not feel

warmth or coldness in the control condition. In the

responses regarding pleasure/discomfort, there was

little difference between conditions. Notably, one

participant answers two out of seven grades

(discomfort) in the questionnaire of

pleasant/discomfort after warming condition,

although his or her RRI changed higher than control

condition. That is, the RRI showed a greater increase

in the warming condition, relative to that observed in

the control condition. Therefore, although the RRI

reaction usually occurs in a relaxed state, this

subjective evaluation was contradictory.

Based on the above results, the p value for the

increase in the RRI was relatively low with foot

warming, and task performance declined. In addition,

the results showed possible conflict between the RRI

result and subjective evaluation. However, there was

some doubt as to whether the RRI increase occurred

as an effect of foot warming. There are three reasons

for this doubt. The first was that, after the experiment,

some participants reported that the feeling of water-

induced pressure on the foot was pleasant. The second

reason was that some participants’ RRIs increased in

both the warming and control conditions (Figure 7).

The third reason was that the RRI increased before

the temperature increase in one instance (Figure 8).

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Therefore, it is possible that the RRI increased

because of the pressure resulting from the water,

rather than foot warming. To clarify this issue, the

factors underlying the RRI increase were clarified in

Study 2.

Figure 7: One participant’s RR interval.

Figure 8: The RR interval and Hottie temperature.

4 STUDY 2: CLARIFICATION OF

THE STRESS-RELIEF FACTOR

To determine whether the increase in the RRI in

Study 1 occurred because of foot warming or the

pressure of the water, Study 2 included the following

two conditions. The experimental procedure and

environment were similar to those of Study 1. Nine

participants (six men and three women aged 22–25

years) were included in Study 2A, and 10 participants

(eight men and two women aged 22–25 years) were

included in Study 2B.

4.1 Foot Warming Method 2a

We used a cardboard form, as shown in Figure 9, to

avoid exposing participants’ feet to the pressure of the

water. The contact temperature with the foot in this

condition is shown in Figure 10. In Study 1, it reached

50° C, and we could not compare the temperature

with only that in the condition in Study 1. Therefore,

we established the condition in Study 2B.

Figure 9: Cardboard mould in the Hottie.

Figure 10: Contact temperature with the foot.

4.2 Foot-Warming Method 2b

To verify that there was an increase in the RRI in the

warming condition with the pressure experienced in

Study 2A, the temperature of the water was adjusted

to that presented in Figure 10. The equipment and

procedures used in Study 2 were the same as those

used in Study 1.

4.3 Results

Each indicator was calculated using the method used

in Study 1. Table 2 shows the increase in the RRI, the

CVI, and task performance in the warming and

control conditions in Studies 2A and 2B; the results

of Study 1 are included for comparison. Figures 11

and 12 show the results of the increase values for the

indices used in the warming and control conditions in

Studies 2A and B. The results obtained via

questionnaire are shown in Figure 14.

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Table 2: Change rates after stimulus presentation.

Figure 11: Change rates after stimulus presentation in Study

2A.

Figure 12: Change rates after stimulus presentation in Study

2B.

Table 3: Questionnaire results.

4.4 Discussion

RRI in the warming condition showed a greater

increase relative to that observed in the control

condition in Study 2A, but there was almost no

difference between conditions in Study 2B (p = .85).

In a previous study (Taelman, 2009), weak warming

and a break in stimulus presentation increased the

RRI to the same extent; therefore, weak warming and

pressure impression presentation did not affect the

RRI. Looking at the questionnaire of the temperature

sense, the warmth is stronger in the order of STUDY

1, 2 - a, 2 - b, and the rate of increase of RRI also

follows this order. So it is suggested that RRI

increases due to feet warming, and the stronger the

temperature sense, the more RRI rise.

CVI scores showed similar trends in Studies 1

and 2. Therefore, regardless of the presentation of

pressure, CVI scores decreased following warming,

suggesting that warming reduced CVI scores

regardless of the extent of the temperature or pressure.

The findings suggested that the RRI increased

and CVI decreased with warming. Further, as CVI

scores decreased, parasympathetic nervous activity

decreased, and sympathetic nervous activity was

considered dominant (Toichi, 1997). Moreover,

because the RRI increased, participants were at rest,

even though they were in a state of

stress/concentration.

The results of the questionnaire and temperature

sensation was the same way as in STUDY 1. With

respect to pleasantness, two participants in Study 2A

and three people in Study 2B showed subjective

evaluation in conflict with the RRI during warming.

Moreover, task performance showed a greater

increase with warming, relative to that observed in the

control condition, in Study 2A (p = .17). However,

there was little difference between conditions in

Study 2B (p = 1.00). Through STUDY 1 - 2, the task

performance was lower only with strong warming

conditions (STUDY 1). Therefore, foot warming

affected task performance only when the warming

temperature was high.

5 CONCLUSIONS

In this study, we examined foot warming as a stress-

relief method that did not disturb task performance.

In Study 1, we warmed participants’ feet during a

two-back task. The results showed that the RRI

increased and CVI scores decreased with foot

warming, regardless of pressure. In addition,

sympathetic nervous activity resulting from foot

warming was similar to that observed with a break

from stimulus presentation. Moreover, the RRI and

subjective evaluation were in conflict. Future

research is required to clarify the causes of

sympathetic dominance and verify reactions with

cold stimulus presentation.

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