Exploring combined vibration and music interventions for acute stress reduction: insights from two experimental studies

Abstract

Work-related stress is increasingly recognized as a systemic issue, impacting employees' well-being and leading to severe health risks. Effective intervention strategies are crucial for addressing mental load and chronic stress. This study investigates the effects of tactile stimulation through vibrations, synchronized with heart rate and gradually decreasing in tempo, on stress reduction. Additionally, the study examines the potential synergistic effects of combining these vibrations with music. 

Study 1: objective: To explore the effectiveness of fixed and progressively slowing vibrations in reducing acute stress induced by mental load.

Methods: Male participants (n=42) were randomized into three groups: variable vibration, fixed vibration, and control. Stress was measured at “baseline”, during stress, and “post-intervention” using subjective assessments (tense arousal (TA) and energetic arousal (EA) scores). Statistical significance was evaluated using p-values.

Results: Participants receiving variable vibration showed a significant decrease in TA scores compared to the control group (p<0.05).

Study 2: Objective: To assess the combined effect of vibrations and music on stress reduction.

Methods: A randomized crossover trial was conducted with 36 participants, each undergoing all three conditions: vibration with music, music only, and control, in a randomized order. Stress levels were measured using subjective evaluations (visual analog scale for stress and annoyance, Jikaku-sho shirabe, emotion and mood inventory) and objective assessment (salivary cortisol). Statistical significance was evaluated using p-values. Based on Study 1, Study 2 incorporated music and extended exposure to enhance effects.

Results: The combination of vibration and music significantly reduced VAS-Stress and VAS-Annoyance scores and increased relaxation and mood scores compared to controls (p<0.05). Salivary cortisol levels also showed a more significant decrease in the combined intervention group (p<0.05). Stratified analysis revealed that participants with heart rates matching the intervention tempo experienced the most significant stress reduction.

Conclusions: Gradually decreasing tempo vibrations, particularly when combined with music, effectively reduce acute stress. These findings suggest that tactile and musical stimulation at a progressively decreasing tempo reduces subjective stress and cortisol levels.

Trial registration: Trial registration: ISRCTN10662834. Registered on 08 August 2025.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40359-025-03293-9.

Introduction

Work-related stress has been increasingly recognized as a systemic problem in recent years because of the adaptation of new types of jobs, increased pressure and demand for work quality and productivity, and increased time restrictions. According to the “Work and Well-being 2021 Survey Report” of the American Psychological Association [1], nearly three out of five employees experience negative impacts of work-related stress. More than two in five employees report health and safety factors that impact stress levels at work, such as physical illnesses and ailments and unpleasant or dangerous physical conditions. Given the significant impact of work-related stress on employees' well-being, finding effective strategies to mitigate this stress is crucial. One promising approach involves addressing the mental load that employees face during their tasks. Mental load refers to the cognitive demands placed on an individual during tasks requiring attention and memory. Several studies have demonstrated the positive effects of relaxing stimuli on reducing mental load and cognitive fatigue [15, 20].

Jiang et al. found that the relaxing effect of music improved performance on stressful cognitive tasks by reducing subjective anxiety induced by mental load. Similarly, Koivisto et al. demonstrated that mental imagery of natural environments elicited stronger positive emotions and greater relaxation compared to urban imagery, suggesting that even visualizing nature can counteract mental strain. Moreover, additional studies have shown the physiological and psychological benefits of visual stimulation with green plant types [13], the physiological benefits of viewing nature [16], and the impact of visual stimuli and properties on restorative effects and human stress [27]. The benefits of reducing mental load and stress through relaxing stimuli extend beyond immediate cognitive relief, as chronic stress is linked to severe health risks. Addressing this connection is crucial for developing effective intervention strategies.

Stress is associated with an increased risk of psychiatric disorders, including depression and schizophrenia, as well as systemic diseases, including cardiovascular disease and insulin resistance [17, 22, 24]. Therefore, attenuating mental stress may decrease the risk of disease [3]. Various intervention strategies have been explored to reduce stress and relax people. Slow-breathing techniques [9–11, 14], listening to music with a gradually decreasing tempo [7], and gentle rocking movements on the bed [34] have been shown to induce relaxation. In this direction, vibroacoustic therapy (VAT) has garnered attention in various research contexts due to its potential therapeutic effects. Research indicates that VAT, which involves the application of low-frequency vibration (20–100 Hz) combined with music, has shown promise in positively impacting mental health conditions such as depression, anxiety, and mood disorders [5, 8, 19]. Mechanisms underlying these effects include the activation of the vagus nerve by low-frequency vibration and the relaxation induced by rhythmic sensory stimulation provided by vibration and music [5]. Despite its potential benefits, the complexity and duration of VAT interventions pose challenges. Therefore, efforts have been made to explore ways to simplify interventions while maintaining effectiveness. One approach involves adjusting the frequency of vibration to align more closely with natural physiological rhythms, such as heart rate, to enhance the recognizability and efficacy of sensory stimulation in a shorter time frame [28]. This rationale led to the design of the present study, which aims to investigate the effects of variable and non-variable vibration experiences on mental care, utilizing heart rate as a reference point (Study 1). Additionally, a multimodal system integrating vibration with music was developed to explore potential synergistic effects (Study 2).

The common denominator of these relaxing experiences is a slow rhythm transmitted to the body. However, there is a paucity of information regarding how the body’s recognition of slowly changing rhythms can trigger relaxation, and no studies have directly or systematically investigated this effect. One study showed that music with a rhythm at the same level as the participant’s heart rate was perceived as pleasant and comfortable [12]. Together with the observation that a slow change in rhythm leads to a relaxed experience, we hypothesized that tactile stimulation applied directly to the body in the form of vibrations at the participant’s heart rate, and a gradual decrease in rhythm would induce a relaxation stimulus. After investigating whether direct tactile stimulation induces relaxation, we examined whether another combination of stimuli enhances the relaxation effect of tactile stimulation.

Methods

Study design

This prospective study was conducted as two studies at a test laboratory in Tokyo, Japan. Participants who met the study selection criteria and volunteered to participate were enrolled. Study 1 investigated the effectiveness of fixed and progressively slowing vibrations in reducing acute stress levels induced by mental load. Study 2 investigated the effectiveness of combining progressively slowing vibrations with progressively decreasing tempo music in reducing acute stress levels induced by mental load. Figure 1 summarizes the evaluation methodologies and timelines followed for both the studies.

Fig. 1.

Experimental procedure for stress measurement study. The figure outlines a two-study design measuring stress levels in three different groups subjected to various mental loads and interventions. Study 1 involves a 5-min arithmetic task, followed by different vibration interventions for each group. Study 2 includes a 14-min ATMT and 2-back task, followed by either vibration/music or music interventions. Stress measures are taken before and after each phase using subjective evaluations and salivary cortisol tests

Both studies were approved by the institutional ethics committee and were retrospectively registered with the ISRCTN registry (ISRCTN10662834).

Study 1

Objective

To investigate whether tactile stimulation through vibrations at the participant’s heart rate with a gradually decreasing rhythm induces relaxation.

Participants

Male participants aged between 20 and 49 years were enrolled. Participants received a sufficient briefing of the objective and content of the present study, and signed a written informed consent form. Participants with serious cardiovascular, hepatic, renal, respiratory, endocrine, or metabolic disorders, those with a medical history of these disorders, those taking pharmaceuticals that could affect the autonomic nervous system, or those who showed signs of distress during pre-study briefings, or who had difficulty following basic instructions were excluded from participation. The participants were randomized into three groups: Group 1 (variable vibration), Group 2 (fixed vibration), or Group 3 (no vibration stimulus). The participants were subjected to a mental load and then experienced an intervention (e.g., variable vibration, fixed vibration, or no vibration). The stress state was measured before mental load (“baseline”), after mental load (“stressed”), and then after intervention (“post-intervention”).

Mental load

Mental arithmetic tasks listed in the Trier Social Stress Test (TSST) [18], which are used to evoke stress in social situations, were performed for 5 min. Specifically, the participants were asked by an interviewer to keep subtracting 17 from 2021 and answer orally (e.g., 2021, 2004, 1987, 1970). If a participant made a mistake during the calculation, the interviewer pointed it out and asked the participant to redo the calculation from the beginning. The procedure followed the standard TSST format, with minor adjustments (e.g., specific starting number and subtraction value) to fit the experimental context. As the primary aim of Study 1 was to evaluate the effects of the post-stress intervention, task performance data were not recorded.

Interventions for stress relief

A vibration device was fabricated with a circular outer frame manufactured using a 3D printer, ensuring ergonomic handling. Its center, designed to house a woofer speaker, facilitated the generation of heartbeat-like vibrations upon activation. The vibration cycle was designed based on the general heart rate of a healthy adult to create beats similar to heartbeat vibrations. The vibration device used for this intervention, is shown in Fig. 2. The intervention was provided for 2 min to the three groups as follows:

1. Group 1 (variable vibration): The frequency of the vibration was set to each subject's heart rate at the start of the test, using a heart rate monitor. Once vibration started, the frequency was gradually decreased until the final frequency was half of the participant’s heart rate or 50 bpm, whichever was higher.

2. Group 2 (fixed vibration): The vibration frequency was fixed at half the participant’s heart rate or 50 bpm, whichever was higher.

3. Group 3 (no vibration, control): No vibration was presented. The participants rested while holding a non-vibrating device during the two-minute interval.

Fig. 2.

Vibration device with a built-in woofer speaker used to generate vibrations for relaxation

Stress measurement

Subjective evaluation of stress was performed in terms of tense arousal (TA) and energetic arousal (EA), based on Thayer’s [31–33] research. Two question items with the highest factor loadings for each factor were selected from 33 question items [26] to create an 8-item questionnaire (TA + : feeling restless or uneasy, reflecting tension arousal; TA-: feeling relaxed or calm, reflecting low tension arousal; EA + : feeling energetic or active, reflecting high energetic arousal; EA-: feeling lethargic or listless, reflecting low energetic arousal). Participants responded to each question based on their mood at the time of evaluation on a scale of 1 (not feeling at all) to 10 (feeling very much), and the sum of the scores was calculated as a representative value for each domain (TA +, TA-, EA +, EA-) and used for subsequent analysis.

Statistical analysis

In Study 1, subjective evaluation data were summarized at three stages: before (“baseline”), after mental load tasks (“stressed”), and after intervention (“post-intervention”). Changes in scores for each domain (TA +, TA-, EA +, EA-) from the “stressed” to “post-intervention” states were calculated. Kruskal–Wallis tests were employed to compare these changes between intervention groups, with post-hoc analysis using the Steel test to identify specific group differences. Group 3 served as the control group for comparison. Data are presented as median (25th to 75th percentile). p-values indicate significant differences from Group 3. Non-parametric tests were chosen due to the likely non-normal distribution of the data, ensuring robustness and reliability in statistical comparisons, particularly with small sample sizes and skewed or ordinal data.

Results

Participant characteristic

Forty-two males were enrolled, with 14 participants in each group. No differences in the baseline scores of TA +, TA-, EA +, and EA- were observed between the groups (Online Resource 1).

Outcomes

Subjective evaluation scores. Table 1 shows the effect of the vibration intervention on the subjective measures of stress. A significantly greater decrease in TA + scores between “stressed” and “post-intervention” states was observed in Group 1 compared to Group 3 (control).

Table 1.

Effect of vibration intervention on subjective measures of stress. Values are shown as the median (25th to 75th percentile) Statistical comparisons were conducted using Kruskal–Wallis test followed by Steel’s test versus Group 3 (control). *p < 0.05. (– indicates not significant)

Measure	Group 1 (n = 12)	Group 2 (n = 13)	Group 3 (n = 11) (Control)	Significance vs Control
TA +	9 (5 to 14.5)*	7 (5 to 12)	3 (1 to 6)	*p < 0.05
TA-	−8 (−13.5 to −3)	−6 (−9.5 to −5)	−3 (−9 to 0)	–
EA +	1.5 (−0.75 to 4)	2 (−1 to 6.5)	0 (−2 to 4)	–
EA-	0.5 (−1.75 to 5.75)	0 (−5 to 2.5)	2 (−1 to4)	–

Study 2: methods

Objective

To evaluate whether the combination of tactile stimulation (vibration) and music enhances relaxation more effectively than either music alone or no intervention.

Participants

Male participants aged 30–49 years were enrolled in this randomized, crossover comparison trial. Participants received a sufficient briefing of the objective and content of the present study, and signed a written informed consent form. Exclusion criteria included serious disorders as in Study 1, audiometry abnormalities, or discomfort with headphones. All participants were randomly assigned to receive the three intervention conditions (Condition 1: vibration plus music, Condition 2: music only, Condition 3: control) in a randomized crossover order. Each session was conducted on a separate day (at least one day apart) to allow for a sufficient washout period and minimize potential carryover effects. The ambient music was employed, specifically composed to synchronize with observed vibration patterns. The chosen piece adhered to stringent criteria: it aligned tempo meticulously with the vibration pattern, employed instrumental elements exclusively, and featured a repetitive melody for continuity and tranquillity.

Mental load

Participants were asked to perform multiple tasks to elicit higher acute stress levels; participants performed three sets of A, B, and C tasks (target number: 11–35) of the advanced trail-making test (ATMT) [23] and a 5-min 2-back task using a personal computer. These tasks were selected for their established effectiveness in inducing acute cognitive stress. The total duration of the mental load tasks was approximately 14 min. While detailed stimulus parameters (e.g., trial number, presentation time) were not the focus of this study, the procedures followed standard implementations as described in the referenced literature.

Interventions for stress relief

The intervention was provided for 3.5 min to the three groups as follows:

1. Condition 1 (vibration plus music): The ambient music and vibration were synchronized, and their frequency gradually decreased from 75 bpm (10 s), 70 bpm (10 s), 65 bpm (30 s), 60 bpm (35 s), 55 bpm (35 s), to 50 bpm (90 s). This vibration pattern was set for all subjects.

2. Condition 2 (music): Only music was provided without any additional vibration.

3. Condition 3 (no vibration or music; control). No vibration or music was provided. Following the intervention, all participants rested on a chair for 10 min.

Music

The music used in this study was an original ambient composition specifically designed for synchronized vibro-acoustic stimulation. It was structured to gradually slow in tempo, matching the vibration stimulus. The composition featured only instrumental elements, with a repetitive and calming melody to ensure a soothing auditory experience. Music was delivered through headphones (ATH-250AV, Audio-Technica) connected to a PC (HP ProBook 450 G7, Hewlett Packard Japan, G.K.), ensuring precise synchronization with the vibration output stimulus. The vibration system, previously developed and described in Study 1 (see Fig. 2), was employed again in Study 2 to provide synchronized tactile stimulation.

This composition was originally developed for use in a smartphone-based vibro-acoustic therapy application for stress management, as described by Motokawa et al. (2025, in review).

Stress measurement

Following the schedule described in Fig. 1, the subjective evaluation of stress was assessed using the visual analog scale (VAS) [21], Jikaku-sho Shirabe [30], emotion and mood inventory, and salivary cortisol levels. VAS scores for stress, tension, and annoyance were obtained using a 0–100-mm scale where 0 meant ‘no symptom’ and 100 meant “the worst possible symptom.” Jikaku-sho shirabe, a 25-item questionnaire proposed by the Industrial Fatigue Research Committee of Japan Occupational Health, was used to assess subjective fatigue symptoms on a 5-point scale [30]. Symptoms were categorized into five factors (feeling of drowsiness [Factor I], instability [Factor II], uneasiness [Factor III], local pain or dullness [Factor IV], and eyestrain [Factor V]. The Emotion and mood inventory is a 13-item self-report questionnaire designed to assess four emotional dimensions: tension, depression, anger, and vigor. Participants rated each item based on their current emotional state using a 4-point Likert scale (1 = not at all; 4 = very much). Total scores for each dimension were calculated and used in subsequent analyses to evaluate emotional changes associated with stress and recovery [25]. For the salivary cortisol assessment, participants provided approximately 1 mL of saliva using a Salivette (SARSTEDT AG & Co. KG) under standardized conditions.

Samples were collected at three time points: (1) before the mental load task (“baseline”), (2) immediately after the mental load task (“stressed”), and (3) 10 min after the intervention (“post-intervention”). Participants were instructed to refrain from eating, drinking, or exercising for at least 1 h prior to each session. Salivary cortisol levels were measured using a Salivary Cortisol Enzyme Immunoassay Kit (Salimetrics, LLC). All samples were stored at − 80 °C and analysed according to the manufacturer’s protocol.

Statistical analysis

In Study 2, both subjective evaluation data and salivary cortisol levels were summarized at the same three stages: before (“baseline”), after mental load tasks (“stressed”), and after intervention (“post-intervention”). Similar analyses to those in Study 1 were conducted, including the calculation of changes in scores for each domain from “stressed” to “post-intervention” states. Additionally, a stratified analysis based on baseline heart rate subgroups (50–75 bpm vs. others) was performed to investigate the effect of intervention tempo on stress recovery in Condition 3. The Wilcoxon signed-rank test was employed for within-condition across different intervention sessions, and the Kruskal–Wallis test with post-hoc Steel test was used for between-condition comparisons. All statistical analyses were conducted using JMP Ver. 17 (SAS Institute) with a significance level set at P < 0.05. Non-parametric tests were chosen to accommodate the likely non-normal distribution of the data, ensuring robustness and reliability in statistical comparisons.

Results

Participant characteristics

A total of 36 males participated in the crossover trial, experiencing each of the three intervention conditions across different sessions. As the VAS-Stress score did not increase between “baseline” and “stressed” states, four, five and three sessions corresponding to Conditions 1, 2, and 3, respectively, were excluded from the analysis. Consequently, 32, 31, and 33 valid sessions remained for Conditions 1, 2, and 3. No differences in baseline VAS, Jikaku-sho Shirabe, or salivary cortisol levels were observed between the groups (Online Resources 2 and 3).

Outcomes

Subjective evaluation scores. Table 2 shows the effect of the vibration intervention on the subjective measures of stress. A significantly larger decrease in VAS-Stress and VAS-Annoyance and a significantly larger increase in emotion and mood inventory scores (Relaxed, Easy, Full of Motivation) between “stressed” and “post-intervention” states were observed in Condition 1 (vibration plus music intervention) compared to Condition 3 (control). A significantly larger decrease in VAS-Stress and VAS-Annoyance, and a significantly larger increase in emotion and mood inventory scores (relaxed) between “stressed” and “post-intervention” states were also observed in Condition 2 (music intervention) compared to Condition 3 (control).

Table 2.

Effects of vibration-plus-music and music-only interventions on subjective measures of stress. Values are shown as the medians (25th to 75th percentiles). Statistical comparisons were conducted using Kruskal– Wallis test followed by Steel’s test versus Group 3 (control). *p < 0.05. (– indicates not significant.)

Measure	Condition 1 (n = 32) (vibration-plus-music)	Condition 2 (n = 31) (music-only)	Condition3 (n = 33) (Control)	Significance vs Control
VAS
Stress	−16* (−25 to −0.8)	−15* (−26 to −7)	−6 (−12.5 to −0.5)	*p < 0.05
Tension	−10.5 (−27.5 to −3.3)	−13 (−25 to −5)	−6 (−16 to 1)	–
Annoyance	−18.5* (−26.8 to −8.5)	−16* (−41 to −6)	−7 (−20.5 to −1)	*p < 0.05
Jikaku-sho Shirabe
Factor I: Feeling of drowsiness	0 (−1 to 1)	0 (0 to 1)	0 (0 to 0.5)	–
Factor II: Feeling of instability	−2 (−2.75 to −0.25)	−2 (−4 to 0)	−1 (−3 to 0)	–
Factor III: Feeling of uneasiness	0 (−1.75 to 0)	0 (−1 to 0)	0 (−1 to 0)	–
Factor IV: Feeling of local pain/dullness	0 (−1 to 0)	−1 (−1 to 0)	0 (−1 to 0)	–
Factor V: Feeling of eyestrain	−1 (−2 to 0)	−1 (−4 to 0)	−1 (−2 to 0)	–
Emotion and Mood Inventory
Relaxed	1* (1 to 2)	1* (0 to 1)	0 (0 to 1)	*p < 0.05
Easy	1* (1 to 2)	1 (0 to 2)	0 (0 to 1)	*p < 0.05
Calm	1 (0 to 1)	1 (0 to 1)	0 (0 to 1)	–
Full of motivation	0* (0 to 0.75)	0 (0 to 0)	0 (−1 to 0)	*p < 0.05
Active	0 (−0.75 to 0.75)	0 (0 to 0)	0 (−0.5 to 0)	–
Energetic	0 (−1 to 0)	0 (0 to 0)	0 (0 to 0)	–
Nimble Agile	0 (0 to 0)	0 (−1 to 0)	0 (0 to 0)	–
Not feel at ease	−1 (−1 to 0)	−1 (−1 to 0)	−1 (−1 to 0)	–
Restless	0 (−1 to 0)	0* (−1 to 0)	0 (0 to 0)	*p < 0.05
Tense	−1 (−1 to 0)	−1 (−2 to 0)	0 (−1 to 0)	–
Inertia	0 (0 to 0)	0 (−1 to 0)	0 (0 to 0)	–
Not in the mood	0 (−1 to 0)	0 (−1 to 0)	0 (−0.5 to 0)	–
Glassy	0 (0 to 0)	0 (0 to 0)	0 (0 to 0)	–

Table 3 shows the effect of the vibration-plus-music and music-only interventions on stress status as measured by salivary cortisol levels. A significantly greater reduction in cortisol level from the “stressed” to the “post-intervention” state was observed in Condition 1 (vibration-plus-music intervention) compared to Condition 3 (control), whereas no significant difference was found for Condition 2 (music-only).

Table 3.

Effects of the vibration-plus-music and music-only interventions on stress status objectively measured by salivary cortisol levels. Values are presented as medians (25th to 75th percentiles). Statistical comparisons were conducted using the Kruskal–Wallis test followed by Steel’s post hoc test versus Group 3 (control). p < 0.05 was considered statistically significant

Measure	Condition 1 (n = 32) (vibration-plus-music)	Condition 2 (n = 31) (music-only)	Condition3 (n = 33) (Control)	Significance vs Control
Cortisol (mg/dL)	−0.079* (−0.120 to −0.026)	−0.052 (−0.083 to −0.011)	−0.045 (−0.072 to 0.011)	p < 0.05

Table 4 shows the results of the stratified analysis based on the tempo of the vibration plus music intervention used in Condition 1. A significant reduction in salivary cortisol levels was observed only in participants whose heart rates (HR) fell within the tempo of the intervention (50–75 bpm), whereas no significant effect was found in other participants with higher HRs. No participants had heart rates below 50 bpm.

Table 4.

Stratified analysis of the effect of the vibration plus music intervention (Condition 1) on stress status, as objectively measured by salivary cortisol levels, based on heart rate ranges. Values are presented as median (25th to 75th percentile). Statistical comparisons were conducted using the Wilcoxon signed-rank test between Condition 1 and Condition 3 (control) within each subgroup. p < 0.05 indicates statistical significance

Group	n	cortisol (mg/dL)	p-value
All subjects
Condition 1	32	−0.079 (−0.120 to −0.026)	0.014*
Condition 3	33	−0.045 (−0.072 to 0.011)
Subgroup (HR 50–75 bpm)
Condition 1	19	−0.079 (−0.122 to −0.051)	0.001*
Condition 3	21	−0.035 (−0.067 to 0.017)
Participants with HR > 75 bpm
Condition 1	13	−0.069 (−0.136 to 0.005)	0.941
Condition 3	12	−0.062 (−0.095 to −0.038)

Discussion

This study hypothesized that relaxation can be induced by the body’s direct tactile perception of vibrations with slowly changing rhythms. For this purpose, in Study 1, we investigated whether tactile experiences of gradually decreasing tempo had an ameliorative effect on acute stress. For comparison, we set up groups with no vibration, fixed vibration, and vibration at a gradually decreasing tempo based on the participant’s heart rate. With respect to the duration of the intervention, the 2-min exposure in Study 1, we included subjective assessments that supported this duration, as preliminary testing indicated it was sufficient to feel the vibration effects. For Study 2, we extended the exposure time based on findings from Study 1 and collaboration with the composer, resulting in a three-and-a-half-minute piece designed to maintain a natural and effective stimulus duration.

The results of the analysis revealed that tactile experiences at a tempo that gradually decreased from the participant’s heart rate facilitated recovery from acute stress. However, this study only evaluated the effects of a single mode of stimulation (tactile) on subjective measures of stress. We hypothesized that cross-modal perception achieved by combining a relaxing tactile stimulus with another synchronized stimulus would have a more effective relaxation effect. We chose to combine the tactile stimulus with the auditory stimulus of listening to music because it has a known relaxing effect [4] and can be easily implemented in society, for example, using smartphones. Smartphones have both vibration and music playback functions, and if the usefulness of the combination of vibration and music can be confirmed, social implementation will be easy and feasible.

The crossover design of Study 2 allowed each participant to serve as their own control, thereby reducing inter-individual variability and enhancing the statistical power to detect the effects of the interventions. Therefore, in Study 2, we chose ambient music with a gradually decreasing tempo synchronized with the tactile stimuli and demonstrated that the combined tactile and auditory stimuli acted synergistically to reduce the mental load. For this purpose, we commissioned a composer to create a piece of music to accompany the change in the tempo of the oscillations. Although musical stimulation alone reduced subjective stress, the combination of tactile (vibration) and auditory (music) stimulation reduced both subjective stress and cortisol levels and might, therefore, be preferable for stress management because of the long-term effects of stress on the endocrine, immune, and central nervous systems. For practical applications, users could hold the smartphone in their hands or place it on their chest to receive vibration stimuli while listening to synchronized music via headphones or speakers, for example during short breaks, before sleep, or while resting.

As mentioned above, we have shown the usefulness of using tactile and auditory senses to make the body perceive gradual changes in rhythm; however, the details of this mechanism are unknown and will be the subject of future research. Several patients in Study 2 commented that they felt that their breathing slowed following cross-modal exposure to music and vibration. It is well known that slow breathing leads to recovery from stress [6, 14]. As mastering breathing techniques takes time (several weeks to several months), the cross-modal relaxation techniques developed in this study could be extremely beneficial for those who have difficulty learning relaxation breathing techniques (children, the elderly, and those who do not have the time to learn the techniques). Nevertheless, this study did not quantitatively assess the breathing rate, and further research is required to investigate the relationship between breathing rates during this experience.

Notably, in Study 2, the effect of the tactile experience at a gradually decreasing tempo varied depending on the participant’s heart rate before the tactile experience. We created a tactile experience that started with the participant’s normal heart rate and then changed to the lowest heart rate. Therefore, in this tactile experience, the initial tempo was set at 75 bpm to align with the average resting heart rate [2], and the final tempo was set at 50 bpm, approximating the minimum heart rate during sleep [35]. Prior studies using vibroacoustic therapy have also shown most beneficial effects at vibration frequencies between 40 and 80 bpm [4]. Our stratified analysis also showed a significant effect only in participants whose heart rates fell within the range of the intervention tempo (50–75 bpm), with no effect observed in those with higher heart rates. These results suggest that monitoring participants’ heart rates and designing individualized experiences accordingly are key to the societal implementation of these findings. Previous studies have explored and developed vibroacoustic therapy using specially designed mattresses, beds and chairs [29]. These have limitations on not being mobile. Based on the results from our study, we are currently developing a smartphone-based application that detects a participant’s heart rate and provides individualized vibration and music stimulation in response to the heart rate. The universal use of smartphones has the potential to provide a readily-accessible experience. These findings support the development of personalized stress-reduction tools via smartphones, which may be beneficial in workplace wellness programs, educational settings, or elder care. Future studies should test their usefulness at a societal level.

This study has some limitations. In particular, the study was based on a small number of participants that included only Japanese males and used only a few methods for stress evaluation and relaxation intervention patterns. Future studies based on a larger population and a systematic investigation of various stress evaluation and relaxation intervention methods are needed to understand the mechanism and optimize the strategy for stress relaxation.

In summary, this study provides empirical support for the effectiveness of gradually decelerating tactile stimuli in reducing acute stress. When combined with synchronized auditory stimuli (vibration plus music), the intervention produced synergistic effects, reducing both subjective stress and physiological markers such as salivary cortisol. These findings support the development of accessible, personalized stress-reduction tools using common devices like smartphones.

Abbreviations

ATMT

Advanced Trail-Making Test

BPM

Beats Per Minute

EA

Energetic Arousal

TA

Tense Arousal

VAS

Visual Analog Scale

VAT

Vibroacoustic Therapy

Authors' contributions

T. Motokawa conceptualized the study, performed formal analysis and investigation, supervised the study, wrote the manuscript, and was responsible for project administration. T. Kato performed the investigation. All authors reviewed the manuscript.

Funding

The research was conducted with the research and development funds from POLA Chemical Industries Inc. (Kanagawa, Japan).

Data availability

The data that support the findings of this study are not openly available due to reasons of sensitivity and are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

This study was approved by the Committee for the Protection of Human Subjects (CPHS) of POLA chemical industries inc. (Kanagawa, Japan); Approval Numbers: 2020 F 132 and 2021 F 22; and was conducted in accordance with the Ethical Guidelines for Medical and Health Research Involving Human Subjects (Ministry of Education, Culture, Sports, Science and Technology and Ministry of Health, Labour and Welfare, 2014, revised in 2017) and the Declaration of Helsinki (revised in 2013).

All participants received a sufficient briefing of the objective and content of the present study, and signed a written informed consent form.

Consent for publication

All the participants have provided their consent for publication.

Competing interests

The authors declare no competing interests.