# Influence of vibratory stimuli on postural muscle tone adjustments and sensation of discomfort in seated posture

Salim Zenasni, Erick Dousset, Mitsuko Aramaki, Richard Kronland-Martinet, Sølvi Ystad, Jocelyn Monnoyer, Tanguy Marqueste

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## To cite this version:

Salim Zenasni, Erick Dousset, Mitsuko Aramaki, Richard Kronland-Martinet, Sølvi Ystad, et al. Influence of vibratory stimuli on postural muscle tone adjustments and sensation of discomfort in seated posture. Results in engineering, 2025, 27, pp.106622. 10.1016/j.rineng.2025.106622. hal-05215172

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Submitted on 19 Aug 2025

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Research paper

# Influence of vibratory stimuli on postural muscle tone adjustments and sensation of discomfort in seated posture

Salim ZENASNI<sup>a,b,c,*</sup> <sup>‡</sup>, Erick DOUSSET<sup>a,1</sup>, Mitsuko ARAMAKI<sup>b,1</sup>, Richard KRONLAND-MARTINET<sup>b</sup>, Sølvi YSTAD<sup>b</sup>, Jocelyn MONNOYER<sup>c</sup>, Tanguy MARQUESTE<sup>a,1</sup>

## ARTICLE INFO

### Keywords:

Vibration perception

RMS EMG

Posture

## ABSTRACT

Prolonged sitting in an automotive context causes discomfort, neuromuscular fatigue, poor blood flow, and potential health issues. Minor posture changes can improve comfort and spinal health, but drivers are constrained by the need for seat stability and focus on driving tasks. While the visual modality is heavily used for driving tasks, tactile modality, particularly vibrations, is underexplored. A proposed solution in this study is to integrate a stimulation system, such as vibratory stimuli, into the seat to induce postural muscles tone adjustments and alleviate aforementioned problems. The influence of vibratory stimuli on such adjustments in sitting posture in relation to the sensation of discomfort has been little investigated. The aim of the present study is to describe and understand this influence by measuring objective parameters (muscular electrical activity) and subjective parameters (sensation of discomfort). We assessed the effect of actuator, vibratory pattern and frequency on muscle activity (surface electromyography, sEMG) induced by static and dynamic stimulations. We found that the perceptual vibratory threshold depended on the area of the body stimulated by the actuator. In addition, we found that vibratory stimulation increased muscle activity, in particular muscles involved in repositioning (p=0.044), without significantly increasing the sensation of perceived discomfort (p=0.094). The results confirm that humans perceive and react to vibrations differently depending on the stimulated zone, with a higher sensitivity on buttocks compared to lower back (p<0.005). This study is a first step towards characterizing multimodal stimulation strategies for postural repositioning in various use contexts.

## 1. Introduction

In our everyday life, we spend a considerable time in sitting position whether at the office in front of our computers for many hours, during the public transport, or even while relaxing in front of the television [1]. In an automotive context, the static and prolonged nature of the sitting posture of the driver becomes a constraint that generates discomfort, neuromuscular fatigue [2,3] and a poor blood flow around the lumbar region of the spine [4] which alters the nutritional contribution to the intervertebral discs [5]. Such situations can therefore cause health problems [6] and require thorough attention in various applications. It is established that blood flow is essential for comfort sensations and well-being [7]. In this context, the understanding of underlying

mechanisms associated with postural readjustments during sitting posture is a key factor in order to better manage the onset of neuromuscular fatigue [8]. During driving, it is hard to move one's body properly not only because the seat must provide stable support for the driver, but also because the driver focuses his attention on road traffic and navigation. Nevertheless, it has been shown that even small changes in sitting posture can have a considerable impact on constraints in the body and that the generated movement has a positive effect on comfort sensation [9]. It also improves the fluidity of nutrient transport and has a beneficial effect on the intervertebral discs [10]. Taking these considerations into account, the integration of a stimulation system into the driver's seat could appear to be the appropriate interface for inducing a postural readjustment that would alleviate such problems [11].

<footnote><sup>a</sup> Aix Marseille Univ, CNRS, ISM, Marseille, France</footnote>

<footnote><sup>b</sup> Aix Marseille Univ, CNRS, PRISM, Marseille, France</footnote>

<footnote><sup>c</sup> Groupe Stellantis, Centre Technique de Velizy, Velizy-Villacoublay, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: salim.zenasni@univ-amu.fr (S. ZENASNI).</footnote>

<footnote n="1">¹ These authors contributed equally to this work.</footnote>
In terms of stimulation, the use of the visual modality can be problematic in terms of safety and efficiency since the driver has to be focused on the road traffic [12]. Indeed, in addition, drivers devote 30 to 50% of their visual activity to secondary tasks such as setting the radio or receiving telephone calls or other notifications [13,14]. As far as the auditory modality is concerned, talking to someone on the phone in particular can generate a major conflict with auditory information, potentially impacting on traffic performance and increasing the risk of collisions [15,16]. Furthermore, several authors are arguing in favour of the use of the vibratory modality in a motor vehicle context over the visual and auditory modalities, particularly as these two senses are already widely used during driving [13–17]. Hence, very few studies give practical and complete guideline for the use of other modalities such as the vibratory modality, perceptible at seat level.

Previous studies reported that vibratory signals are generally characterised by their amplitudes (often in m.s⁻² or mm) and frequencies for sinusoidal signals. They can be perceived differently depending on the parts of the body being stimulated [18]. Frequency range (0.1-20 Hz) is well known in the literature and is characteristic of road vibrations [19]. Indeed, the latter showed that vertical vibrations in this range induce discomfort for both drivers and passengers. Moreover, most studies investigated discomfort induced by vibration originating from the road notably [20]. According to Mansfield, studies focusing on localized vibrations on the body were mainly concerned with vibrations transmitted to the hand or finger, because these are among the most sensitive parts of the body [21]. Indeed, the human body is composed of a wide variety of sensory receptors, making the response from our perceptual systems (visual, tactile...) complex and challenging to assess. This is partly due to their dependence on the conditions of stimulation and the differing responses from one individual to another.

Vibration perception is essentially picked up by cutaneous mechanoreceptors called Ruffini, Pacini, Meissner and Merkel with Meissner and Pacini playing a predominant role in vibration perception. Muscles mechanoreceptors can also be sensitive to vibration. The neuromuscular spindles are sensitive to the degree and speed of stretching, but they also respond to vibrations, with an optimum at 70-80 Hz [22]. The neuromuscular spindles are involved in the detection of our own position (statesthesia) and movement (kinesthesia). For the current study, stimuli thresholds will be characterized as absolute thresholds which are classically described in psychophysics as the smallest quantity of physical energy capable of provoking a reaction.

The influence of mechanical vibration on humans has therefore been addressed by a large number of studies. These studies have explored the link between vibratory stimulation on behavioural response strategies in terms of postural regulation (mainly standing), muscular activity and its effects on the sensation of discomfort. However, there is no real consensus for studies focusing essentially on the perception of vibration, its amplitude and its frequency [18,23,24]. Indeed, there is also a large difference between protocols used the various studies and in the observed inter-individual and intra-individual differences related to the postures adopted during vibratory stimulations. Thus, depending on the area of the body stimulated by the vibration and the vibratory frequency in particular, the perception of the vibration and the effect of the vibratory modality vary [25,26]. Most studies examine whole-body vibration (WBV) in the standing or sitting position without a backrest and focus mainly on vibration perception thresholds [18]. It has been shown that below 80 Hz the sensitivity increases to the interface of the seat and in the vertical direction than in other directions [27]. There are several inter-study discrepancies in this area due to complex relationship between vibratory characteristics and differences in experimental protocols.

Self et al. [28] suggest that the vibration characteristics to be considered are amplitude, frequency, and direction. In their opinion, it is also important to consider the position of the vibratory stimulus and these authors suggest the consideration and use of vibratory patterns in relation to rhythm and temporal as well as spatial summation

phenomena. In addition, Erp [29] suggests examining the link with comfort following stimulation, since poorly controlled exposure to vibration can cause discomfort [30]. Ji et al. [31] examined seat vibration focusing on intensity, position, direction, rhythm and distance between actuators. Thus, for vibration characteristics ranging from 3.5 V to 4V ([25;30 Hz]; seat cushion) and ranging from 4 V to 4.5 V ([35;40 Hz]; backrest), as well as stimulation durations of 3 seconds, seat vibrations are the most adequate while being perceptible for drivers. However, some of the results differed from those of [27], particularly for the vibration directions. This can be explained by the fact that Ji's study established vibration thresholds and suggestions heavily based on the participant's comfort, and not on perception only.

Regarding the literature on the effects of vibration on muscles responses, the majority of studies investigated standing posture based on WBV [32–34] and few were interested in sitting posture [35–37]. Despite differences in protocols (WBV, absence of back support, direct skin vibration, etc.) and contexts (rehabilitation, sportsmen with muscular exercises, etc.), the authors seem to agree on a frequency range of [40;80 Hz] and vibration amplitude of [2;4 mm] for optimal stimulation. In particular, Zimmerman et al. [35] showed an interesting link between EMG (electromyography) activity in the erectors of the spine and posture in the seated position. However, this postural and frequency dependence observed by WBV does not lead to homogeneous results across all studies [38–40]. Then, the studies agree in showing that the perceived discomfort sensation increases with time, but they often diverge on the frequency dependence question without mentioning the differences in experimental contexts. This could be explained by the fact that perceived discomfort is subjective [41–43].

Various studies indicated that the characteristics of vibration can impact postural responses [44–47]. For instance, stimulation of the tibialis anterior or of the forefoot can cause the standing body to tilt forwards or backwards, respectively. The direction of postural shifts also depends on the frequency of vibration. Participants tended to tilt backwards at frequencies of 20 and 40 Hz, while tilting forwards at 60 and 80 Hz [26]. In another example, erectors muscles stimulation led to postural shifts in the direction of the stimulus [47]. Thus, combining vibrotactile stimuli for an area on the body through different vibration sources can improve perceptual and postural responses through spatial summation properties [48]. Furthermore, when standing, sinusoidal vibrations on the skin have a low chance of activating the muscle stretch sensors at frequencies above 80 Hz [49,50].

These findings highlight the influence of vibration on muscle tone response. It is also difficult to find practical and comprehensive guidelines in the literature on the use of vibratory modalities in seating, although there have been studies pointing out the need to use this modality in relation to visual and auditory interfaces in vehicles [29,27,46]. The aim of this exploratory work is therefore to identify the characteristics of vibratory stimuli inducing changes in muscle tone. These muscles have been selected because they are involved in postural changes for which the sensation of perceived discomfort will be minimal. Muscle activity (EMG) and the perception of discomfort were measured during static or dynamic vibratory stimulation.

## 2. Materials and methods

### 2.1 Ethical approval

Ethic approval was given by the ethics committee of CERSTAPS (N°IRB00012476-2023-15-05-249). Participants were enrolled from July 2023 until August 2023. They gave their written informed consent before starting the experiment.

### 2.2. Participants

Fourteen healthy volunteers were recruited for the experiment (32.43±10.65 years, 10 men and 4 women). The inclusion criteria were
that volunteers had to be at least 20 years old, have held a driving licence for at least two years, and be naive about the scientific context of the experiment. The study excluded people with illnesses, disabilities or handicaps of a cognitive and/or locomotor nature at the time of the experiment, hearing problems, and those who had suffered from back pain or musculoskeletal disorders in the previous six months, pregnant women, women in labour or breast-feeding, in accordance with article L1121-5 of the French Public Health Code, as well as those who have consumed alcohol in the 24 h preceding the day of the test. The objective of the present study was to achieve a medium effect size. In accordance with this aim, it was determined that the total number of subjects required for the study was 14. This calculation was made so that an appropriate statistical power could be reached [51].

## 2.3. Experimental synopsis

The experiment consisted of a calibration part (for EMG measurement and for vibratory stimulation) followed by a main experimental part subdivided into 2 sessions. It took place at the PRISM laboratory. Before taking part in the experiment, each volunteer signed an informed consent form, attesting to his or her informed participation. The whole synopsis is presented in Fig. 1 and the different parts of the experiment are described in the following sections.

### 2.3.1. sEMG measurement and calibration

Participants were equipped with surface electrodes bilaterally to record the activity of the stabilizing muscles of the sitting posture, i.e., the right (TD<sub>R</sub>) and the left (TD<sub>L</sub>) Trapezius Descendens muscles, the right (LR) and left (LL) Erector Spinae Longissimus muscles and the right (RF<sub>R</sub>) and left (RF<sub>L</sub>) Rectus Femoris muscles for the thighs. The sEMG electrodes were positioned in accordance with international Seniam recommendations (bipolar electrodes (EL503, Biopac®) spaced 2 cm apart with a recording diameter of 10 mm; [52]). A chair and table were used to prepare the participant and place the electrodes. If necessary, the skin was shaved. It was then decreased using 70° isopropyl alcohol pre-applied to wipes. To facilitate the installation, participants were asked to wear shorts and a tank top provided by the experimenters. The electrodes were placed according to the diagram in Fig. 2. For the trapezius, the electrodes were positioned on the shoulder in the middle of the axis linking the acromion to the c7 vertebra. For the longissimus, the electrodes were positioned 2-finger widths laterally to the spinal process of L1. For the rectus femoris, the electrodes were positioned on the upper thigh in the middle of the axis running from the anterior iliac

spine to the upper part of the patella. EMG activity was recorded at a sampling frequency of 2000 Hz using the BIOPAC® - MP150WSW acquisition system and the Acqknowledge® software. The signal was amplified (*10000) and high-pass filtered with a cut-off frequency of 50 Hz to eliminate ECG (electrocardiogram) artefacts [53] (see Fig 3).

The EMG data was calibrated using the following procedure. Each participant was placed on a weight bench to obtain reference measurements for each muscle. Firstly, he/she was asked to maintain a maximum sustained voluntary isometric contraction of the Rectus femoris. This procedure was also carried out on a chair where the knee angle was fixed at 90° and the participant had to counter the force applied by the experimenter's body weight, which was applied to his shoulders, thus obtaining a maximum voluntary isometric contraction of the Trapezius descendens. Finally, the participant was lying on his stomach on a mat and had to produce a maximum voluntary isometric contraction of the Longissimus by raising arms and legs so that they were not in contact with the ground. Each muscle contraction lasted 3 S. For each stage, the participant repeated these contractions 3 times, corresponding to 3 trials per pair of muscles, for a total of 9 trials. The muscular activity corresponding to these maximum voluntary contractions was recorded.

### 2.3.2. Vibratory stimulation and calibration

Participants were then placed on the experimental set up comprising a car seat that was adjustable for height and depth and fixed to a mobile and rigid platform. The seat was a Citroën type b9 seat fitted with 8 Tectonic TEAX32C30-4/B actuators positioned uniformly (cf. Fig 4): four actuators were arranged in a square in the backrest (i.e., 2 in the upper back and 2 in the lower back) and 4 actuators were also arranged in a square in the seat (i.e., 2 under each thigh). They provided vibratory stimulation and were individually computer-controllable using Max Msp 8.2.2 & Dante Controller software via inter-M 300PLUS and a MOTU 2408mkII. Participants were asked to hold a Logitech driving force GT PS3 steering wheel comfortably in accordance with standard driving guidelines, with their arms neither too tense nor too bent. They were then equipped with Sennheiser HD 650 headphones, which emitted pink noise to mask any disturbing external sound sources. The calibration regarding the vibratory stimulation was therefore conducted to ensure that participants perceived homogeneous sensation of vibrations across the 8 actuators of the seat. The calibration consisted in 2 phases: the perception thresholds for the 8 actuators on the seat were determined (first phase), and then the perceived vibration was equalised using the least powerful actuator as a reference (second phase).

<figure><img src="image_3.png" /><figcaption>**Fig 1.** Schematic diagram of the experimental protocol. A) Consent, Questionnaire; B) Seat adjustment; C) EMG calibration for Trapezius descendens, Erector Spinae Longissimus & Rectus femoris muscles; D) Identification of perceptual vibratory thresholds and equalization of vibratory sensations; E) Main experiment; F) Equipment removal & end of experiment snack.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig 2. Electrode placement on muscles: Trapezius descendens (position A), Longissimus (position B) & Rectus femoris (position C).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig 3. Example of a Left Trapezius Descendens (TD_L) EMG signal, Panel A represents the raw TD_L signal, panel B represents the filtered TD_L signal and panel C represents the filtered and rectified TD_L EMG signal (blue) with the RMS envelope (red curve).</figcaption></figure>

Concerning the first phase, the perception thresholds were defined at the vibratory frequency values fixed for the main experiment (i.e., 45, 50, 55, 60 and 65 Hz). For each frequency and each actuator, the method used was an adaptive method with "3 down-2up" slope adjustment, inspired by the adaptive forced-choice procedure which follows the 1-up 1 down rule [54–56]. The session started with an initial intensity of the vibratory stimulation produced by an actuator, and the participant pressed the "O" button on the steering wheel for as long as he/she perceived the vibration. The intensity of the stimulus was then reduced as long as the participant kept the button pressed. Once the participant released the button, the intensity of the stimulation increased. This process was repeated until three press values and two release values were obtained. To refine the threshold measurement, the variation in intensity during the increasing and decreasing phases was slowed down as the participant approached the threshold. Finally, these values were averaged to obtain the final threshold value.

Concerning the second phase, the equalisation is intended to

harmonise the vibratory sensation across all the actuators, by reaching the point of subjective equality with respect to a vibratory reference defined as the Actuator 3. Indeed, in pretesting, this actuator was perceived much later than all the other actuators. The session started with the Actuator 3 running at a constant intensity. Then, a second actuator, randomly chosen, was activated at a supra-threshold level. Participants had to adjust the intensity level of the stimulation generated by this second actuator so that they perceived a vibratory sensation similar to the reference vibration. The adjusted intensity levels, noted G, were therefore normalized for each participant (between 0 and 1, without unit). The following formula was used to convert our values, which already give information about vibration perception thresholds and our sensitivity, into vibration amplitude:

$$V = 20 \log_{10} (G)$$

V is the intensity in decibels (dB) and G corresponds to the normalised value in a standard interval [0;1] computed from the actual value (between 0 and 127) used in Max 8.2.2 software to set the output intensity of the actuator. 0 dB corresponds to the maximum amplitude of vibration whereas -65 dB is to be considered as the minimum amplitude of vibration.

### 2.3.3. Main experiment

After these calibration phases, the main experimental part was conducted. It was composed of two experimental sessions corresponding to the 'static' and 'dynamic' conditions. The 'static' condition consisted of sending 3 S vibratory stimulations [31] on each actuator in a randomized manner at 5 frequencies (45, 50, 55, 60, 65 Hz) with an inter-stimulus interval (ISI) of 2.5 S [57]. The 5 chosen frequencies are selected based on previous studies on the perception of vibrations, the perceived discomfort, (duration of stimuli and inter-stimuli intervals) as well as the effect of vibrations on muscular and postural activity [57,26, 58,59,27,31]. In total, the 'static' condition lasted 3 min 40 S = (3 S + 2.5 S) x 8 actuators x 5 frequencies. The 'dynamic' condition consisted of delivering a vibration moving in the seat according to four vibration patterns (see Fig 4) at the 5 same frequencies (45, 50, 55, 60, 65 Hz): patterns along vertical axis, i.e. downward (Pattern 1) and upward (Pattern 2) directions; patterns along horizontal axis, i.e. rightward (Pattern 3) and leftward (Pattern 4) directions. In total, the 'dynamic' condition lasted 4 min 50 S = (3 S + 2.5 S) x 4 patterns x 5 frequencies. The order of presentation of conditions as well as the trials within condition were randomized across participants.

During the experimental sessions, participants were placed in a sitting position on the seat. They were required to maintain a fixed position throughout the experiment to be as close as possible to the driving position, with no specific task. At the beginning of each session, they were asked not to move for a few minutes during which basal EMG activity was recorded. The EMG activity was then recorded during the main experimental part. To synchronise the vibratory stimulations to the EMG activity recordings, a synchronization program was developed on Python. The program ran from a local port on the following basis: as soon as the randomization of the experimental conditions was
<figure><img src="image_6.png" /><figcaption>Fig 4. Positions of the 8 actuators (Ac) and the 4 patterns of dynamic vibration: Patterns 1 and 2 represent vibrations moving along the vertical axis, Patterns 3 and 4 represent vibrations moving along the horizontal axis.</figcaption></figure>

performed via Streamlit, Python retrieved this information. This then allowed to launch the first condition via Streamlit, which simultaneously launched the corresponding vibratory experiment and the EMG recording.

Participants were asked to report the perceived global discomfort using a visual analogue scale (VAS) at the beginning of the experiment, between the experimental conditions and after the last experimental condition. The VAS scale was presented as a digital interface with no visible graduation.

At the end of the experiment, the equipment was taken away from the participant, who was given a snack and was compensated. The participant was released once the experimenter had given the go-ahead, for safety reasons.

## 3. Data processing and statistical analyses

The collected scores of perceived discomfort (VAS) were processed using Matlab® programming software (version R2021, MathWorks, Natick, MA, USA) and Microsoft Excel 2020 (Microsoft Corporation, Impressa systems, Santa Rosa, California, USA). Statistical analyses were performed using JASP (Version 0.16) [Computer software]. The recording of muscle activity was transmitted using Acqknowledge 5® software and then processed using Matlab®. The EMG signals were rectified, filtered, and normalized in relation to the reference level of muscle electrical activity obtained during the calibration phase at the beginning of the experiment. The envelopes of the EMG signal (Root Mean Square-RMS) were analysed as a function of session time, as well as the maximum signal amplitude peaks for all periods. Integration of the EMG signal squared (quadratic) allows us to measure the energy of the signal and eliminate negative values. A measure derived from this integration is the root mean square (RMS) value. Each signal was converted into RMS values over a temporal window, as follows:

$$ \text{RMS}\{m(t)\} = \left( \frac{1}{T} \int_{t-}^{t+T} m^2(\tau) d\tau \right)^{1/2} $$

The EMG RMS during the experimental session was expressed as a percentage of the baseline EMG RMS values, noted ΔRMS:

$$ \Delta \text{RMS} = (\text{RMSStimulation} - \text{RMSRest}) / \text{RMSRest} \times 100 $$

Data were expressed as the mean ± standard deviation. Correlations were performed on the vibration versus ΔRMS using Pearson or Spearman tests, depending on the normality of data. The strength of the effect is reported as Kendall's W (W). For the RMS data, repeated measures analysis of variance (ANOVA) was performed with the “vibration” as factor (2 levels: rest & stimulation). For vibratory threshold data, a repeated measures ANOVA was performed with the “actuator” (8 levels equivalent to 8 positions on the seat: 1 to 8) and “frequency” (5 levels: 45, 50, 55, 60, & 65 Hz) as factors. For the discomfort scores, a repeated measures ANOVA was performed with the “moment of experimentation” as factor (3 levels: pre-conditions, inter-conditions, post-conditions). The postulates of normal distribution of the data and homogeneity of the variances were verified. The significance level chose was 5% (p<0.05). Effect sizes are reported as eta squared (η²). To assess the relationship between discomfort scores and muscle activity, a linear regression test was conducted.

For the static condition, a repeated measures ANOVA was performed with the factors: “actuator”, “frequency” and “muscles” (6 levels: left and right trapezius, left and right longissimus & left and right rectus femoris). For the dynamic condition, a repeated measures ANOVA was performed with the factors: “pattern” (4 levels), “frequency” and “muscles”.

We computed the ΔRMS of muscle activity for which the ANOVA revealed a statistically significant effect of the vibration factor relative to the experimental conditions. The means of the ΔRMS variables are
reported with their standardized 95% confidence intervals.

If the normality of data was not verified, a non-parametric Friedman analysis was conducted. All statistical differences were considered significant when *p*<0.05. Conover's post hoc tests were conducted when main effects are significant.

### 3.1. Additional statistical analysis section

To further examine whether sample size had an influence on statistical power and thus on statistical significances reflected by *p* values due to the nature of the statistical test of ANOVA with repeated measures and multiple factors, we applied Mauchly's test to verify the sphericity of the data when it was necessary. In the case where sphericity was violated, we applied a Greenhouse Geiser correction factor to verify *p*-values. If no significant differences were observed with the correction factor, *p*-values resulting from this supplementary analysis verification step, were not reported.

## 4. Results and discussion

The first aim of this study was to investigate the perceptual responses of participants to vibratory stimuli delivered in a seated position at different vibration frequencies and at different actuator positions. We assumed that the vibratory perception differs according to the actuators and the vibration frequencies. The second aim of this study was to get conceptual evidence that we could induce changes in muscle tone, with vibrations stimuli, which could subsequently lead to postural micro-repositioning in a controlled environment. We investigated effects on variations in EMG activity depending on the following factors: vibration frequencies, muscles as well as actuator position for the “static” and pattern for the “dynamic” condition. We assumed that the ΔRMS differs according to all these factors. Thirdly, we studied the link between perceived discomfort and ΔRMS, as well as the link between vibration perception and ΔRMS. We therefore expected changes in EMG response

to be linked to our discomfort, but also to our sensitivity to vibrations in the seat. We expected this link to be stronger for the “dynamic” condition than for the “static” condition.

### 4.1. Vibration perception on the seat

Results firstly showed that when the participants were asked to report the perceived vibrations emitted by the eight actuators, the threshold values differed according to the area of the body solicited. In particular, a significant effect of the actuator factor on vibration perception threshold was showed (F(7,84) = 9.86, *p*≤0.001, η²<sub>p</sub>= 0.221): participants were more sensitive to actuator 5 compared to actuator 1 (*p*=0.05), actuator 3 (*p*<0.001), actuator 4 (*p*=0.037), actuator 7 (*p*=0.020) and actuator 8 (*p*=0.022). These results confirmed the observations reported in [18]. A greater sensitivity at the level of the buttocks compared with the trunk and thighs was reported, in line with the works of Thomas & Mise [60] who showed a better ability to discriminate actuator signals in the seat rather than at the level of the backrest and that this ability varied according to the position of the actuators. These results are also in line with recent literature guidelines regarding personalization of vibration stimuli to optimize vibration perception [61].

In addition, a significant interaction between frequency and actuators was shown (F(28,336) = 3.109, *p*≤0.001, η²<sub>p</sub>= 0.068). Participants were more sensitive at 55 Hz (*p*=0.008) and 60 Hz (*p*<0.001) compared to 45 Hz for actuator 7 (Fig 5). Our results were not in agreement with most of the observations regarding the frequency dependency reported in the literature for the seated posture condition, and therefore suggest that the frequency interval of our study [45;65 Hz], which is included in the 1st identified interval in the literature of [30;80 Hz] [26,27,31,62], corresponds to an interval for which the participants are already very sensitive. However, in this specific context of the sitting posture, recent studies focusing on finding the right frequency range in order to effectively alert the car drivers through seat vibrations, report similar results

<figure><img src="image_7.png" /><figcaption>Fig 5. Vibratory perception thresholds for stimulated parts of the body. Significant differences between stimulated zones are indicated by their *p*-values.</figcaption></figure>
regarding human perception of vibrations on a seat [63]

## 4.2. EMG response

A significant effect of the vibration on the RMS was demonstrated ($F_{(1,13)}=4.984, p=0.044, \eta_p^2=0.277$). The vibrations in static and dynamic conditions led to an increase in RMS. In particular, a significant effect of the vibration condition on EMG activity was revealed ($Z=3.4, p=0.005$, effect size = 0.953). The EMG response was significantly higher for the static (40.17 ± 16.22) than for the dynamic condition (31.57 ± 11.07).

In the ‘static’ condition, results showed a significant effect of the actuator on the ΔRMS ($\chi^2(7)=29.741, p<0.001$, Kendall’s W = 0.017. They also showed a significant effect of the muscles on the ΔRMS ($\chi^2(5)=572.234, p<0.001$, Kendall’s W = 0.223) with stronger EMG responses for trapezius and longissimus muscles than for rectus femoris muscles ($p<0.001$) (see Fig 6). There was no significant effect of the frequency on ΔRMS ($\chi^2(4)=1.234, p=0.873$, Kendall’s W = 0.005).

In the “dynamic” condition, results did not reveal effect of the vibratory pattern ($\chi^2(3)=0.293, p=0.961$, Kendall’s W = 0.006, nor the frequency ($\chi^2(4)=0.298, p=0.990$, Kendall’s W = 0.001) on the EMG responses. Elsewhere, they showed a significant effect of the muscles ($\chi^2(5)=376.937, p<0.001$, Kendall’s W = 0.305) (see Fig 7) [17,64]. We found greater activity in the left rectus femoris than in the

right rectus femoris ($p=0.011$). These results are interesting regarding the works of [65–67] since these studies showed increased sensitivity for non-dominant limbs. This would suggest a sort of compensation on the part of the non-dominant limb. As for the dominant limb, it has been shown that the perception of vibration was weaker than that of the non-dominant limb [68].

Overall, we can conclude that the vibratory modality plays a significant role in the EMG response. Humans are capable of perceiving vibratory stimuli and are more sensitive for certain parts of the body. For both static and dynamic conditions, we found a muscle tone due to the perception-motricity relation and therefore here vibration - EMG response. These results also supported that vibratory stimuli reinforced proprioceptive input to the trunk and thighs via skin receptors that integrate information from vibrations. This information would then be used and interpreted by our body, inducing muscular responses [22]. This has been confirmed in more recent medical literature in the rehabilitation context [69]. The same information could be used in other contexts, such as in on-board warning systems as suggested by [17].

In both conditions and contrary to our expectations and to previous studies [18,70,62], there was no difference in EMG response according to the frequency of vibration. This could be due to the fact that the frequency range of vibration chosen is already very sensitive to vibration [27,71,39]. It can also be due to the differences in experimental protocols between ours and the others in which more frequencies were

<figure><img src="image_8.png" /><figcaption>**Fig 6.** RMS EMG for the stabilising muscles with respect to the 8 actuators in static condition. Left muscles (Left Trapezius Descendens(TD<sub>L</sub>), Left Longissimus(L<sub>L</sub>) & Left Rectus Femoris(RF<sub>L</sub>)) are represented in panel A and right muscles (Right Trapezius Descendens(TD<sub>R</sub>), Right Longissimus(L<sub>R</sub>) & Right Rectus Femoris(RF<sub>R</sub>)) are represented in panel B. The graph represents the % of increase of EMG activity compared to rest before stimulation. Significant differences between muscles are indicated by *** ($p<0.001$).</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig 7.** ΔRMS EMG for the stabilising muscles with respect to the 4 patterns in dynamic condition. Left muscles (Left Trapezius Descendens(TD<sub>L</sub>), Left Longissimus(L<sub>L</sub>) & Left Rectus Femoris(RF<sub>L</sub>)) are represented in panel A and right muscles (Right Trapezius Descendens(TD<sub>R</sub>), Right Longissimus(L<sub>R</sub>) & Right Rectus Femoris(RF<sub>R</sub>)) are represented in panel B. The graph represents the % of increase of EMG activity compared to rest before stimulation. Significant differences between muscles are indicated by *** (*p*<0.001) or * (*p*<0.05).</figcaption></figure>

investigated [24] and predefined vibratory intensities were used [31].

### 4.3. Relationship between vibratory responses and discomfort scores

The mean scores of the perceived discomfort reported by the participants were 10.1 (± 6.7) before the experimental conditions, 11.7 (± 7.4) between the conditions and 16.2 (± 9.4) at the end of the experiment (Fig 7). These scores did not significantly differ throughout the experiment (F (2,24) = 2.61, *p*=0.094, η²<sub>p</sub>= 0.179). Correction coefficient of Greenhouse-Geiser has been applied (*p* after correction = 0.104). It allows us to conclude that the EMG response was induced by the vibratory stimuli and not by discomfort due to uncomfortable sensations. Some studies showed that the sensation of perceived discomfort increased with time [40,42], however compared to them, our experiment duration was relatively short.

For the analysis of perceived discomfort, our design led to an estimated effect size of approximately *f* ≈ 0.25, which is considered a medium effect and corresponds to a statistical power of around 60% with

our sample size [51].

Fig 8

Results did not reveal significant links between perceived discomfort and ΔRMS EMG (*p*=0.785) as well as between vibratory perceptual threshold and ΔRMS EMG (*p*=0.565). This lack of link could be interpreted as a frequency range that is too specific and narrowed around the sensitivity optimum [24,28]. Furthermore, rather than a global sensation of discomfort, a measure of local discomfort could be collected to focus on specific parts of the body and thus, on which corresponding seat parts as suggested in recent driving context studies [20,72]. Such measure allows participants to report their discomfort sensation for each body part separately. This is strengthened by the fact that perceptual responses and EMG activity differed according to the areas of the human body stimulated.

### 4.4. Future direction

Regarding automotive context, numerous studies on massage
<figure><img src="image_10.png" /><figcaption>Fig 8. Mean values of discomfort scores reported by participants throughout the experiment.</figcaption></figure>

systems in car seats (see for example [73]) reported that principles used to make such seats are empirical. On the other hand, our study provides an objective source of results which leads to induce postural adjustments to reduce discomfort notably. Our present study showed successfully that vibrations in the 45-65 Hz range induced increases in postural muscle tone. Thus, our study is a preliminary step to find seat able to reduce neuromuscular fatigue and discomfort that is also to reduce the risk of accident in the automotive context. To this end, we notably intend to use a proven objective measure in the literature regarding movement recording by using pressure mats for the calculation of the center of pressure (CoP) which informs us on potential micro/macro - movements [74].

It is noteworthy that our work could concern in the future other contexts such as transportation, work office [75] and e-sports [76,77].

## 5. Conclusion

This seminal study highlighted the effective influence of vibratory stimulations on the surface EMG response. Firstly, our results showed that vibrations are perceived differently depending on the zones stimulated, as the participants were more sensitive for the gluteal region for instance. Secondly, they demonstrated that these vibrations increased muscular activity, in particular muscles involved in repositioning, without significantly increasing the sensation of perceived discomfort. Therefore, they mean that vibrations could influence our ability for postural repositioning in the seated position.

The proposed methodology also suggests several improvements for future works [29,31]. One of the perspectives would be to shorten the stimuli and inter-stimuli durations to be more in line with [57,31]. Moreover, a precise description of the interactions between our body, the seat and the vibration stimuli [78] should be provided since vibration acceleration (in m.s⁻² or in mm) is a relevant descriptor to better understand perception mechanisms. To assess postural repositioning, the current apparatus can be further combined with a posture detection system based on pressure sensors located on the entire seat. Such device allows us to develop interactive protocols in which vibratory stimulations would be adapted according to our posture on the seat [79].

This study confirms the human ability to perceive and react muscularly to vibratory stimulations. Going further, we aim at integrating different modalities, such as auditory stimulations due to the complementary nature of the sound and vibratory modalities [80,81]. Therefore, multimodal stimulation strategies can be designed for efficient muscle tone changes that could be associated with postural

readjustments in different domains of applications (transport, workstations ...).

## Funding

This work was funded by Stellantis. The funder provided support in the form of salaries for the authors Salim ZENASNI and Jocelyn MONNOYER but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

## Data statement

The data that support the findings of this study are available from the corresponding author upon request and on: https://osf.io/sjdnf/?view_only=500d31b4cf3e4885b01d457539c02502

## CRediT authorship contribution statement

**Salim ZENASNI:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Erick DOUSSET:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Mitsuko ARAMAKI:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Richard KRONLAND-MARTINET:** Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology, Investigation, Conceptualization. **Solvi YSTAD:** Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology. **Jocelyn MONNOYER:** Writing – review & editing, Resources, Funding acquisition. **Tanguy MARQUESTE:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Salim Zenasni reports financial support was provided by Stellantis NV. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors express their sincere thanks to Jocelyn Rozé (PRISM), COMMUTE Project (ANR-22-CE-33-0009).

## Data availability

I have shared the link to my data at the attach file step.

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