Surface electromyography as a tool to assess the responses of car passengers to lateral accelerations. Part II: Objective comparison of vehicles

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Abstract

The purpose of this study was to objectively assess the response of car passengers to lateral accelerations. Surface EMG signals were collected bilaterally from the cervical erector spinae (CES), latissimus dorsi (LD), erector spinae (ES), external oblique (EO), and vastus lateralis (VL) muscles of 10 subjects. Lateral acceleration was also recorded. Three chassis-seat configurations AA, BA and BB were tested, with the first letter denoting the chassis and the second the seat. SEMG signals were often contaminated by noise, and were, therefore, denoised using the methods explained in part I. Reciprocal phasic activity was observed for all muscles except for the EO, and the reaction of passengers to lateral accelerations was interpreted as a bust torsion. The RMS of EMG segments was used as an indication of muscle activity. Muscle activation of VL and ES were significantly affected by the configuration tested (p < 0.05), with greater activation levels observed for the chassis A than for the chassis B. Such a finding implies that greater roll requires greater muscle activity, thus resulting in less comfortable vehicles. Therefore, SEMG can be used to provide an objective measure of discomfort in passengers subjected to lateral accelerations in a car seat.

Introduction

In the automotive world, comfort has become an important criterion for user choice. The automotive industry places a big emphasis on ergonomic works in order to satisfy user demands such as security and comfort. In this context, comfort means not only air conditioning or shock absorption, but also the minimal perception of accelerations transmitted through the seats. Comfort assessment remains complex and has generally consisted of two methods: subjective methods based on subjects’ responses to questions about the comfort of different seats while driving, and mechanical methods using sensors, in particular accelerometers, mounted on seats or other parts of the vehicle. Previous studies used both methods to derive a better representation of comfort [4], [10]. There have been few objective methods developed in order to assess dynamic comfort. Andersson et al. [1] used surface electromyography (SEMG) to analyze muscle activation levels, while Cholewicki et al. [2] used the Center of Pressure (CoP) to quantify postural control in lumbar muscles. Some studies combined several methods to evaluate comfort. For instance, Grabish et al. [9] demonstrated that fuzzy measures and integrals provided a powerful methodology to model complex (multidimensional) subjective sensations. Kolich and Taboun [14] demonstrated that seat comfort could be assessed using SEMG data as well as subjective responses for long duration trials. Other studies combined acceleration and SEMG measurements [5], [19], [20]. Therefore, SEMG has been widely used for comfort assessment.

In ergonomics, SEMG is a non-invasive method that can be used to provide information on muscle activity. Some examples, of ergonomic applications of SEMG include the response of the trunk muscles to postural perturbations in sitting subjects [21], the assessment of head rest comfort in cars [15], or the comparison of muscle activity between different car seats due to changing backrest inclination, lumbar support position and seat inclination [1], [12]. Hostens and Ramon [13] observed a decrease in mean frequency of SEMG signals after one hour of car driving with a corresponding increase in muscle stiffness reported.

Although SEMG is a useful tool for objective comfort evaluation, there are a number of problems associated with SEMG recordings in ergonomic settings that make data analysis difficult. The recording environment introduces additional noise that alters EMG characterization and the extraction of relevant parameters. For instance, El Falou et al. [6] showed that classical signal processing methods were unable to identify muscular fatigue in SEMG recordings despite the presence of subject discomfort, whereas a specific process of elimination of non-postural EMG segments performed on the same data enabled the identification of muscular fatigue [7].

The aim of this paper is to objectively assess the response of car passengers to lateral accelerations on a 5-km circuit (similar to a state highway), with an average velocity of 110 km/h. The present study was conducted as an application of the methodology of SEMG extraction described in part I. The purpose is to obtain an objective evaluation of discomfort when turning in order to compare different chassis-seat configurations subjected to lateral accelerations.

Section snippets

Subjects

Eighteen male subjects participated in the study. Subjects’ mean age, height and mass were 43 ± 8 y, 180 ± 7 cm and 83 ± 14 kg, respectively. All subjects were passengers during the trials.

Electromyography

Surface EMG signals were recorded bilaterally from five muscles using a bipolar electrode configuration. The muscles chosen, after pilot testing, were external oblique (EO), latissimus dorsi (LD), erector spinae iliocostalis (ES), cervical erector spinae (CES) and vastus lateralis (VL). When an additional letter L or

Results

The mean lateral acceleration value, recorded during left turns was approximately equal to 0.31 g (with acceleration range from 0.22 to 0.38 g depending on the turns), and 0.30 g for right turns (acceleration range from 0.17 to 0.41 g depending on the turns). All segments classified as EMG activity were found to be induced by lateral accelerations. When cars were in the straight sections of road on the testing circuit, no phasic EMG segments were detected. Muscle activity was significantly

Discussion

The experimental protocol used in the present study proved to be reliable. Given that the study did not aim to evaluate the effect of fatigue, a three-minute circuit with six turns was sufficient to detect significant differences between the configurations. The use of passengers as subjects, who were required to maintain a pre-imposed position, enabled differences in activation levels required to maintain a steady position to be identified. Such an experimental constraint was chosen in order to

Conclusion

The present study was able to identify differences in lateral support between different vehicles using SEMG recordings. Muscle activity was found to be highly correlated with lateral acceleration and significant differences were observed with respect to the acceleration direction. Patterns of muscle activation indicated that passengers counteracted lateral accelerations by using a bust torsion, at the same time as an activation of their quadriceps. It could be concluded that greater roll

Georges Farah was born in Lebanon in 1979. He received a diploma in Telecommunication and Computer Engineering from the Lebanese University of Tripoli in 2002 and a Masters in System Optimization and Security from the University of Technology of Troyes (France) in 2003. He is currently studying for a PhD in the University of Technology of Troyes in collaboration with the Ergonomics Research Division of Renault. His main interests are data fusion, data mining, signal processing, and

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Georges Farah was born in Lebanon in 1979. He received a diploma in Telecommunication and Computer Engineering from the Lebanese University of Tripoli in 2002 and a Masters in System Optimization and Security from the University of Technology of Troyes (France) in 2003. He is currently studying for a PhD in the University of Technology of Troyes in collaboration with the Ergonomics Research Division of Renault. His main interests are data fusion, data mining, signal processing, and electromyography.

Claire Petit-Boulanger received her master of Neurophysiology in 1986 from Paris VI University and since this date has been working for the research Division of the car manufacturer Renault. She began studying alertness and problems of drowsiness at the wheel until 1990. Thereafter, she participated in the creation of a new team working on driver behaviour, using parameters of the Autonomic Nervous System. Since 1990, she has gained experience in the study of mental and emotional loads associated to the use of safety and comfort systems, using electrodermal responses.

David Hewson worked as a research physiologist for the Royal New Zealand Air Force between 1994 and 2000, and received his PhD from the University of Auckland in 2000. He is now an associate professor at the University of Technology of Troyes. His research interests are ergonomic and clinical applications of surface electromyography, and the development of new methodologies for fall prevention in the elderly.

Jacques Duchêne received the M.Sc. degree in electronic engineering from the Ecole Supérieure d’Electricité (France) in 1973, and the doctorat d’état in sciences in 1983. He joined the University of Technology of Troyes in 1994, where he is charge of the research management and post-graduate school direction. His main research interests are signal processing and pattern recognition with specific reference to new methodologies for pattern classification. He now focuses on signal segmentation as well as characterisation of time-frequency and time-scale distributions. The main domains of application in biomedical engineering are ergonomics (comfort in cars), medical monitoring (uterine EMG) and EMG characterisation (frequency parameters and conduction velocity).

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