Driver sitting comfort and discomfort (part I): Use of subjective ratings in discriminating car seats and correspondence among ratings

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Abstract

Several subjective rating schemes were investigated to determine which might be the most effective for use in designing and evaluating car seats, and what relationships exist among these schemes. Participants (n=27) completed short-term driving sessions, in six combinations of seats (from vehicles ranked high and low on overall comfort), vehicle class (sedan and SUV), and driving venue (lab-based and field). Overall ratings were obtained, as well as separate measures of comfort and discomfort of the whole body and local body parts. No association was found between subjective ratings and a publicly available overall vehicle comfort score (J.D. Power and Associates’ Comfort Score), implying that other factors besides sitting comfort/discomfort (and car seats) account for overall vehicle comfort. Other major results were that contemporary car seats appear to best accommodate those of middle stature, that packages/seats of sedans were preferred over those of SUVs, that separate processes appeared to be involved in determining whole body comfort and discomfort, and that ratings of comfort were most effective at differentiating among the car seats. Finally, a scheme for the use of subjective ratings was suggested: discomfort ratings for ensuring basic seat requirements (pain prevention-oriented) and comfort ratings for promoting advanced seat requirements (pleasure promotion-oriented).

Relevance to industry

Evidence regarding the advantages and disadvantages of different subjective rating schemes can facilitate future design and evaluation of automotive seats.

Introduction

Seated postures have been regarded as potentially unhealthy and considered as one of the major contributing factors for several musculoskeletal disorders such as pain in the lower back (LB) (Ebe and Griffin, 2001), neck (Schneider and Ricci, 1989), and shoulder (Magnusson and Pope, 1998). Due to increased exposures to seated postures (Grieco, 1986), especially in the car (Rajput and Abboud, 2007), sitting comfort has become an important issue that demands adequate ergonomic interventions (Dunk and Callaghan, 2005).

A driver's sitting comfort, however, needs to be distinguished from sitting comfort in home or office chairs, or in the non-vehicular workplace (Andreoni et al., 2002). The former involves more restrictions to posture in a more limited space, several controlling tasks, and vibration from the road that can lead to a higher risk of musculoskeletal disorders. Indeed, an epidemiological study by Gyi (1996) showed that people exposed to over 4 h of driving per day were more than twice as likely to suffer from low back pain compared to those with over 4 h of sedentary work per day. Annett (2002) defined a generalized relationship between ergonomics constructs (in our case, sitting comfort) and challenge factors (i.e., task, object, environment), and regarded a construct as a complex (i.e., behavioral, verbal, or physiological) response to a challenge. This author's view also supports that in-vehicle sitting comfort needs to be distinguished, as it is under different challenge factors (e.g., different levels of confined space, vibration, and visuomotor demands). Despite this, relatively less attention has been paid to sitting comfort in a car (Harrison et al., 2000).

In addition, sitting comfort needs to be divided into sitting comfort and discomfort. Several studies have suggested that comfort and discomfort be treated as complementary but independent entities (Sauter et al., 2005; Zhang et al., 1996). Similarly, using Maslow's hierarchy of needs (1943), Hancock and Pepe (2005) showed that discomfort and comfort are at different stages of needs, the latter being placed at a higher stage than the former. Most studies that have quantified subjective responses in seated postures, however, have included only a discomfort scale (e.g., Hsu and Wang, 2003; Jung and Choe, 1996; LeBlanc et al., 2003; Straker et al., 1997), largely due to their primary focus on pain prevention (Hancock and Pepe, 2005). Other studies have used one scale to measure mixed feelings by placing comfort and discomfort on opposite end of a continuum (e.g., Genaidy et al., 1995; Genaidy and Karwowski, 1993; Kee and Karwowski, 2001, Kee and Karwowski, 2003, Kee and Karwowski, 2004). In other studies, comfort was not measured and only a discomfort scale was used with supplemental objective measures such as electromyography (EMG), center of pressure (COP), or interface pressure (e.g., Fenety et al., 2000). In their review of literature on sitting comfort and discomfort, de Looze et al. (2003) observed that in none of the reviewed studies had comfort and discomfort been simultaneously but separately rated using two different scales.

Driving postures are related to both comfort and discomfort. In a study by Hanson et al. (2006), participants described their preferred driving posture using adjectives. A total of 119 descriptions were collected and classified into four groups: mental (comfortable, relaxing, restful, nice, peaceful, and calm), environment (adjustable, adaptable, flexible supportive, spacious, and good field of vision), generally positive (good, perfect, fantastic, and wonderful), and generally negative (troublesome and less good). Compared to the descriptors given in the study of Zhang et al. (1996) on sitting comfort and discomfort (Fig. 1), their findings suggest that the driving posture indeed is related to both comfort and discomfort. From this, it can again be argued that subjective responses to driving postures should be rated in terms of comfort and discomfort using two separate scales.

Zhang et al. (1996) denoted that comfort and discomfort are not merely opposing constructs, and identified well-being as one component of comfort. According to Warr (1999), well-being should also be treated as an independent construct. He showed that well-being can be measured on the three axes of comfort–anxiety, pleasure–displeasure, and enthusiasm–depression. A good state of well-being is partially related to comfort (e.g., at ease, relaxed), but not inclusively (e.g., enthusiastic and pleased are not part of comfort). Likely, a bad state of well-being is partially related to discomfort (e.g., uneasy, fatigued), but again not inclusively (e.g., anxious and depressed). A model combining the views of Zhang et al. and Warr (Fig. 1) indicates that some components of comfort and discomfort indeed have opposite meanings (e.g., supported, at ease vs. unsupported, uneasy), or are common (e.g., sleepy, drowsy), and that well-being is related to both comfort and discomfort, but is not merely part of comfort or discomfort.

Hanson et al. (2006) showed that preferred driving postures are not bilaterally symmetric, or that different comfort/discomfort ratings may be obtained for paired body parts. However, previous studies have tended to assume a symmetrical driving posture between the two sides (e.g., Porter and Gyi, 1998; Reed et al., 2002). In addition, most previous studies on sitting comfort/discomfort have made restrictions on seat configurations (e.g., seat back/cushion reclining angle) and/or sitting postures (e.g., torso/knee angle), which may have prohibited participants from being in the most comfortable (preferred) postures possible according to their sitting strategy (Andreoni et al., 2002), and interrupting necessary adjustments of the seat and steering wheel.

The primary objective of this study was to compare among several subjective ratings and to suggest their appropriate usage in the assessment of subjective responses in driving posture. As addressed above, designing a car seat is a challenging task that must meet multiple requirements; within a confined space where vibration is generally present, the car seat is required to accommodate diverse groups of people by firmly supporting and physically fitting their preferred postures as well as by allowing freedom to change postures. In order to support some of these requirements, this study investigated which, among several, subjective rating schemes might be the most effective for use in designing and evaluating car seats, and what relationships exist among these schemes. In addition, the association between sitting comfort/discomfort in the car seat and a publicly available overall vehicle comfort score was examined.

Section snippets

Overview of experiment and participants

The experiment involved a number of driving sessions, in which a variety of subjective comfort and discomfort responses were obtained. Of particular interest were how these responses were influenced by stature, vehicle segment, specific seat, and whether driving was real (on-road) or simulated (lab-based), each of which were independent variables. In addition to the subjective responses, other measures were obtained (posture and pressure) but are not presented here.

Twenty-seven volunteers were

Whole body overall rating

There was a significant (p=0.0056) interaction effect of Stature×Seat Condition on whole body overall ratings. Stature was not a significant main effect, while the effect of Seat Condition was marginal (p=0.058). Overall ratings tended to be higher in the seats of lower-rated vehicles (S2 and U2; Fig. 3). A significant difference between vehicle classes was found (p=0.018), wherein sedan conditions were rated higher than SUVs (Fig. 3), with means (S.D.) of 84.1 (13.3) and 80.7 (15.1),

Discussion

Three rating schemes were used to quantify subjective responses to driving postures. Of primary interests were their inter-relationships and their relative effectiveness in distinguishing car seats, with a longer-term goal of determining suitable methods for obtaining such subjective responses. Toward these aims, several factors were included (seats, driving venues, vehicle classes, and stature groups), and a variety of analyzes were employed. Effects of each factor are described and

Conclusions

Three different rating scales (overall, comfort, and discomfort ratings) were employed to obtain subjective responses to car seats from vehicles that were rated differently in terms of overall vehicle comfort (J.D. Power and Associates’ Comfort Score). No significant associations were found between the Comfort Score and the three ratings. Additional factors (e.g., package, dynamic factor, and aesthetics of other parts in the vehicle) are thus likely being used in determining a vehicle's overall

Acknowledgments

Hyundai Kia Motors is acknowledged for the support they provided for this study. The paper's contents are solely the responsibility of the authors and do not necessarily represent the official views of the sponsor. In addition, the authors thank Mr. Randy Waldron and his colleagues for developing the driving rig.

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