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HFSXXX10.1177/0018720815584866Human FactorsSelf-Selected LSP
Spine Posture and Discomfort During Prolonged Simulated Driving With Self-Selected Lumbar Support Prominence Diana E. De Carvalho and Jack P. Callaghan, University of Waterloo, Waterloo, Canada Objective: We examined magnitude preference, subjective discomfort, and spine posture during prolonged simulated driving with a self-selected amount of lumbar support. Background: The general use of lumbar supports has been associated with decreased reports of lowback pain during driving exposures; however, minimal data exist regarding occupant magnitude preference. Method: Participants chose between five discrete levels of lumbar support (0–4 cm). Time-varying postural and discomfort responses were then monitored throughout 2 hr of simulated driving. Results: There were no significant effects of gender or time on posture. Women preferred larger amounts of support than men (3.25 cm ± 0.71 and 2.56 cm ± 0.88, respectively, p = .048). All participants exhibited significant increases (p = .003) in pelvic discomfort throughout the 2-hr trial regardless of the level of support chosen. Discomfort related to various aspects of the lumbar support increased significantly over time. Retrospectively, no participants desired a setting beyond 4 cm, and the majority of respondents indicate had they been able to change their initial selection, they would choose a setting between 2 and 3 cm. Conclusion: The results suggest that occupants would prefer increasing the excursion capability of automobile lumbar supports beyond 2 cm. Application: Excursion capability and adjustability of automobile lumbar supports are important features to better meet end-user preference and to reducing lumbar flexion in sitting. Keywords: spine, physical ergonomics, interventions, biomechanics, vehicle design, gender
Address correspondence to Jack P. Callaghan, Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, 200 University Ave West, Waterloo, ON N2L3G1, Canada; e-mail: [email protected]. HUMAN FACTORS Vol. XX, No. X, Month XXXX, pp. 1–12 DOI: 10.1177/0018720815584866 Copyright © 2015, Human Factors and Ergonomics Society.
Introduction
Low-back pain associated with prolonged driving is a well-documented problem worldwide (Akinbo, Odebiyi, & Osasan, 2008; AlperovitchNajenson et al., 2010; Anderson, 1992; Andrusaitis, Oliveira, & Barros Filho, 2006; Boshuizen, Bongers, & Hulshof, 1990; Bovenzi, 2010; Brown, Wells, Trottier, Bonneau, & Ferris, 1998; Chen, Chang, Chang, & Christiani, 2005; Gyi & Porter, 1998; Lyons, 2002; Magnusson, Pope, Wilder, & Areskoug, 1996; Miyamoto, Shirai, Nakayama, Gembun, & Kaneda, 2000; Okunribido, Magnusson, & Pope, 2006; Okunribido, Shimbles, Magnusson, & Pope, 2007; Prado-Leon, Aceves-Gonzalez, & Avila-Chaurand, 2008; Waters, Genaidy, Barriera Viruet, & Makola, 2008). While there are numerous factors that impact the low back during driving, sustained flexion of the low back (loss of lumbar lordosis) likely plays a role in pain generation. Vehicle seat design itself has the direct ability to improve spine posture in automobile sitting. Lumbar supports have been shown to quantitatively increase the lumbar lordosis (Andersson, Murphy, Ortengren, & Nachemson, 1979; De Carvalho & Callaghan, 2012; Hazard & Reinecke, 1995) and reduce disc pressure and muscle activity (Andersson, Ortengren, Nachemson, & Elfstrom, 1974; Kingma & van Dieen, 2009). Their use in automobile seats has been associated with decreased reports of low-back pain during driving exposures (Chen, Dennerlein, Chang, Chang, & Christiani, 2005). Mechanical lumbar supports, beyond extra cushioning and trim, that are adjustable and provide massage-type or programmed movement have been shown to reduce vibration transmission and muscle activity, improve blood flow, and dramatically reduce driver discomfort (Donnelly, Callaghan, & Durkin, 2009; Durkin, Harvey, Hughson, & Callaghan, 2006; Kingma & van Dieen, 2009).
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Our research group has conducted a radiographic investigation of the prototype lumbar support used in this current paper (De Carvalho & Callaghan, 2012) and confirmed improved lumbar spine posture with increasing lumbar support prominence (LSP). The lumbar lordosis angles were found to increase from 20° with 0 cm or no support to 30° with 4 cm LSP (De Carvalho & Callaghan, 2012; De Carvalho, Soave, Ross, & Callaghan, 2010). However, the experimental design was limited to male participants, and we investigated only three levels of magnitude (0 cm, 2 cm, and 4 cm) for extremely short durations to limit radiographic exposure. Moreover, data pertaining to user preference or the effect of the support levels during a prolonged exposure were not ascertained. Most automobile seats are designed for the 50th percentile adult male (Kolich, 2003), decreasing the likelihood that the majority of the population will find appropriate support for the low back. It has also been discussed that fixed lumbar prominences, when they do not match the contour of the occupant’s back, can contribute to discomfort (Reed & Schneider, 1996). Although no current standards exist regarding the exact amount of support that should be provided, an LSP of 2 cm for fixed lumbar supports has been suggested in the literature (Reed, Schneider, & Ricci, 1994). However, what would the end user prefer? Kolich and colleagues document user preference for lumbar support apex height: a range of 90 to 123 mm above the H-point (Kolich, 2003), a theoretical point based on a mannequin measurement to represent the hip of a 50th percentile male, yet virtually no work has been conducted with regard to occupant preference of lumbar support excursion. The results of one study on 15 men concluded that 1 cm of LSP is favorable for short-duration (10 min) simulated driving (Lim, Chung, & Na, 2000). However, caution must be taken when interpreting perceived automobile seat comfort based on shortterm exposures (Gyi & Porter, 1999); therefore, support preferences for prolonged exposures and women specifically remain unexamined. There is limited work aimed at determining the support magnitude preferred by occupants and the resulting postural and discomfort effect imparted by these choices. In this study we examined the responses of seated users during a 2-hr prolonged
period of simulated driving with self-selected LSP magnitudes. The specific goals of the study were (a) to establish preliminary data on the lumbar support preferences of men and women (both initial and retrospective) for prolonged simulated driving and (b) to explore gender differences in spine posture, self-perceived comfort (Automotive Seating Discomfort Questionnaire [ASDQ]; Smith, Andrews, & Wawrow, 2006), and discomfort (Visual Analogue Scale [VAS]) throughout a prolonged period of simulated driving with a selfselected amount of lumbar support. The null hypothesis, that there would be no difference between the magnitude preference of men and women, was expected for self-selected LSP. Since participants were given the choice to self-select the amount of lumbar support, no differences in posture were expected between genders or throughout the trial. Discomfort, however, was hypothesized to increase over the prolonged simulated driving trial, given the results of prior work (Callaghan, Coke, & Beach, 2010). Method Participants
Seventeen participants, nine men and eight women, with no recent history of low-back pain were recruited from a university population (men: average age 24.9 years ± 4.3, height 1.84 m ± 0.08, weight 90.4 kg ± 12.3; women: 23.8 years ± 2.8, height 1.61 m ± 0.24, weight 59.2 kg ± 5.7). Ethics approval was obtained from the Office of Research Ethics at the University of Waterloo. Data Collection
Lumbar spine flexion and pelvic angles were obtained using triaxial accelerometers (S2-10G-MF, NexGen Ergonomics, Montreal, QC, Canada) as tilt sensors. Sensors were mounted with the +y-axis directed downward over the L1 and S2 spinous processes with double-sided tape and secured with flexible medical tape. Two 5-s normalization trials were collected: upright standing and maximum lumbar flexion standing. Participants then completed a baseline rating of perceived discomfort (RPD). Levels of discomfort for seven areas of the body (upper lumbar spine,
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Figure 1. Configuration of driving simulator setup and detail image of the prototype lumbar support with vertical opening (inset).
lower lumbar spine, bilateral iliac crests, sacrum, and bilateral buttocks) were rated on a 100-mm VAS with anchors of no discomfort (0 mm) and worst discomfort imaginable (100 mm). Participants were then seated in a driving simulator consisting of the following: seat (Crown Victoria Model EN114 2007, Lear Seating Corporation, Southfield, MI, USA), gaming steering system with a Ford Crown Victoria steering wheel attached, pedals, and modified dashboard. All components of the simulator were mounted according to the internal configuration and dimensions of a standard Ford Crown Victoria (Figure 1). This same seat and lumbar support system were previously used for a radiographic study (De Carvalho & Callaghan, 2012). Each occupant adjusted the fore/aft placement of the seat individually such that he or she could reach the pedals and steering wheel comfortably. The backrest angle, orientation of the upper part of the seat with respect to the vertical, was fixed at 20° for all participants. This angle corresponds to approximately 100° with respect to the seat cushion. This angle was chosen based on the work of Andersson et al. (1979) that suggests that 100°
of backrest inclination relieves back muscle activity and reduces low-back flexion. Backrest angle was measured at LSP 0 cm using the HPM-II ASPECT manikin according to SAE J4002 specifications, and this inclination was locked in the calibrated position. The vertical location of the lumbar support apex was centered at the third lumbar vertebrae for each participant based on palpation by the examiner (prior to being seated, surface landmarks were identified and marked with a pen and were then reconfirmed visually once the participant was seated). The posterior aspect of the seat trim was removable, revealing the lumbar support mechanism (apex marked). A thin vertical opening in the seat foam and trim ensured visual confirmation of skeletal landmarks and provided space to ensure that the accelerometers did not contact the seat (Figure 1, inset). The LSP for the seat was determined by the manufacturer using an HPM-II manikin and measured in millimeters of horizontal shell deflection (Michida, Okiyama, & Matsuhashi, 2005; Schneider et al., 1999). A controller specific to this study was created to adjust LSP magnitude discretely
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between five settings using buttons (0 cm through 4 cm). To our knowledge, this is the first study that has used the HPM-II manikin for the quantification of LSP. Past work on lumbar supports has involved methods that are difficult to replicate; thus, the use of this standard tool is a great strength of this work. Settings beyond 4 cm LSP were not included in this study due to limitations in the mechanical design of the lumbar support unit. The driving simulator (STISIM Drive™ v2.0, Systems Technology Inc., Hawthorne, CA, USA) course was intended to represent highway driving and was projected onto a screen in front of the driver. After the seat adjustments were complete, the participants remained in the seat and were then given a lumbar support controller and a chance to try each level. To standardize the experience of each participant, each level from 0 cm to 4 cm was then presented for a period of 2 min each. Following this procedure, participants were asked to choose the level of support they would like to use for a long drive. Once this level was selected, and a 2-min adjustment period had passed, participants completed a pain-rating VAS and ASDQ (Smith et al., 2006). For the 2-hr driving trial, participants were instructed to keep both hands on the steering wheel at the 10 and 2 o’clock positions, stay on the simulation course, and maintain a constant speed of 100 km/h. Throughout the prolonged trial, postural data were collected continuously. For ease of data processing and analyzing, data were separated into eight 15-min intervals, and the first 2 min of each 15-min trial were analyzed. RPDs were completed at 15-min intervals throughout the prolonged driving trial for a total of nine measures over the 2 hr. At the end of the simulated driving period, a second ASDQ and final RPD were completed. Following collection, an outtake questionnaire regarding the lumbar support was completed. Participants were asked (a) whether or not they would have changed their chosen lumbar support excursion if given a chance, (b) which direction (increasing or decreasing support) they would have chosen, and (c) how large this change would have been. Data Reduction
Accelerometer signals were A/D converted at rates of 256 Hz with a 16-bit A/D system
(Optotrak Data Acquisition Unit II, Northern Digital Inc., Waterloo, ON, Canada). A secondorder low-pass Butterworth filter with an effective cutoff frequency of 1 Hz was used to filter these data (Dunk & Callaghan, 2010). Using the inclinations of the sensitive axes (y and z) of the accelerometers with respect to gravity, lumbar and pelvic angles were calculated according to their corresponding radiographic measures using custom written software (Matlab Version R2012b, MathWorks, Natick, MA, USA). Specifically, lumbar angles were taken as the difference between the inclination of the top and bottom accelerometers, respectively, and pelvic angle was taken as the inclination of the bottom accelerometer with respect to vertical. Negative angles represent backward tilt. These data were then normalized for each participant: Lumbar flexion angles were normalized to a percentage of standing maximum flexion, and pelvic angle was presented relative to the angle achieved in upright standing. RPDs for seven areas of the back (upper lumbar spine, lower lumbar spine, right iliac crest, left iliac crest, sacrum, right buttock, and left buttock) were expressed relative to the baseline values reported at the start of the collection protocol. Questions 13 to 20 of the ASDQ were analyzed as these questions dealt directly with the lumbar support and overall perceived comfort of the automobile seat. Gyi and Porter (1999) showed that initial ratings of seat comfort are not usually indicative of true seat comfort, so the responses to the postdrive ASDQ were compared to the predrive questionnaire to evaluate changes in perception. Statistics
A two-way analysis of variance, with time as the within factor and gender as the between factor, were completed to compare dependent variables during the prolonged driving trial (SAS Version 9.2, SAS Institute Inc., Cary, NC, USA). Tukey’s studentized range test post hoc was used on all significant effects within the time factor. A two-tailed Student’s T test was used to compare lumbar support choice for the prolonged driving trial between men and women. In all statistical tests, a p value of less than .05 was accepted as statistically significant. To evaluate the effect size of gender differences
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Figure 2. Normalized lumbar flexion angles (% range of motion) throughout the prolonged driving trial for men and women. Error bars are standard deviations. No significant differences were found for time or gender.
in posture, Cohen’s ds was calculated using Hedge’s gs correction to account for the small sample size (Lakens, 2013). Results
Women selected larger support levels than men (women: average 3.25 cm ± 0.71 cm [SD], range 2 cm to 4 cm; men: average 2.56 cm ± 0.88 cm [SD], range 1 cm to 4 cm, p = .048), and no participants selected the 0-cm support level. Analysis of time-varying posture data throughout the simulated driving trial revealed that lumbar and pelvic angles remained consistent throughout the 2-hr exposure with no significant main effects of time (lumbar, p = .4527; pelvic, p = .521; Figures 2 and 3). There were no significant differences between the lumbar (p = .2882) or pelvic angles (p = .0885) of men and women. There was a trend for women to sit with more lumbar flexion and posterior pelvic rotation than men. Significant main effects of time were found for RPD, with an increase of discomfort over the prolonged driving for the following body areas: sacrum (p = .003), bilateral iliac crests (p < .0001),
left buttock (p = .0002), and right buttock (p < .0001; Figure 4). Although there was a tendency for women to report lower levels of low-back discomfort (baseline removed), there was no significant main effect of gender (p = .1286). Results from the ASDQ showed significant increases in discomfort associated with certain aspects of the seat, notably, the backrest contour (p = .0051), lumbar support stiffness (p = .0123), lumbar prominence (p = .0039), vertical location of lumbar support (p = .0241), and pressure from lumbar support (p = .0030). Further, overall seat discomfort (p = .0016) was also found to increase significantly over time. Outtake Questionnaire
Subjective outtake questionnaire responses indicated that no participants would have desired a setting beyond 4 cm, and 76% of the participants would have preferred to change their LSP setting at some point during the driving trial if they were able to. Taking this hindsight preference into account, it appears that the majority of participants would choose a magnitude of support greater than 2 cm and most
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Figure 3. Relative pelvic angles (with respect to upright standing) throughout the prolonged driving trial for men and women. Error bars are standard deviations. No significant differences were found for time or gender.
Figure 4. Average perceived discomfort (baseline removed) throughout 2 hr of driving for women (A) and men (B). Downloaded from hfs.sagepub.com at GEORGIAN COURT UNIV on May 9, 2015
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importantly would want the ability to modify support level during a prolonged driving exposure. Considering these preliminary findings, it appears an adjustable support exceeding 2 cm LSP would be a desired capability in automobile seat designs. Discussion
Lumbar and pelvic postures remained relatively unchanged throughout the 2 hr of simulated driving, and postural angles were not significantly different between men and women. Therefore we can accept our hypothesis that there would be no differences in lumbar or pelvic posture throughout the driving trial or between genders. Small and moderate corrected effect sizes were found for these measures, respectively, between genders (lumbar angle, Hedge’s gs = .2854; pelvic angle, Hedge’s gs = .6582), suggesting there is potential that gender differences in pelvic posture could emerge with a larger sample size (Lakens, 2013). The normalized lumbar and pelvic angle magnitudes found in this study are consistent with prior work (Beach, McDonald, Coke, & Callaghan, 2008; Callaghan et al., 2010; De Carvalho & Callaghan, 2011). Although the postures do appear extreme (approaching or even exceeding 100% of normalized forward flexion capability), past findings have shown average automobile seat posture consistently ranging from 50% to 80%, with individual scores commonly approaching or exceeding 100%. Further, there is radiographic evidence that shows the lumbar lordosis angle (measured from L1-S1) in sitting ranges from 20° to 40° in the same automobile seat tested in this study (De Carvalho & Callaghan, 2012). Miyasaka, Ohmori, Suzuki, and Inoue (2000) radiographically measured an average range of motion of 78.8° ± 10° for the lumbar lordosis angle (L1-S1) through maximum extension to maximum flexion in 90 adults. Considering the normal ranges of motion of the lumbar spine are 40° to 60° in flexion and 20° to 35° in extension (Magee, 2002), it does appear that the magnitudes of lumbar flexion achieved in an automobile seat could approach end-range flexion for the lumbar spine. Other potential explanations for these extreme numbers could be as follows: (a) Creep of passive
tissues in the low back may lead to increased range of motion in flexion occurring after a period of 20 min (participants in our study were seated for approximately 20 min prior to the start of the prolonged trial) as documented by McGill and Brown (1992), or (b) the “maximum flexion” normalization posture achieved was less than a true maximum. We did try to standardize the instructions given to participants during the normalization trials, encouraging motion at the spine in flexion as opposed to hinging at the hip, however, as both the low back and hips are inherently linked and normally both involved in forward flexion, isolating pure lumbar flexion can be difficult. Self-selected lumbar support, with settings beyond the lumbar support deflections typically available commercially, likely contributed to the lack of gender differences that have been previously reported (Beach et al., 2008; Callaghan et al., 2010). Differences in preferred seated postures of men and women (Dunk & Callaghan, 2005) have been previously documented in office chairs. However, the physical constraints of driving (interacting with pedals and steering wheel) and the size of the seat itself remove the ability of women to adopt their preferred posture. From past studies, this preference appears to be sitting with less lumbar flexion, a more anteriorly rotated pelvis, and center of mass positioned more anteriorly on the seat (Beach et al., 2008; Callaghan et al., 2010; Dunk & Callaghan, 2005). Attempting to reduce the increased lumbar flexion imposed by sitting in a driver’s seat might be one explanation for the tendency of women to choose higher amounts of lumbar support in this study. Participants adopted fairly static postures throughout the prolonged driving trial as reflected in the lack of significant differences in average spine angles over time. The static nature of torso posture during prolonged driving has been documented previously (De Carvalho & Callaghan, 2011; Reed, Manary, Flannagan, & Schneider, 2000). This phenomenon is likely due to the constraints imposed by interacting with the pedals, steering wheel, and seat. There is evidence of significant increases in tension of the posterior spine elements starting with 75% of maximum lumbar flexion (Adams, McNally, Chinn, & Dolan, 1994). Further, increases of 2.5° in
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Figure 5. Low-back discomfort ratings stratified by lumbar support prominence chosen for women (A) and men (B) over the 2-hr driving trial.
active range of flexion has been documented after 1 hr of static sitting at 70% range of motion, reflecting the effect of viscoelastic creep in posterior spine structures (Sanchez-Zuriaga, Adams, & Dolan, 2010). Such passive loading has been shown to have a direct adverse effect on musculoligamentous health of the lumbar spine (Solomonow, 2009; Solomonow, Zhou, Baratta, &, Burger, 2003). Therefore, the degree of lumbar spine flexion exhibited in this study, even with added lumbar support, could be cause for concern when maintained for extended periods of time. The third hypothesis, that discomfort would increase over the prolonged simulated driving trial, was accepted. Not surprising was that significant increases in discomfort over time were found in both the RPD and ASDQ questionnaires as projected. This finding is consistent with past studies on prolonged driving (Callaghan et al., 2010; De Carvalho & Callaghan, 2011; Donnelly et al., 2009; Durkin et al., 2006; El Falou et al., 2003; Kyung & Nussbaum, 2008; Na, Lim, Choi, & Chung, 2005; Porter, Gyi, & Tait, 2003). It was interesting to note the regional differences in perceived discomfort, with only areas of the pelvis (buttock, sacrum, etc.) exhibiting this increase. There is the potential that the
presence of a lumbar support played a role in minimizing low-back discomfort while contributing to discomfort at the pelvis. The design of this study prohibits this conclusion; however, this would be an interesting research question to pursue in the future. In the discussion of the radiographic results of our previous study, we surmised that the increased extension imparted at the low back by the lumbar support coupled with the lack of support at the pelvis could place increased stress at the lumbosacral junction (De Carvalho & Callaghan, 2012). Pain referral from the lower lumbar facet joints has been documented into the gluteal region (Windsor et al., 2002); thus, it does appear realistic that this lack of direct pelvic support could generate the observed discomfort pattern through the pelvis. When the discomfort of the low back was stratified by the LSP setting chosen by participants for the 2 hr of prolonged driving, there was a clear trend of a lower level of discomfort developing in the 3-cm group (Figure 5). Statistical analyses could not be completed on the LSP grouping due to small sample numbers in each selection group and with the 0-cm setting not chosen at all. There was also a trend for discomfort levels to decrease as LSP magnitude increased from 1 cm to 3 cm and then for discomfort to
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increase with the 4-cm LSP setting. Given the high rate of selection for the 3-cm setting (8 of 17 participants) and the directional responses of users who chose 2 cm and would have increased the LSP excursion if allowed (2 of 4 participants) and those who started with 4 cm and would have preferred a lower setting (2 of 4), it appears reasonable that an adjustable LSP with excursion capacity greater than 2 cm and less than 5 cm would accommodate the preferences of most automobile seat users; however, further research is warranted to confirm this. Potentially more important than the amount of support is the allowance of adjustability of the support, which is indirectly supported by the outtake questionnaire. Reed and Schneider (1996) discussed the importance of lumbar support adjustability as a way to ensure support prominence appropriately matches the contours of the occupant’s back. It appears relevant, then, that changes to LSP might be necessary over prolonged periods of time as deformations occur in both the seat (foam) and occupant (creep of tissues; De Carvalho & Callaghan, 2011; Kolich, 2009). Prolonged static postures are associated with increased discomfort over time (Tissot, Messing, & Stock, 2009; Van Nieuwenhuyse et al., 2004); therefore movement, either of the lumbar support itself or by the occupant taking a break and getting out of the vehicle, remain vital for preserving the overall health of the seated individual (Dunstan, Howard, Healy, & Owen, 2012; Proper, Singh, van Mechelen, & Chinapaw, 2011). Limitations
Although permitting participants to selfselect the amount of lumbar support provided the evaluation of realistic information regarding prospective and retrospective lumbar support preferences, it has limited the ability to conduct a complete statistical analysis on each support level. This design was a trade-off that was made a priori. Given that there is the preliminary indication that support level has a significant effect on discomfort and posture, larger followup studies need to be conducted to explore this effect more thoroughly. In order to minimize variability and confounding effects, the seat back angle was fixed and the apex of the lumbar support was directed
at the third lumbar vertebrae for all subjects. These factors raise two important limitations of this study. First, it has been shown that the size of the occupant and the amount of lumbar support used will result in a range of self-selected seat back angles (Reed, Schneider, & Eby, 1995). Therefore, the fixed backrest angle could have altered the degree of lumbar support chosen by participants. Similarly, the degree of preferred LSP could have been influenced by the vertical position of the support. In this study, adjustments to the LSP were made with the participant seated. This method was used to eliminate variability in whole-body postures that would occur with exiting/reentering the seat. There is evidence that differing effects on the occupant will occur with LSP adjustments in loaded (seated) or unloaded (unoccupied) scenarios. Specifically, the effect of a lumbar support on low-back posture is greater when the adjustment is made in the loaded condition (Reed & Schneider, 1996). It could be argued that people generally set their seat adjustment and do not change it once exiting/reentering the vehicle, perhaps limiting the applicability of our results. However, the results of this study indicated that 76% of individuals would have altered the lumbar support setting during the driving trial if allowed, suggesting that for longer driving exposures, individuals are likely to adjust lumbar support settings while seated. To limit variability, and in accordance with safety guidelines, we instructed participants to maintain two hands on the wheel. Recent naturalistic studies have indicated that gripping the wheel with two hands is not necessarily the norm (Jonsson, 2011). Therefore, following this instruction could have been unnatural for some of the participants in this study. Further, shoulder angle has been shown to change lumbar spine posture (Stagnara et al., 1982), so there is the potential that the spine angles found in this study could have varied with different steering wheel gripping methods. The impact of various self-selected driving postures on low-back accommodations should be targeted in future studies. Conclusions
When participants were given the choice to select the amount of lumbar support they
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thought would be comfortable for a long car drive, all participants selected at least some amount of support, and women tended toward choosing larger amounts of support than men. Seventy-one percent of the study population chose a support level above what has previously been recommended in the literature (2 cm). Further, self-selected lumbar support resulted in an absence of significant changes to lumbar spine and pelvic postures over time and between genders. Acknowledgments Jack P. Callaghan is supported by a Canada Research Chair in Spine Biomechanics and Injury Prevention. Diana E. De Carvalho was supported by a Canadian Institute of Health Research Doctoral Award.
Key Points •• Women tend to prefer larger amounts of lumbar support than men. •• Self-selected lumbar support during prolonged driving removes previously documented gender effects on spine posture. •• Adjustable lumbar supports appear to be an important feature for automobile seats.
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Jonsson, B. (2011). Hand position on steering wheel during driving. Traffic Injury Prevention, 12, 187–190. Kingma, I., & van Dieen, J. H. (2009). Car driving with and without a movable back support: Effect on transmission of vibration through the trunk and on its consequences for muscle activation and spinal shrinkage. Ergonomics, 52, 830–839. Kolich, M. (2003). Automobile seat comfort: Occupant preferences vs. anthropometric accommodation. Applied Ergonomics, 34, 177–184. Kolich, M. (2009). Repeatability, reproducibility, and validity of a new method for characterizing lumbar support in automotive seating. Human Factors, 51, 193–207. Kyung, G., & Nussbaum, M. A. (2008). Driver sitting comfort and discomfort (Part II): Relationships with and prediction from interface pressure. International Journal of Industrial Ergonomics, 38, 526–538. Lakens, D. (2013). Calculating and reporting effect sizes to facilitate cumulative science: A practical primer for t-tests and ANOVAs. Frontiers in Psychology, 4, 863. Lim, S., Chung, J., & Na, S. (2000). The effect of lumbar support prominence on driver’s comfort and body pressure distribution. In Proceedings of the IEA/HFES 2000 Congress (pp. 5-308–5-311). Santa Monica, CA: Human Factors and Ergonomics Society. Lyons, J. (2002). Factors contributing to low back pain among professional drivers: A review of current literature and possible ergonomic controls. Work, 19, 95–102. Magee, D. (2002). Orthopedic physical assessment (4th ed.). St. Louis, MO: Saunders. Magnusson, M. L., Pope, M. H., Wilder, D. G., & Areskoug, B. (1996). Are occupational drivers at an increased risk for developing musculoskeletal disorders? Spine, 21, 710–717. McGill, S. M., & Brown, S. (1992). Creep response of the lumbar spine to prolonged full flexion. Clinical Biomechanics, 7, 43–46. Michida, N., Okiyama, M., & Matsuhashi, K. (2005). Seat lumbar support evaluation with ASPECT manikin (SAE Technical Paper 2005-01-1007). doi:10.4271/2005-01-1007 Miyamoto, M., Shirai, Y., Nakayama, Y., Gembun, Y., & Kaneda, K. (2000). An epidemiologic study of occupational low back pain in truck drivers. Journal of Nippon Medical School, 67, 186–190. Miyasaka, K., Ohmori, K., Suzuki, K., & Inoue, H. (2000). Radiographic analysis of lumbar motion in relation to lumbosacral stability. Investigation of moderate and maximum motion. Spine, 25, 732–737. Na, S., Lim, S., Choi, H., & Chung, M. K. (2005). Evaluation of driver’s discomfort and postural change using dynamic body pressure distribution. International Journal of Industrial Ergonomics, 35, 1085–1096. Okunribido, O. O., Magnusson, M., & Pope, M. (2006). Delivery drivers and low-back pain: A study of the exposures to posture demands, manual materials handling and whole-body vibration. International Journal of Industrial Ergonomics, 36, 265–273. Okunribido, O. O., Shimbles, S. J., Magnusson, M., & Pope, M. (2007). City bus driving and low back pain: A study of the exposures to posture demands, manual materials handling and whole-body vibration. Applied Ergonomics, 38, 29–38. Porter, J. M., Gyi, D. E., & Tait, H. A. (2003). Interface pressure data and the prediction of driver discomfort in road trials. Applied Ergonomics, 34, 207–214.
Prado-Leon, L. R., Aceves-Gonzalez, C., & Avila-Chaurand, R. (2008). Occupational driving as a risk factor in low back pain: A case-control study in a Mexican population. Work, 31, 387–396. Proper, K. I., Singh, A. S., van Mechelen, W., & Chinapaw, M. J. (2011). Sedentary behaviors and health outcomes among adults: A systematic review of prospective studies. American Journal of Preventive Medicine, 40, 174–182. Reed, M. P., Manary, M. A., Flannagan, C. A., & Schneider, L. W. (2000). Effects of vehicle interior geometry and anthropometric variables on automobile driving posture. Human Factors, 42, 541–552. Reed, M., & Schneider, L. (1996). Lumbar support in auto seats: Conclusions from a study of preferred driving posture (SAE Technical Paper 960478). Warrendale, PA: SAE International. Reed, M., Schneider, L., & Eby, B. (1995). Some effects of lumbar support contour on driver seated posture (SAE Technical Paper 950141). Warrendale, PA: SAE International. Reed, M., Schneider, L., & Ricci, L. (1994). Survey of auto seat design recommendations for improved comfort (UMTRI-946). Technical report, University of Michigan Transportation Research Institute, Ann Arbor. Sanchez-Zuriaga, D., Adams, M. A., & Dolan, P. (2010). Is activation of the back muscles impaired by creep or muscle fatigue? Spine, 35, 517–525. Schneider, L., Reed M., Roe, R., Manary, M., Flannagan, C., Hubbard, R., & Rupp, G. (1999). ASPECT: The next-generation H-point machine and related vehicle and seat design and measurement tools (SAE Technical Paper 1999-01-0962). doi:10.4271/1999-01-0962 Smith, D. R., Andrews, D. M., & Wawrow, P. T. (2006). Development and evaluation of the Automotive Seating Discomfort Questionnaire (ASDQ). International Journal of Industrial Ergonomics, 36, 141–149. Solomonow, M. (2009). Ligaments: A source of musculoskeletal disorders. Journal of Bodywork and Movement Therapies, 13, 136–154. Solomonow, M., Zhou, B. H., Baratta, R. V., & Burger, E. (2003). Biomechanics and electromyography of a cumulative lumbar disorder: Response to static flexion. Clinical Biomechanics, 18, 890–898. Stagnara, P., De Mauroy, J. C., Dran, G., Gonon, G. P., Costanzo, G., Dimnet, J., & Pasquet, A. (1982). Reciprocal angulation of vertebral bodies in a sagittal plane: Approach to references for the evaluation of kyphosis and lordosis. Spine, 7, 335–342. Tissot, F., Messing, K., & Stock, S. (2009). Studying the relationship between low back pain and working postures among those who stand and those who sit most of the working day. Ergonomics, 52, 1402–1418. Van Nieuwenhuyse, A., Fatkhutdinova, L., Verbeke, G., Pirenne, D., Johannik, K., Somville, P. R., Mairiaux, P., Moens, G. F., & Masschelein, R. (2004). Risk factors for first-ever low back pain among workers in their first employment. Occupational Medicine, 54, 513–519. Waters, T., Genaidy, A., Barriera Viruet, H., & Makola, M. (2008). The impact of operating heavy equipment vehicles on lower back disorders. Ergonomics, 51, 602–636. Windsor, R. E., King, F. J., Roman, S. J., Tata, N. S., Cone-Sullivan, L. A., Thampi, S., Acebey, M., Gilhool, J. J., Rao, R., & Sugar, R. (2002). Electrical stimulation induced lumbar medial branch referral patterns. Pain Physician, 5, 347–353.
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12 Month XXXX - Human Factors Diana E. De Carvalho is a PhD candidate in the Department of Kinesiology, Faculty of Applied Health Sciences, at the University of Waterloo. She graduated as a doctor of chiropractic from the Canadian Memorial Chiropractic College in 2006 and with a master of science (kinesiology– biomechanics) in 2008 at the University of Waterloo.
Jack P. Callaghan is a professor in the Department of Kinesiology, Faculty of Applied Health Sciences, at the University of Waterloo. He completed his doctor of philosophy (kinesiology–biomechanics) in 1999 at the University of Waterloo. Date received: May 29, 2014 Date accepted: April 1, 2015
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HFSXXX10.1177/0018720815584866Human FactorsSelf-Selected LSP
Spine Posture and Discomfort During Prolonged Simulated Driving With Self-Selected Lumbar Support Prominence Diana E. De Carvalho and Jack P. Callaghan, University of Waterloo, Waterloo, Canada Objective: We examined magnitude preference, subjective discomfort, and spine posture during prolonged simulated driving with a self-selected amount of lumbar support. Background: The general use of lumbar supports has been associated with decreased reports of lowback pain during driving exposures; however, minimal data exist regarding occupant magnitude preference. Method: Participants chose between five discrete levels of lumbar support (0–4 cm). Time-varying postural and discomfort responses were then monitored throughout 2 hr of simulated driving. Results: There were no significant effects of gender or time on posture. Women preferred larger amounts of support than men (3.25 cm ± 0.71 and 2.56 cm ± 0.88, respectively, p = .048). All participants exhibited significant increases (p = .003) in pelvic discomfort throughout the 2-hr trial regardless of the level of support chosen. Discomfort related to various aspects of the lumbar support increased significantly over time. Retrospectively, no participants desired a setting beyond 4 cm, and the majority of respondents indicate had they been able to change their initial selection, they would choose a setting between 2 and 3 cm. Conclusion: The results suggest that occupants would prefer increasing the excursion capability of automobile lumbar supports beyond 2 cm. Application: Excursion capability and adjustability of automobile lumbar supports are important features to better meet end-user preference and to reducing lumbar flexion in sitting. Keywords: spine, physical ergonomics, interventions, biomechanics, vehicle design, gender
Address correspondence to Jack P. Callaghan, Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, 200 University Ave West, Waterloo, ON N2L3G1, Canada; e-mail: [email protected]. HUMAN FACTORS Vol. XX, No. X, Month XXXX, pp. 1–12 DOI: 10.1177/0018720815584866 Copyright © 2015, Human Factors and Ergonomics Society.
Introduction
Low-back pain associated with prolonged driving is a well-documented problem worldwide (Akinbo, Odebiyi, & Osasan, 2008; AlperovitchNajenson et al., 2010; Anderson, 1992; Andrusaitis, Oliveira, & Barros Filho, 2006; Boshuizen, Bongers, & Hulshof, 1990; Bovenzi, 2010; Brown, Wells, Trottier, Bonneau, & Ferris, 1998; Chen, Chang, Chang, & Christiani, 2005; Gyi & Porter, 1998; Lyons, 2002; Magnusson, Pope, Wilder, & Areskoug, 1996; Miyamoto, Shirai, Nakayama, Gembun, & Kaneda, 2000; Okunribido, Magnusson, & Pope, 2006; Okunribido, Shimbles, Magnusson, & Pope, 2007; Prado-Leon, Aceves-Gonzalez, & Avila-Chaurand, 2008; Waters, Genaidy, Barriera Viruet, & Makola, 2008). While there are numerous factors that impact the low back during driving, sustained flexion of the low back (loss of lumbar lordosis) likely plays a role in pain generation. Vehicle seat design itself has the direct ability to improve spine posture in automobile sitting. Lumbar supports have been shown to quantitatively increase the lumbar lordosis (Andersson, Murphy, Ortengren, & Nachemson, 1979; De Carvalho & Callaghan, 2012; Hazard & Reinecke, 1995) and reduce disc pressure and muscle activity (Andersson, Ortengren, Nachemson, & Elfstrom, 1974; Kingma & van Dieen, 2009). Their use in automobile seats has been associated with decreased reports of low-back pain during driving exposures (Chen, Dennerlein, Chang, Chang, & Christiani, 2005). Mechanical lumbar supports, beyond extra cushioning and trim, that are adjustable and provide massage-type or programmed movement have been shown to reduce vibration transmission and muscle activity, improve blood flow, and dramatically reduce driver discomfort (Donnelly, Callaghan, & Durkin, 2009; Durkin, Harvey, Hughson, & Callaghan, 2006; Kingma & van Dieen, 2009).
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Our research group has conducted a radiographic investigation of the prototype lumbar support used in this current paper (De Carvalho & Callaghan, 2012) and confirmed improved lumbar spine posture with increasing lumbar support prominence (LSP). The lumbar lordosis angles were found to increase from 20° with 0 cm or no support to 30° with 4 cm LSP (De Carvalho & Callaghan, 2012; De Carvalho, Soave, Ross, & Callaghan, 2010). However, the experimental design was limited to male participants, and we investigated only three levels of magnitude (0 cm, 2 cm, and 4 cm) for extremely short durations to limit radiographic exposure. Moreover, data pertaining to user preference or the effect of the support levels during a prolonged exposure were not ascertained. Most automobile seats are designed for the 50th percentile adult male (Kolich, 2003), decreasing the likelihood that the majority of the population will find appropriate support for the low back. It has also been discussed that fixed lumbar prominences, when they do not match the contour of the occupant’s back, can contribute to discomfort (Reed & Schneider, 1996). Although no current standards exist regarding the exact amount of support that should be provided, an LSP of 2 cm for fixed lumbar supports has been suggested in the literature (Reed, Schneider, & Ricci, 1994). However, what would the end user prefer? Kolich and colleagues document user preference for lumbar support apex height: a range of 90 to 123 mm above the H-point (Kolich, 2003), a theoretical point based on a mannequin measurement to represent the hip of a 50th percentile male, yet virtually no work has been conducted with regard to occupant preference of lumbar support excursion. The results of one study on 15 men concluded that 1 cm of LSP is favorable for short-duration (10 min) simulated driving (Lim, Chung, & Na, 2000). However, caution must be taken when interpreting perceived automobile seat comfort based on shortterm exposures (Gyi & Porter, 1999); therefore, support preferences for prolonged exposures and women specifically remain unexamined. There is limited work aimed at determining the support magnitude preferred by occupants and the resulting postural and discomfort effect imparted by these choices. In this study we examined the responses of seated users during a 2-hr prolonged
period of simulated driving with self-selected LSP magnitudes. The specific goals of the study were (a) to establish preliminary data on the lumbar support preferences of men and women (both initial and retrospective) for prolonged simulated driving and (b) to explore gender differences in spine posture, self-perceived comfort (Automotive Seating Discomfort Questionnaire [ASDQ]; Smith, Andrews, & Wawrow, 2006), and discomfort (Visual Analogue Scale [VAS]) throughout a prolonged period of simulated driving with a selfselected amount of lumbar support. The null hypothesis, that there would be no difference between the magnitude preference of men and women, was expected for self-selected LSP. Since participants were given the choice to self-select the amount of lumbar support, no differences in posture were expected between genders or throughout the trial. Discomfort, however, was hypothesized to increase over the prolonged simulated driving trial, given the results of prior work (Callaghan, Coke, & Beach, 2010). Method Participants
Seventeen participants, nine men and eight women, with no recent history of low-back pain were recruited from a university population (men: average age 24.9 years ± 4.3, height 1.84 m ± 0.08, weight 90.4 kg ± 12.3; women: 23.8 years ± 2.8, height 1.61 m ± 0.24, weight 59.2 kg ± 5.7). Ethics approval was obtained from the Office of Research Ethics at the University of Waterloo. Data Collection
Lumbar spine flexion and pelvic angles were obtained using triaxial accelerometers (S2-10G-MF, NexGen Ergonomics, Montreal, QC, Canada) as tilt sensors. Sensors were mounted with the +y-axis directed downward over the L1 and S2 spinous processes with double-sided tape and secured with flexible medical tape. Two 5-s normalization trials were collected: upright standing and maximum lumbar flexion standing. Participants then completed a baseline rating of perceived discomfort (RPD). Levels of discomfort for seven areas of the body (upper lumbar spine,
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Figure 1. Configuration of driving simulator setup and detail image of the prototype lumbar support with vertical opening (inset).
lower lumbar spine, bilateral iliac crests, sacrum, and bilateral buttocks) were rated on a 100-mm VAS with anchors of no discomfort (0 mm) and worst discomfort imaginable (100 mm). Participants were then seated in a driving simulator consisting of the following: seat (Crown Victoria Model EN114 2007, Lear Seating Corporation, Southfield, MI, USA), gaming steering system with a Ford Crown Victoria steering wheel attached, pedals, and modified dashboard. All components of the simulator were mounted according to the internal configuration and dimensions of a standard Ford Crown Victoria (Figure 1). This same seat and lumbar support system were previously used for a radiographic study (De Carvalho & Callaghan, 2012). Each occupant adjusted the fore/aft placement of the seat individually such that he or she could reach the pedals and steering wheel comfortably. The backrest angle, orientation of the upper part of the seat with respect to the vertical, was fixed at 20° for all participants. This angle corresponds to approximately 100° with respect to the seat cushion. This angle was chosen based on the work of Andersson et al. (1979) that suggests that 100°
of backrest inclination relieves back muscle activity and reduces low-back flexion. Backrest angle was measured at LSP 0 cm using the HPM-II ASPECT manikin according to SAE J4002 specifications, and this inclination was locked in the calibrated position. The vertical location of the lumbar support apex was centered at the third lumbar vertebrae for each participant based on palpation by the examiner (prior to being seated, surface landmarks were identified and marked with a pen and were then reconfirmed visually once the participant was seated). The posterior aspect of the seat trim was removable, revealing the lumbar support mechanism (apex marked). A thin vertical opening in the seat foam and trim ensured visual confirmation of skeletal landmarks and provided space to ensure that the accelerometers did not contact the seat (Figure 1, inset). The LSP for the seat was determined by the manufacturer using an HPM-II manikin and measured in millimeters of horizontal shell deflection (Michida, Okiyama, & Matsuhashi, 2005; Schneider et al., 1999). A controller specific to this study was created to adjust LSP magnitude discretely
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between five settings using buttons (0 cm through 4 cm). To our knowledge, this is the first study that has used the HPM-II manikin for the quantification of LSP. Past work on lumbar supports has involved methods that are difficult to replicate; thus, the use of this standard tool is a great strength of this work. Settings beyond 4 cm LSP were not included in this study due to limitations in the mechanical design of the lumbar support unit. The driving simulator (STISIM Drive™ v2.0, Systems Technology Inc., Hawthorne, CA, USA) course was intended to represent highway driving and was projected onto a screen in front of the driver. After the seat adjustments were complete, the participants remained in the seat and were then given a lumbar support controller and a chance to try each level. To standardize the experience of each participant, each level from 0 cm to 4 cm was then presented for a period of 2 min each. Following this procedure, participants were asked to choose the level of support they would like to use for a long drive. Once this level was selected, and a 2-min adjustment period had passed, participants completed a pain-rating VAS and ASDQ (Smith et al., 2006). For the 2-hr driving trial, participants were instructed to keep both hands on the steering wheel at the 10 and 2 o’clock positions, stay on the simulation course, and maintain a constant speed of 100 km/h. Throughout the prolonged trial, postural data were collected continuously. For ease of data processing and analyzing, data were separated into eight 15-min intervals, and the first 2 min of each 15-min trial were analyzed. RPDs were completed at 15-min intervals throughout the prolonged driving trial for a total of nine measures over the 2 hr. At the end of the simulated driving period, a second ASDQ and final RPD were completed. Following collection, an outtake questionnaire regarding the lumbar support was completed. Participants were asked (a) whether or not they would have changed their chosen lumbar support excursion if given a chance, (b) which direction (increasing or decreasing support) they would have chosen, and (c) how large this change would have been. Data Reduction
Accelerometer signals were A/D converted at rates of 256 Hz with a 16-bit A/D system
(Optotrak Data Acquisition Unit II, Northern Digital Inc., Waterloo, ON, Canada). A secondorder low-pass Butterworth filter with an effective cutoff frequency of 1 Hz was used to filter these data (Dunk & Callaghan, 2010). Using the inclinations of the sensitive axes (y and z) of the accelerometers with respect to gravity, lumbar and pelvic angles were calculated according to their corresponding radiographic measures using custom written software (Matlab Version R2012b, MathWorks, Natick, MA, USA). Specifically, lumbar angles were taken as the difference between the inclination of the top and bottom accelerometers, respectively, and pelvic angle was taken as the inclination of the bottom accelerometer with respect to vertical. Negative angles represent backward tilt. These data were then normalized for each participant: Lumbar flexion angles were normalized to a percentage of standing maximum flexion, and pelvic angle was presented relative to the angle achieved in upright standing. RPDs for seven areas of the back (upper lumbar spine, lower lumbar spine, right iliac crest, left iliac crest, sacrum, right buttock, and left buttock) were expressed relative to the baseline values reported at the start of the collection protocol. Questions 13 to 20 of the ASDQ were analyzed as these questions dealt directly with the lumbar support and overall perceived comfort of the automobile seat. Gyi and Porter (1999) showed that initial ratings of seat comfort are not usually indicative of true seat comfort, so the responses to the postdrive ASDQ were compared to the predrive questionnaire to evaluate changes in perception. Statistics
A two-way analysis of variance, with time as the within factor and gender as the between factor, were completed to compare dependent variables during the prolonged driving trial (SAS Version 9.2, SAS Institute Inc., Cary, NC, USA). Tukey’s studentized range test post hoc was used on all significant effects within the time factor. A two-tailed Student’s T test was used to compare lumbar support choice for the prolonged driving trial between men and women. In all statistical tests, a p value of less than .05 was accepted as statistically significant. To evaluate the effect size of gender differences
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Figure 2. Normalized lumbar flexion angles (% range of motion) throughout the prolonged driving trial for men and women. Error bars are standard deviations. No significant differences were found for time or gender.
in posture, Cohen’s ds was calculated using Hedge’s gs correction to account for the small sample size (Lakens, 2013). Results
Women selected larger support levels than men (women: average 3.25 cm ± 0.71 cm [SD], range 2 cm to 4 cm; men: average 2.56 cm ± 0.88 cm [SD], range 1 cm to 4 cm, p = .048), and no participants selected the 0-cm support level. Analysis of time-varying posture data throughout the simulated driving trial revealed that lumbar and pelvic angles remained consistent throughout the 2-hr exposure with no significant main effects of time (lumbar, p = .4527; pelvic, p = .521; Figures 2 and 3). There were no significant differences between the lumbar (p = .2882) or pelvic angles (p = .0885) of men and women. There was a trend for women to sit with more lumbar flexion and posterior pelvic rotation than men. Significant main effects of time were found for RPD, with an increase of discomfort over the prolonged driving for the following body areas: sacrum (p = .003), bilateral iliac crests (p < .0001),
left buttock (p = .0002), and right buttock (p < .0001; Figure 4). Although there was a tendency for women to report lower levels of low-back discomfort (baseline removed), there was no significant main effect of gender (p = .1286). Results from the ASDQ showed significant increases in discomfort associated with certain aspects of the seat, notably, the backrest contour (p = .0051), lumbar support stiffness (p = .0123), lumbar prominence (p = .0039), vertical location of lumbar support (p = .0241), and pressure from lumbar support (p = .0030). Further, overall seat discomfort (p = .0016) was also found to increase significantly over time. Outtake Questionnaire
Subjective outtake questionnaire responses indicated that no participants would have desired a setting beyond 4 cm, and 76% of the participants would have preferred to change their LSP setting at some point during the driving trial if they were able to. Taking this hindsight preference into account, it appears that the majority of participants would choose a magnitude of support greater than 2 cm and most
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Figure 3. Relative pelvic angles (with respect to upright standing) throughout the prolonged driving trial for men and women. Error bars are standard deviations. No significant differences were found for time or gender.
Figure 4. Average perceived discomfort (baseline removed) throughout 2 hr of driving for women (A) and men (B). Downloaded from hfs.sagepub.com at GEORGIAN COURT UNIV on May 9, 2015
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importantly would want the ability to modify support level during a prolonged driving exposure. Considering these preliminary findings, it appears an adjustable support exceeding 2 cm LSP would be a desired capability in automobile seat designs. Discussion
Lumbar and pelvic postures remained relatively unchanged throughout the 2 hr of simulated driving, and postural angles were not significantly different between men and women. Therefore we can accept our hypothesis that there would be no differences in lumbar or pelvic posture throughout the driving trial or between genders. Small and moderate corrected effect sizes were found for these measures, respectively, between genders (lumbar angle, Hedge’s gs = .2854; pelvic angle, Hedge’s gs = .6582), suggesting there is potential that gender differences in pelvic posture could emerge with a larger sample size (Lakens, 2013). The normalized lumbar and pelvic angle magnitudes found in this study are consistent with prior work (Beach, McDonald, Coke, & Callaghan, 2008; Callaghan et al., 2010; De Carvalho & Callaghan, 2011). Although the postures do appear extreme (approaching or even exceeding 100% of normalized forward flexion capability), past findings have shown average automobile seat posture consistently ranging from 50% to 80%, with individual scores commonly approaching or exceeding 100%. Further, there is radiographic evidence that shows the lumbar lordosis angle (measured from L1-S1) in sitting ranges from 20° to 40° in the same automobile seat tested in this study (De Carvalho & Callaghan, 2012). Miyasaka, Ohmori, Suzuki, and Inoue (2000) radiographically measured an average range of motion of 78.8° ± 10° for the lumbar lordosis angle (L1-S1) through maximum extension to maximum flexion in 90 adults. Considering the normal ranges of motion of the lumbar spine are 40° to 60° in flexion and 20° to 35° in extension (Magee, 2002), it does appear that the magnitudes of lumbar flexion achieved in an automobile seat could approach end-range flexion for the lumbar spine. Other potential explanations for these extreme numbers could be as follows: (a) Creep of passive
tissues in the low back may lead to increased range of motion in flexion occurring after a period of 20 min (participants in our study were seated for approximately 20 min prior to the start of the prolonged trial) as documented by McGill and Brown (1992), or (b) the “maximum flexion” normalization posture achieved was less than a true maximum. We did try to standardize the instructions given to participants during the normalization trials, encouraging motion at the spine in flexion as opposed to hinging at the hip, however, as both the low back and hips are inherently linked and normally both involved in forward flexion, isolating pure lumbar flexion can be difficult. Self-selected lumbar support, with settings beyond the lumbar support deflections typically available commercially, likely contributed to the lack of gender differences that have been previously reported (Beach et al., 2008; Callaghan et al., 2010). Differences in preferred seated postures of men and women (Dunk & Callaghan, 2005) have been previously documented in office chairs. However, the physical constraints of driving (interacting with pedals and steering wheel) and the size of the seat itself remove the ability of women to adopt their preferred posture. From past studies, this preference appears to be sitting with less lumbar flexion, a more anteriorly rotated pelvis, and center of mass positioned more anteriorly on the seat (Beach et al., 2008; Callaghan et al., 2010; Dunk & Callaghan, 2005). Attempting to reduce the increased lumbar flexion imposed by sitting in a driver’s seat might be one explanation for the tendency of women to choose higher amounts of lumbar support in this study. Participants adopted fairly static postures throughout the prolonged driving trial as reflected in the lack of significant differences in average spine angles over time. The static nature of torso posture during prolonged driving has been documented previously (De Carvalho & Callaghan, 2011; Reed, Manary, Flannagan, & Schneider, 2000). This phenomenon is likely due to the constraints imposed by interacting with the pedals, steering wheel, and seat. There is evidence of significant increases in tension of the posterior spine elements starting with 75% of maximum lumbar flexion (Adams, McNally, Chinn, & Dolan, 1994). Further, increases of 2.5° in
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Figure 5. Low-back discomfort ratings stratified by lumbar support prominence chosen for women (A) and men (B) over the 2-hr driving trial.
active range of flexion has been documented after 1 hr of static sitting at 70% range of motion, reflecting the effect of viscoelastic creep in posterior spine structures (Sanchez-Zuriaga, Adams, & Dolan, 2010). Such passive loading has been shown to have a direct adverse effect on musculoligamentous health of the lumbar spine (Solomonow, 2009; Solomonow, Zhou, Baratta, &, Burger, 2003). Therefore, the degree of lumbar spine flexion exhibited in this study, even with added lumbar support, could be cause for concern when maintained for extended periods of time. The third hypothesis, that discomfort would increase over the prolonged simulated driving trial, was accepted. Not surprising was that significant increases in discomfort over time were found in both the RPD and ASDQ questionnaires as projected. This finding is consistent with past studies on prolonged driving (Callaghan et al., 2010; De Carvalho & Callaghan, 2011; Donnelly et al., 2009; Durkin et al., 2006; El Falou et al., 2003; Kyung & Nussbaum, 2008; Na, Lim, Choi, & Chung, 2005; Porter, Gyi, & Tait, 2003). It was interesting to note the regional differences in perceived discomfort, with only areas of the pelvis (buttock, sacrum, etc.) exhibiting this increase. There is the potential that the
presence of a lumbar support played a role in minimizing low-back discomfort while contributing to discomfort at the pelvis. The design of this study prohibits this conclusion; however, this would be an interesting research question to pursue in the future. In the discussion of the radiographic results of our previous study, we surmised that the increased extension imparted at the low back by the lumbar support coupled with the lack of support at the pelvis could place increased stress at the lumbosacral junction (De Carvalho & Callaghan, 2012). Pain referral from the lower lumbar facet joints has been documented into the gluteal region (Windsor et al., 2002); thus, it does appear realistic that this lack of direct pelvic support could generate the observed discomfort pattern through the pelvis. When the discomfort of the low back was stratified by the LSP setting chosen by participants for the 2 hr of prolonged driving, there was a clear trend of a lower level of discomfort developing in the 3-cm group (Figure 5). Statistical analyses could not be completed on the LSP grouping due to small sample numbers in each selection group and with the 0-cm setting not chosen at all. There was also a trend for discomfort levels to decrease as LSP magnitude increased from 1 cm to 3 cm and then for discomfort to
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increase with the 4-cm LSP setting. Given the high rate of selection for the 3-cm setting (8 of 17 participants) and the directional responses of users who chose 2 cm and would have increased the LSP excursion if allowed (2 of 4 participants) and those who started with 4 cm and would have preferred a lower setting (2 of 4), it appears reasonable that an adjustable LSP with excursion capacity greater than 2 cm and less than 5 cm would accommodate the preferences of most automobile seat users; however, further research is warranted to confirm this. Potentially more important than the amount of support is the allowance of adjustability of the support, which is indirectly supported by the outtake questionnaire. Reed and Schneider (1996) discussed the importance of lumbar support adjustability as a way to ensure support prominence appropriately matches the contours of the occupant’s back. It appears relevant, then, that changes to LSP might be necessary over prolonged periods of time as deformations occur in both the seat (foam) and occupant (creep of tissues; De Carvalho & Callaghan, 2011; Kolich, 2009). Prolonged static postures are associated with increased discomfort over time (Tissot, Messing, & Stock, 2009; Van Nieuwenhuyse et al., 2004); therefore movement, either of the lumbar support itself or by the occupant taking a break and getting out of the vehicle, remain vital for preserving the overall health of the seated individual (Dunstan, Howard, Healy, & Owen, 2012; Proper, Singh, van Mechelen, & Chinapaw, 2011). Limitations
Although permitting participants to selfselect the amount of lumbar support provided the evaluation of realistic information regarding prospective and retrospective lumbar support preferences, it has limited the ability to conduct a complete statistical analysis on each support level. This design was a trade-off that was made a priori. Given that there is the preliminary indication that support level has a significant effect on discomfort and posture, larger followup studies need to be conducted to explore this effect more thoroughly. In order to minimize variability and confounding effects, the seat back angle was fixed and the apex of the lumbar support was directed
at the third lumbar vertebrae for all subjects. These factors raise two important limitations of this study. First, it has been shown that the size of the occupant and the amount of lumbar support used will result in a range of self-selected seat back angles (Reed, Schneider, & Eby, 1995). Therefore, the fixed backrest angle could have altered the degree of lumbar support chosen by participants. Similarly, the degree of preferred LSP could have been influenced by the vertical position of the support. In this study, adjustments to the LSP were made with the participant seated. This method was used to eliminate variability in whole-body postures that would occur with exiting/reentering the seat. There is evidence that differing effects on the occupant will occur with LSP adjustments in loaded (seated) or unloaded (unoccupied) scenarios. Specifically, the effect of a lumbar support on low-back posture is greater when the adjustment is made in the loaded condition (Reed & Schneider, 1996). It could be argued that people generally set their seat adjustment and do not change it once exiting/reentering the vehicle, perhaps limiting the applicability of our results. However, the results of this study indicated that 76% of individuals would have altered the lumbar support setting during the driving trial if allowed, suggesting that for longer driving exposures, individuals are likely to adjust lumbar support settings while seated. To limit variability, and in accordance with safety guidelines, we instructed participants to maintain two hands on the wheel. Recent naturalistic studies have indicated that gripping the wheel with two hands is not necessarily the norm (Jonsson, 2011). Therefore, following this instruction could have been unnatural for some of the participants in this study. Further, shoulder angle has been shown to change lumbar spine posture (Stagnara et al., 1982), so there is the potential that the spine angles found in this study could have varied with different steering wheel gripping methods. The impact of various self-selected driving postures on low-back accommodations should be targeted in future studies. Conclusions
When participants were given the choice to select the amount of lumbar support they
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thought would be comfortable for a long car drive, all participants selected at least some amount of support, and women tended toward choosing larger amounts of support than men. Seventy-one percent of the study population chose a support level above what has previously been recommended in the literature (2 cm). Further, self-selected lumbar support resulted in an absence of significant changes to lumbar spine and pelvic postures over time and between genders. Acknowledgments Jack P. Callaghan is supported by a Canada Research Chair in Spine Biomechanics and Injury Prevention. Diana E. De Carvalho was supported by a Canadian Institute of Health Research Doctoral Award.
Key Points •• Women tend to prefer larger amounts of lumbar support than men. •• Self-selected lumbar support during prolonged driving removes previously documented gender effects on spine posture. •• Adjustable lumbar supports appear to be an important feature for automobile seats.
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12 Month XXXX - Human Factors Diana E. De Carvalho is a PhD candidate in the Department of Kinesiology, Faculty of Applied Health Sciences, at the University of Waterloo. She graduated as a doctor of chiropractic from the Canadian Memorial Chiropractic College in 2006 and with a master of science (kinesiology– biomechanics) in 2008 at the University of Waterloo.
Jack P. Callaghan is a professor in the Department of Kinesiology, Faculty of Applied Health Sciences, at the University of Waterloo. He completed his doctor of philosophy (kinesiology–biomechanics) in 1999 at the University of Waterloo. Date received: May 29, 2014 Date accepted: April 1, 2015
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