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The effect of a backrest and seatpan inclination on sitting discomfort and trunk muscle activation in subjects with extension-related low back pain a
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Máire Curran , Wim Dankaerts , Peter O'Sullivan , Leonard O'Sullivan & Kieran O'Sullivan a
Department of Clinical Therapies, University of Limerick, Limerick, Ireland
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Department of Rehabilitation Sciences, Catholic University, Leuven, Belgium
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Department of Physiotherapy, Curtin University, Perth, Australia
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Department of Design and Manufacturing Technology, University of Limerick, Limerick, Ireland Published online: 27 Mar 2014.
To cite this article: Máire Curran, Wim Dankaerts, Peter O'Sullivan, Leonard O'Sullivan & Kieran O'Sullivan (2014) The effect of a backrest and seatpan inclination on sitting discomfort and trunk muscle activation in subjects with extension-related low back pain, Ergonomics, 57:5, 733-743, DOI: 10.1080/00140139.2014.897378 To link to this article: http://dx.doi.org/10.1080/00140139.2014.897378
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Ergonomics, 2014 Vol. 57, No. 5, 733–743, http://dx.doi.org/10.1080/00140139.2014.897378
The effect of a backrest and seatpan inclination on sitting discomfort and trunk muscle activation in subjects with extension-related low back pain Ma´ire Currana, Wim Dankaertsb, Peter O’Sullivanc, Leonard O’Sullivand and Kieran O’Sullivana* a
Department of Clinical Therapies, University of Limerick, Limerick, Ireland; bDepartment of Rehabilitation Sciences, Catholic University, Leuven, Belgium; cDepartment of Physiotherapy, Curtin University, Perth, Australia; dDepartment of Design and Manufacturing Technology, University of Limerick, Limerick, Ireland
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(Received 27 November 2013; accepted 12 February 2014) Few studies have demonstrated that seating modifications reduce low back pain (LBP). One recent study found that a forward-inclined seatpan reduced low back discomfort (LBD), however this was only examined in people with flexionrelated LBP. No study has yet investigated its effectiveness among people with extension-related LBP. This crossover study examined 12 subjects with extension-related LBP. Sitting discomfort and surface electromyography of three trunk muscles were recorded during a 10-minute typing task while sitting with two different seatpan inclinations, both with and without a backrest. LBD ( p , 0.001) and overall body discomfort (OBD) ( p ¼ 0.016) were significantly greater on the forwardinclined seatpan. The backrest did not alter trunk muscle activation or sitting discomfort. The results demonstrate that in a specific subgroup of people with extension-related LBP, increasing forward seatpan inclination significantly increased LBD and OBD. Future research should consider matching ergonomics prescriptions according to the individual presentation of people with LBP. Practitioner Summary: Sitting on a forward-inclined seatpan resulted in greater low back discomfort and overall body discomfort than sitting on a flat seatpan during a typing task among people with extension-related low back pain (LBP). Future research should examine matching ergonomics prescriptions to the individual presentation of people with LBP. Keywords: back pain; musculoskeletal disorders; seating; office ergonomics
1.
Introduction
Low back pain (LBP) is a very prevalent and costly musculoskeletal disorder (Woolf and Pfleger 2003). Sitting is frequently reported as an aggravating factor for LBP (Womersley and May 2006). As prolonged sitting alone does not seem to be associated with the development of LBP (Bakker et al. 2009; Lis et al. 2007; Roffey et al. 2010), there has been much research investigating various seating modifications in the management of this condition. There is a common belief that lordotic postures are favourable, which is supported by healthcare professionals (O’Sullivan et al. 2012d), the general public (O’Sullivan et al. 2013a) and ergonomics recommendations (Corlett 2006). Further to this, the use of lumbar supports (Williams et al. 1991) or devices (Reinecke, Hazard, and Coleman 1994) aiming to increase seated lumbar lordosis has been advocated for the management of LBP. This increase in seated lumbar lordosis may reduce LBP temporarily (Williams et al. 1991). However, hyper-lordotic sitting postures increase levels of trunk muscle activation in pain-free populations (O’Sullivan et al. 2006a), and this may result in discomfort and fatigue (Carcone and Keir 2007; Lander et al. 1987). There is also evidence that some people with persistent LBP provoked by sitting present with lordotic postures (Dankaerts et al. 2006b, 2009). Dynamic sitting devices that cause slight spinal movement have been advocated as an approach to reduce LBP (van Dieen, de Looze, and Hermans 2001). It has been proposed that LBP subjects maintain more sustained, static postures during sitting, with large infrequent postural shifts, as opposed to regular spinal movements (Dankaerts et al. 2006b; Telfer, Spence, and Solomonidis 2009). However, recent systematic reviews (O’Sullivan et al. 2012c, 2013b) revealed that there was no evidence that dynamic sitting alone significantly alters trunk muscle activation or reduces LBP. The effect of altering seatpan inclination has been investigated in a number of studies. Forward-inclined seatpans have been reported to encourage lumbar lordosis (Gadge and Innes 2007; O’Sullivan et al. 2012b). However, it is unclear as to whether low back discomfort (LBD) is decreased (Gadge and Innes 2007) or increased (Bendix, Jensen, and Bendix 1988) with a forward-inclined seatpan. Additionally, it has been documented that although LBD may be reduced, there can actually be an increase in discomfort in other body areas (Gadge and Innes 2007).
*Corresponding author. Email: [email protected] q 2014 Taylor & Francis
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Another commonly advocated seating modification is the use of a backrest (Carcone and Keir 2007; Kingma and van Dieen 2009). Backrests have become customary in the workplace, although their use may be suboptimal among seated occupational tasks (Vergara and Page 2000). It had been suggested that backrests reduce trunk muscle activation (Gregory, Dunk, and Callaghan 2006; Hardage, Gildersleeve, and Rugh 1983; Lander et al. 1987). However, it is unclear if a backrest affects sitting discomfort as most studies investigated pain-free populations. Therefore, this should be further analysed among LBP subjects. Therefore, there is very limited evidence to validate most seating modifications in the management of LBP (Driessen et al. 2010). This suggests either these modifications are ineffective, or have been applied to wrongly selected subjects, or possibly a combination of these factors. There is evidence that people with LBP provoked by sitting may not present with the same spinal postures and motor patterns (Dankaerts et al. 2006a, 2006b). It is proposed therefore that seating modifications should reflect the individual clinical presentation of a person with LBP, which might involve greater or less lumbar lordosis (Dankaerts et al. 2006b). One recent study (O’Keeffe et al. 2013) took into account the existence of distinct movement subgroups among people with LBP, and investigated an ergonomics intervention among those with flexion-related LBP. This study examined a forward-inclined seatpan (‘Back App’) that also features a dynamic component and does not have a backrest, and investigated its effect on sitting discomfort among subjects with flexion-related LBP. Results demonstrated a significant reduction in LBD while sitting on the forward-inclined seatpan when compared to a flat seatpan with a backrest. As the forward-inclined seatpan encourages lumbar lordosis (O’Sullivan et al. 2012b), it was therefore proposed that LBD reduced as a result of the directional nature of the disorder. However, it is also possible that there may have been an enhanced placebo effect due to the novel appearance of the forward-inclined seatpan and the dynamic component. Additionally, this study involved patients with low levels of functional disability, no dominant psychosocial factors and a primarily mechanical behaviour of their LBP (aggravated/eased by spinal postures and movements). This study raised a number of issues to be investigated. These include examining whether seated LBD would also be reduced in a different LBP population, e.g. those with extension-related LBP. This would clarify if the effects seen in the previous study (O’Keeffe et al. 2013) were primarily related to the directional nature of the disorder (flexion-related LBP) and/or the fact that subjects had low disability levels, or simply reflected the placebo effect. Additionally, the forward-inclined seatpan may have reduced sitting discomfort as a result of an increase in lumbar lordosis and altering trunk muscle activation (O’Keeffe et al. 2013). As a backrest is proposed to reduce trunk muscle activation in pain-free subjects, it is therefore beneficial to determine whether a backrest could be as effective in reducing sitting discomfort and altering trunk muscle activation, among those with LBP. Therefore, in order to address these varying factors, another study is warranted, which includes subjects who are possibly less likely to benefit from a forward-inclined seatpan which increases lumbar lordosis, i.e. people with extensionrelated LBP. Additionally, as previously highlighted, there is limited evidence conducted on the effect of backrests among LBP patients; therefore, it needs to be investigated to determine any effect backrests may have on muscle activation and sitting discomfort. The initial hypothesis of this study was that LBD and overall body discomfort (OBD) among this subgroup would be increased rather than decreased on the forward-inclined seatpan. It was further hypothesised that trunk muscle activity would be reduced on the forward-inclined seatpan and the presence of the backrest may reduce LBD, OBD and trunk muscle activity. Therefore, the aims of this study were to compare LBD, OBD and trunk muscle activity when sitting on a forwardinclined seatpan with sitting on a flat seatpan during a controlled typing task in subjects with extension-related LBP, and also to determine the effect of a backrest on these parameters. 2. 2.1
Methods Study design
A single session, repeated measures, crossover design was conducted, corresponding to previous, similar studies (O’Keeffe et al. 2013). The dependent variables were trunk muscle activation and sitting discomfort, and the independent variables were backrest (present or absent) and seatpan inclination (flat or forward inclined). All subjects completed the same protocol on a single day, with the order of testing randomised using sealed opaque envelopes. Ethical approval was obtained from a local Research Ethics Committee. 2.2
Subjects
Subjects who were referred to a local pain medicine centre with LBP were recruited. Subjects were given an information sheet and written informed consent was obtained from all subjects prior to testing. Subjects were informed that they were free to withdraw from the study at any time. Subjects were included if they were aged over 18 years and had reported that their LBP was aggravated by extension-related activities (e.g. standing/walking/upright sitting) and eased by spinal flexion
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such as slump sitting. This is consistent with an active extension pattern classification (Dankaerts et al. 2006b). Subjects were excluded if they were pregnant, had LBP for less than three months, had any red flags, had a specific spine disorder (e. g. fracture, tumor stenosis) or had previous spinal surgery. The primary pain location was between L1 and the buttocks, to eliminate those whose extension-related LBP might have been related to spinal stenosis. Subjects whose pain was aggravated by flexion were also excluded. Based on these criteria, 12 (7 female, 5 male) subjects were recruited. This was a single-group study, with no pain-free control group. Subject demographics were obtained prior to testing (Table 1). The Numerical Rating Scale (NRS) was used to measure average LBP over the previous week (Childs, Piva, and Fritz 2005); fear of physical activity was determined using the physical activity subscale of the fear avoidance beliefs questionnaire (Waddell et al. 1993); functional disability was measured using the Oswestry Disability Index (ODI) (Fairbank and Pynsent 2000) and depression, anxiety and stress were measured using the Depression, Anxiety and Stress Scale (DASS-21) (Lovibond and Lovibond 1995).
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2.3
Trunk muscle activation
Surface electromyography (sEMG) was used to analyse the activation of three trunk muscles. The Motion Lab Systems MA-300 multi-channel EMG system (Motion Lab Systems Inc., Baton Rouge, LA, USA) was used to collect the sEMG data. Self-adhesive disposable Ag/AgCl disc surface electrodes were used which have an electrical contact surface of 1 cm2 and a centre-to-centre spacing of 2.5 cm. The sampling rate was 2000 Hz per channel, with a gain of 2000 and a common mode rejection ratio of . 115 dB at 60 Hz (O’Sullivan et al. 2006a). Prior to electrode placement, the skin was prepared to reduce skin impedance. Each electrode site was lightly abraded with fine sandpaper, body hair was shaved if necessary and the skin was cleansed with isopropyl alcohol solution, consistent with recommendations (Hermens et al. 2000). The muscles studied were superficial lumbar multifidus (level of L5, parallel to a line connecting the posterior superior iliac spine and L1 –L2 interspinous space), iliocostalis lumborum pars thoracis (ICLT) (level of L1, midway between the midline and lateral aspect of the subject’s body) and external oblique (EO) (below the rib cage, along a line connecting the most inferior costal margin and the contralateral pubic tubercle). These three muscles were selected based on pilot testing and previous research demonstrating that these muscles are most affected by changes in seatpan inclination (O’Sullivan et al. 2012a, 2012b). The surface electrodes were positioned parallel to the muscle fibre direction of each muscle unilaterally (O’Sullivan et al. 2006a). The side of dominant pain was chosen for the application of electrodes among subjects with bilateral and central LBP. For eight subjects, this was the right-hand side and for four subjects this was the left-hand side. A common ground electrode was placed at the ulnar styloid. One researcher applied the electrodes for all subjects. Since normalisation to maximal voluntary contraction has been reported as unreliable in LBP patients (Dankaerts et al. 2004), sEMG data were amplitude normalised to two standardised activities to elicit a sub-maximal voluntary isometric contraction (sub-MVIC) (O’Sullivan et al. 2002). For the abdominal muscles, the crook lying double leg raise was used. For this, the subject was in crook lying with the hips flexed to 458 and the knees flexed to 908. The subject raised both legs 1 cm off the supporting surface for 3 seconds. For the paraspinal muscles, the prone lying ‘double leg raise’ was used. For this, the subject was in a prone lying position with the knees bent to 908 and both knees lifted 5 cm off the ground for 3 seconds. The highest generated contraction from three repetitions was taken as sub-MVIC for each specific muscle (Dankaerts et al. 2006a). Each test was recorded for 5 seconds duration with a one-minute rest period between tests to reduce fatigue (O’Sullivan et al. 2002). The middle 3 seconds were analysed (Soderberg and Knutson 2000).
Table 1.
Mean (^ SD) subject demographics.
Measure
Mean(^ SD)
Age (years) Height (m) Mass (kg) Body mass index (kg/m2) Low back pain duration (years) Average weekly pain (Numerical Rating Scale) Fear-avoidance beliefs Oswestry Disability Index (%) Depression Anxiety Stress
41.7 (^ 9.3) 1.7 (^ .1) 84.8 (^ 23.4) 30.2 (^ 9) 9.2 (^ 6.4) 5.7 (^ 2) 19.8 (^ 6.7) 42.8 (^ 13.3) 20.5 (^ 15.1) 15.5 (^ 14.8) 22.7 (^ 15.1)
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2.4 Sitting conditions: forward-inclined seatpan versus flat seatpan A flat seatpan and a forward-inclined seatpan (‘Back App’) were compared for the purpose of this study (Figures 1 and 2). The ‘Back App’ was adjusted to allow a hip flexion angle of 558 with subjects placing their feet on the footplate (Figure 2), in line with previous studies (O’Keeffe et al. 2013; O’Sullivan et al. 2012a, 2012b). The ‘Back App’ can operate as a stable or dynamic design through adjusting the stability component which is located at the base of the seat. There are three different stability levels which comprise of the ‘green zone’ which is stable, the ‘black zone’ which is slightly unstable and the ‘red zone’ which involves greater instability. The stability level was maintained at the ‘black zone’ for this study which allows a mild degree of motion in three planes, in line with previous, similar studies (O’Keeffe et al. 2013; O’Sullivan et al. 2012a, 2012b). The flat seatpan was height adjustable and had wheels. The seatpan was controlled at a 908 hip and knee angle for each subject ensuring both feet were placed firmly on the floor (Gregory, Dunk, and Callaghan 2006). The flat seatpan differed slightly in size and material texture from that of the forward-inclined seatpan. Goniometry was used to measure both hip and knee angles as this has been reported to have high intra-rater and inter-rater reliability (Ekstrand et al. 1982). 2.5 Backrest A standard backrest was used for all subjects. The height and distance of the backrest was standardised (Figures 1a and 2a). During the backrest condition, the backrest was secured to a plinth, which was located 22 cm behind the subjects’ greater trochanter at the level of posterior superior iliac spine. 2.6
Discomfort
Subjects rated their discomfort at the end of each sitting condition using the Body Part Discomfort Scale (Corlett and Bishop 1976), as this has been extensively used for seat evaluation (Fenety, Putnam, and Walker 2000). This scale involves subjects subjectively rating their discomfort in each of 12 body areas on a six-point verbal numerical rating scale, where 0 ¼ ‘no discomfort’ and 5 ¼ ‘extreme discomfort’. LBD was rated using a single LBD score. OBD was examined separately as previous studies have demonstrated a reduction in LBD can be associated with increased discomfort in other body regions
Figure 1. Layout of the simulated workstation with subject sitting on the flat seatpan, with backrest (left) and without backrest (right).
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Figure 2. Layout of the simulated workstation with subject sitting on the forward-inclined seatpan with backrest (left) and without backrest (right).
when sitting on forward-inclined seatpans (Gadge and Innes 2007). OBD consisted of the mean discomfort of the 12 body areas to give a single total body perceived discomfort value (Fenety, Putnam, and Walker 2000). 2.7
Procedure
A simulated workstation was devised using an adjustable plinth as a desk (Figures 1 and 2). As self-selecting workstation set-ups can be linked with poor sitting postures (Gadge and Innes 2007), the height and distance from the workstations was standardised. Subjects were positioned by the same researcher for each sitting condition to enhance consistency. Subjects’ elbows were at 908 and kept in line with the trunk during the typing task (Kingma and van Dieen 2009). The distance between the subject and the plinth was standardised so that subjects’ greater trochanter was 30 cm from the edge of the plinth (O’Keeffe et al. 2013). A laptop was placed 10 cm from the edge of the plinth, directly in front of subjects. A paperweight was used to position a standardised document to be typed on the right-hand side of the laptop screen. The set-up and testing protocol were piloted prior to the study to enhance consistency and accuracy. Subjects were instructed to ‘sit as you normally would’ on the flat seatpan (O’Sullivan et al. 2006b), to ‘maintain your balance’ on the forward-inclined seatpan (O’Keeffe et al. 2013; O’Sullivan et al. 2012b) and for both seats to ‘rest against the backrest’ when the backrest was available. Subjects typed for 10 minutes in each sitting condition with sEMG data recorded for 5 seconds every 3 minutes, similar to previous research (McGill, Kavcic, and Harvey 2006; O’Sullivan et al. 2012b). Pilot testing demonstrated that longer periods of sitting in this population resulted in unacceptable levels of discomfort. Subjects were blinded as to when muscle activity was being recorded. Subjects rated their discomfort for each body region at the end of the 10-minute testing periods. A one-minute rest period between sitting conditions was provided to minimise the confounding effects of fatigue (O’Sullivan et al. 2012b). 2.8
Data analysis
Minimal filtering of sEMG data was undertaken. A high-pass RC Butterworth filter (–3 dB/octave at 10 Hz) and a linear phase Bessel low-pass filter (– 24 dB/octave at 500 Hz) were applied to the data. The middle 3 seconds of raw sEMG data from each five-second testing period (O’Sullivan 2006) were analysed using a root-mean-square (RMS) algorithm (using
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Motion Lab Systems Analysis software). These RMS sEMG values were normalised to sub-MVIC for the three muscles and the raw data was expressed as percentage of muscle activity. The mean, and variation (SD), from the three time intervals was assessed for all three muscles and was used for statistical analysis. There was little variance between each time interval, corroborating previous studies which demonstrated that time had no significant effect on muscle activity in short sitting tasks (McGill, Kavcic, and Harvey 2006). All raw sEMG files were visually inspected and data from channels with signal interference was removed. Data were analysed using the Statistical Package for Social Science (SPSS version 20.0). Data were tested for normality using the Shapiro – Wilks test, and homogeneity of variances using Levene’s test. All dependent variables (EO, ICLT, LM, LBD and OBD) were either normally distributed (Shapiro – Wilks, p . 0.05), or their plots indicated minimal variation from normality of distribution, allowing for parametric data analysis. A two-way ANOVA (with post-hoc Tukey) was conducted to compare each of the dependent variables (EO, ICLT, LM, LBD and OBD) between two independent variables (presence of backrest and seatpan inclination). Statistical significance was set at p , 0.05. 3. Results 3.1 Subjects All 12 subjects completed the protocol. Subjects reported at least moderate pain (mean ^ SD NRS ¼ 5.7 ^ 2), disability (mean ^ SD ODI ¼ 42.8 ^ 13.3) and psychological distress (Table 1). Surface EMG data was missing for one subject due to signal interference. 3.2
Low back discomfort
There was no significant interaction (F(1, 44) ¼ 0.000, p ¼ 1.000) between the effects of a backrest and seatpan inclination on LBD. Simple main effects analysis showed that LBD did not differ significantly (F(1, 44) ¼ 0.076, p ¼ 0.785) according to the presence or absence of a backrest. However, LBD was significantly greater with the forward-inclined seatpan when compared to the flat seatpan (F(1, 44) ¼ 19.354, p ¼ 0.000) (Figure 3). 3.3 Overall body discomfort There was no significant interaction (F(1, 44) ¼ 0.477, p ¼ 0.493) between the effects of a backrest and seatpan inclination on OBD. Simple main effects analysis showed that OBD did not differ significantly (F(1, 44) ¼ 0.068, p ¼ 0.795) according to the presence or absence of a backrest but did differ significantly depending on seatpan inclination, with OBD significantly greater on the forward-inclined seatpan (F(1, 44) ¼ 6.24, p ¼ 0.016) (Figure 4).
Figure 3. Change in the mean (^SD) of low back discomfort (LBD) for all sitting conditions. Note: Negative values indicate that discomfort reduced in that sitting condition.
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Figure 4. Change in the mean (^SD) of overall body discomfort (OBD) for all sitting conditions. Note: Negative values indicate that discomfort reduced in that sitting condition.
3.4 Trunk muscle activation There was no significant interaction between the effects of a backrest and seatpan inclination for mean activation of any of the three muscles analysed ( p . 0.05). Additionally, simple main effects analysis showed no significant difference ( p . 0.05) in mean trunk muscle activity according to the presence or absence of a backrest, or depending on seatpan inclination. There was also no significant difference in the variation of trunk muscle activity between any of the sitting conditions ( p . 0.05). 4. Discussion 4.1 Sitting discomfort The findings of this study demonstrate that in a subgroup of people with extension-related LBP, consistent with an active extension pain disorder (Dankaerts et al. 2006b), LBD and OBD were significantly increased on a forward-inclined seatpan in comparison to a flat seatpan. A number of studies have previously investigated the effect of forward-inclined seatpans on sitting discomfort. However contrasting results have been obtained with both decreased (Bridger 1988; Gadge and Innes 2007) and increased (Bendix, Jensen, and Bendix 1988; Bishu et al. 1991) LBD reported with a forward-inclined seatpan. However, most studies investigating seatpan inclination included pain-free populations, and those that did include LBP patients did not consider their individual clinical presentation. The importance of investigating subgroups of LBP patients has been previously highlighted (Dankaerts et al. 2006b). Therefore, this current study deliberately selected subjects with extension-related LBP. Previous research demonstrates that this subgroup tend to sit in a hyperextended posture which is correlated with their symptoms (Dankaerts et al. 2009). In other words, rather than assuming the most comfortable sitting posture available to them, their sitting posture seems to exacerbate their discomfort, and this direction-specific behaviour appears to be related to deficits in repositioning sense and body schema (Luomajoki and Moseley 2011; O’Sullivan et al. 2003, 2013c; Sheeran et al. 2012). Based on previous research using the same forward-inclined seatpan, the most likely explanation for an increase in LBD in this study is an increase in lumbar lordosis (O’Sullivan et al. 2012b). Although this previous study was conducted among pain-free subjects, it is in line with other research which has demonstrated that similar forward-inclined seatpans can increase lumbar lordosis (Gadge and Innes 2007; Gale et al. 1989; Koskelo, Vuorikari, and Ha¨nninen 2007). Therefore, the directional nature of the disorder among the active extension pattern group included resulted in an increase in LBD, as hypothesised. In contrast to this, it was recently found that LBD was reduced on the same forward-inclined seatpan (O’Keeffe et al. 2013). However, that study examined those with flexion-related LBP. Therefore, matching the forward-inclined seatpan according to the flexion-pattern of their LBP most likely reduced LBD as a result of the directional nature of the disorder (O’Keeffe et al. 2013). This highlights the potential importance of matching ergonomics interventions to the clinical presentation of
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individual patients rather than generic prescription, as it is evident that different subgroups can respond with either significantly increased or reduced LBD, based on their individual LBP disorder. The increase in OBD in this study corroborates the results of a previous study which demonstrated that forward-inclined seatpans increased discomfort levels in other body areas, such as the hips and buttocks (Gadge and Innes 2007). It is also somewhat consistent with the lack of a decrease in OBD in the recent study using the same forward-inclined chair among people with flexion-related LBP (O’Keeffe et al. 2013). Another possible reason for the differences in OBD between sitting conditions was the foot position. The forward-inclined seatpan required subjects to distribute more weight through the front of their feet, with increased ankle plantar flexion. While research is limited on the effect of foot position, it has been shown to affect lower leg muscle activation which could contribute to lower leg discomfort (Carlsoo 1961). Therefore, this may have led to an increase in OBD observed on the forward-inclined seatpan, although since foot position does not significantly increase trunk muscle activation (Carlsoo 1961) it most likely had no effect on LBD. Contrary to the hypothesis, the addition of a backrest to both of the seatpan conditions did not significantly reduce LBD or OBD. Previous research has demonstrated very slight reductions in sitting discomfort when using a backrest (Gregory, Dunk, and Callaghan 2006; Kingma and van Dieen 2009; Lander et al. 1987). However, these other studies were all among pain-free subjects, providing limited evidence for backrests having any effect on sitting discomfort in LBP patients. The lack of effect may also reflect an inability among this group of patients to relax their paraspinal muscles (Dankaerts et al. 2006a). 4.2
Trunk muscle activation
In contrast to our hypothesis, there was no significant difference in trunk muscle activity for any of the sitting conditions in this study. Previous research indicated that trunk muscle activity may be reduced among pain-free subjects with a backrest (Gregory, Dunk, and Callaghan 2006; Kingma and van Dieen 2009). However, very little research has been conducted on the effect of a backrest on muscle activation among people with LBP, and data using participants with LBP are confounded by differences in the degree of hip flexion between the comparison chairs (Cram and Vinitzky 1995). There is also considerable inconsistency in research examining the effect of inclined seatpans on trunk muscle activation, with both increased (Cram and Vinitzky 1995; Lander et al. 1987) and decreased (Koskelo, Vuorikari, and Ha¨nninen 2007) muscle activation reported. However, one recent study (O’Sullivan et al. 2012b) investigating the effect of the same forwardinclined seatpan used in the current study among pain-free subjects demonstrated reduced paraspinal muscle activity. There are a number of likely explanations as to why muscle activity was not altered between sitting conditions in the current study. As highlighted, most previous studies have been done using pain-free populations. A meta-analysis (Geisser et al. 2005) identified higher muscle activity levels during static postures in LBP subjects when compared to pain-free subjects, and a reduced ability to relax the paraspinal muscles. Elevated levels of trunk muscle activation and an inability to relax the back muscles in sitting has been demonstrated in extension-related LBP patients previously (Dankaerts et al. 2009). Therefore, this group may be less able to relax into anterior pelvic tilt and lumbar lordosis on a forward-inclined seatpan than pain-free subjects (O’Sullivan et al. 2012b). Unfortunately, the lack of kinematic data does not allow us to confirm this hypothesis, though it appears reasonable based on data from people without LBP (O’Sullivan et al. 2012b). Similarly, this group may be less able to relax their lordosis and use a backrest in the manner that pain-free participants can. Another possible reason for no change in muscle activation in the current study is the greater levels of functional disability (mean ODI ¼ 42.8%) than in the previous study using this forward-inclined seatpan (O’Keeffe et al. 2013). Subjects with more disabling LBP, such as those included in this study, are reported to have altered patterns of muscle activity during sitting when compared to control subjects and those with non-disabling LBP (DeGood et al. 1994). Therefore, this may explain why trunk muscle activation was reduced amongst pain-free subjects on the forward-inclined seatpan (O’Sullivan et al. 2012b) and not in this group. Finally, the dynamic component of the forward-inclined seatpan is another factor which must be considered in this study. This may increase spinal movement in sitting (Kingma and van Dieen 2009; O’Sullivan et al. 2006b), yet most research suggests dynamic sitting in isolation has a negligible effect on LBD (O’Sullivan et al. 2012c), spinal posture (O’Sullivan et al. 2006b) and trunk muscle activation (O’Sullivan et al. 2013b). Therefore, while we cannot separate the dynamic element from the forward-inclined seatpan, it seems unlikely that the dynamic element contributed to the results obtained in this study. 4.3
Implications for clinical practice and research
The results of this study may have important implications for the management of people with LBP and research in this area. It has been reported that seating modifications are ineffective in the management of musculoskeletal disorders (Driessen et al. 2010). Despite the fact that sitting discomfort actually increased among the specific LBP population included in this study, there are many aspects worth noting towards the role of seating modifications.
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Subjects in this study were actively seeking medical treatment for their LBP. They were specifically selected as they subjectively reported pain with extension-related postures such as walking/standing and upright sitting. This was conducted so that subjects who were sensitive to lumbar extension were matched to a seating device that increased lumbar extension. A particular subgroup of LBP subjects were selected, rather than prescribing it to all people with LBP. Although this approach is clinically relevant, the consideration of matching interventions to subgroups of LBP patients is not frequently investigated (Vibe Fersum et al. 2010). This is important as the response of LBP patients to seating modifications may vary according to their individual clinical presentation (Dankaerts et al. 2006b). This has been highlighted in the current study, with an increase in LBD, which is in direct contrast to previous results (O’Keeffe et al. 2013) which found reduced LBD when using the same forward-inclined seatpan among those with flexion-related LBP. Although lordotic sitting is commonly considered as optimal for the lumbar spine (O’Sullivan et al. 2013a, 2012d), this study provides evidence that seating modifications may need to vary between subgroups of LBP populations to prevent the ‘washout’ effect (Dankaerts et al. 2009). In particular, the results of this study suggest that forward-inclined seatpans should not be recommended for patients with extension-related LBP. Additionally, subjects in this study had at least moderately disabling LBP, with high levels of fear avoidance. As such, there was likely to be a greater degree of central nervous system sensitisation involved in their pain (Smart et al. 2010). Therefore, while their pain did demonstrate at least some mechanical pain behaviour, they may be less likely to benefit from a physical ergonomics intervention in isolation. For example, the level of pain and fear among participants in the current study was twice that of the subjects with flexion-related LBP who reported a positive response to sitting on the same forward-inclined seatpan in a previous study (O’Keeffe et al. 2013). It is likely that a biopsychosocial disorder such as chronic LBP requires some matching of interventions (Fersum et al., 2013; Hill et al. 2011; O’Sullivan 2005), both for psychosocial components as well as physical components. The current study demonstrates that if ergonomics interventions are to be effective, they should be selected with the individual patient presentation in mind. 4.4
Limitations
This study investigated a small sample of a particular subgroup of people with LBP, and therefore results may not be generalised for other LBP populations. The sitting task was of short duration as subjects only typed for 10 minutes in each sitting condition. However, pilot testing revealed that 10 minutes of testing was sufficient to observe increases in LBD and OBD in this subgroup of LBP patients. Ideally, these ergonomics studies should be investigated over a much longer time period and in typical occupational settings, as the standardised nature of the office task in this study does not reflect the true office environment. The evaluation of additional outcome measures such as spinal kinematics could have enhanced interpretation of the likely mechanism of effect. Future studies should further attempt to identify the mechanism(s) through which LBD was increased on the forward-inclined seatpan by examining sEMG, spinal kinematics and contact with the backrest. Based on previous research, facilitation of lumbar lordosis was the likely cause of effect for increased LBD (O’Sullivan et al. 2012b). There were a limited number of muscles analysed, with only three muscles, on one side of the trunk, investigated in this study. Investigating more trunk muscles on both sides could allow for more detailed evaluation of the degree of muscle co-contraction (McGill, Kavcic, and Harvey 2006), and whether this changed between seating conditions. However, the muscles tested were selected based on previous research (O’Sullivan et al. 2012a, 2012b) demonstrating that they are most affected by changes in seatpan inclination. Furthermore, the assessor was not blinded. The size, and texture of the material, differed slightly between the two seatpans, and since this could cause differences in local buttock discomfort, it should be considered when interpreting the results. However, the main difference between the seatpans was LBD, suggesting this was not a major issue. 5. Conclusion In a specific subgroup of people with extension-related LBP, there was a significant increase in LBD and OBD while sitting on a forward-inclined seatpan when compared to a flat seatpan. The lack of investigations into subgroups of people with LBP may explain the inability to detect effective ergonomics interventions, as the individual clinical presentation of people with LBP has not been taken into account. Therefore, future research should target interventions to the clinical presentations of subgroups of LBP patients. LBP is a complex multidimensional disorder and future studies should investigate seating modifications as part of a biopsychosocial management plan. Funding BackApp provided the chair, but did not fund, this study. One author (KOS) was funded by a research fellowship from the Health Research Board of Ireland. One author (MC) was supported by a bursary from the Occupational Health and Ergonomics clinical interest group of the Irish Society of Chartered Physiotherapists.
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Ergonomics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/terg20
The effect of a backrest and seatpan inclination on sitting discomfort and trunk muscle activation in subjects with extension-related low back pain a
b
c
d
a
Máire Curran , Wim Dankaerts , Peter O'Sullivan , Leonard O'Sullivan & Kieran O'Sullivan a
Department of Clinical Therapies, University of Limerick, Limerick, Ireland
b
Department of Rehabilitation Sciences, Catholic University, Leuven, Belgium
c
Department of Physiotherapy, Curtin University, Perth, Australia
d
Department of Design and Manufacturing Technology, University of Limerick, Limerick, Ireland Published online: 27 Mar 2014.
To cite this article: Máire Curran, Wim Dankaerts, Peter O'Sullivan, Leonard O'Sullivan & Kieran O'Sullivan (2014) The effect of a backrest and seatpan inclination on sitting discomfort and trunk muscle activation in subjects with extension-related low back pain, Ergonomics, 57:5, 733-743, DOI: 10.1080/00140139.2014.897378 To link to this article: http://dx.doi.org/10.1080/00140139.2014.897378
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Ergonomics, 2014 Vol. 57, No. 5, 733–743, http://dx.doi.org/10.1080/00140139.2014.897378
The effect of a backrest and seatpan inclination on sitting discomfort and trunk muscle activation in subjects with extension-related low back pain Ma´ire Currana, Wim Dankaertsb, Peter O’Sullivanc, Leonard O’Sullivand and Kieran O’Sullivana* a
Department of Clinical Therapies, University of Limerick, Limerick, Ireland; bDepartment of Rehabilitation Sciences, Catholic University, Leuven, Belgium; cDepartment of Physiotherapy, Curtin University, Perth, Australia; dDepartment of Design and Manufacturing Technology, University of Limerick, Limerick, Ireland
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(Received 27 November 2013; accepted 12 February 2014) Few studies have demonstrated that seating modifications reduce low back pain (LBP). One recent study found that a forward-inclined seatpan reduced low back discomfort (LBD), however this was only examined in people with flexionrelated LBP. No study has yet investigated its effectiveness among people with extension-related LBP. This crossover study examined 12 subjects with extension-related LBP. Sitting discomfort and surface electromyography of three trunk muscles were recorded during a 10-minute typing task while sitting with two different seatpan inclinations, both with and without a backrest. LBD ( p , 0.001) and overall body discomfort (OBD) ( p ¼ 0.016) were significantly greater on the forwardinclined seatpan. The backrest did not alter trunk muscle activation or sitting discomfort. The results demonstrate that in a specific subgroup of people with extension-related LBP, increasing forward seatpan inclination significantly increased LBD and OBD. Future research should consider matching ergonomics prescriptions according to the individual presentation of people with LBP. Practitioner Summary: Sitting on a forward-inclined seatpan resulted in greater low back discomfort and overall body discomfort than sitting on a flat seatpan during a typing task among people with extension-related low back pain (LBP). Future research should examine matching ergonomics prescriptions to the individual presentation of people with LBP. Keywords: back pain; musculoskeletal disorders; seating; office ergonomics
1.
Introduction
Low back pain (LBP) is a very prevalent and costly musculoskeletal disorder (Woolf and Pfleger 2003). Sitting is frequently reported as an aggravating factor for LBP (Womersley and May 2006). As prolonged sitting alone does not seem to be associated with the development of LBP (Bakker et al. 2009; Lis et al. 2007; Roffey et al. 2010), there has been much research investigating various seating modifications in the management of this condition. There is a common belief that lordotic postures are favourable, which is supported by healthcare professionals (O’Sullivan et al. 2012d), the general public (O’Sullivan et al. 2013a) and ergonomics recommendations (Corlett 2006). Further to this, the use of lumbar supports (Williams et al. 1991) or devices (Reinecke, Hazard, and Coleman 1994) aiming to increase seated lumbar lordosis has been advocated for the management of LBP. This increase in seated lumbar lordosis may reduce LBP temporarily (Williams et al. 1991). However, hyper-lordotic sitting postures increase levels of trunk muscle activation in pain-free populations (O’Sullivan et al. 2006a), and this may result in discomfort and fatigue (Carcone and Keir 2007; Lander et al. 1987). There is also evidence that some people with persistent LBP provoked by sitting present with lordotic postures (Dankaerts et al. 2006b, 2009). Dynamic sitting devices that cause slight spinal movement have been advocated as an approach to reduce LBP (van Dieen, de Looze, and Hermans 2001). It has been proposed that LBP subjects maintain more sustained, static postures during sitting, with large infrequent postural shifts, as opposed to regular spinal movements (Dankaerts et al. 2006b; Telfer, Spence, and Solomonidis 2009). However, recent systematic reviews (O’Sullivan et al. 2012c, 2013b) revealed that there was no evidence that dynamic sitting alone significantly alters trunk muscle activation or reduces LBP. The effect of altering seatpan inclination has been investigated in a number of studies. Forward-inclined seatpans have been reported to encourage lumbar lordosis (Gadge and Innes 2007; O’Sullivan et al. 2012b). However, it is unclear as to whether low back discomfort (LBD) is decreased (Gadge and Innes 2007) or increased (Bendix, Jensen, and Bendix 1988) with a forward-inclined seatpan. Additionally, it has been documented that although LBD may be reduced, there can actually be an increase in discomfort in other body areas (Gadge and Innes 2007).
*Corresponding author. Email: [email protected] q 2014 Taylor & Francis
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Another commonly advocated seating modification is the use of a backrest (Carcone and Keir 2007; Kingma and van Dieen 2009). Backrests have become customary in the workplace, although their use may be suboptimal among seated occupational tasks (Vergara and Page 2000). It had been suggested that backrests reduce trunk muscle activation (Gregory, Dunk, and Callaghan 2006; Hardage, Gildersleeve, and Rugh 1983; Lander et al. 1987). However, it is unclear if a backrest affects sitting discomfort as most studies investigated pain-free populations. Therefore, this should be further analysed among LBP subjects. Therefore, there is very limited evidence to validate most seating modifications in the management of LBP (Driessen et al. 2010). This suggests either these modifications are ineffective, or have been applied to wrongly selected subjects, or possibly a combination of these factors. There is evidence that people with LBP provoked by sitting may not present with the same spinal postures and motor patterns (Dankaerts et al. 2006a, 2006b). It is proposed therefore that seating modifications should reflect the individual clinical presentation of a person with LBP, which might involve greater or less lumbar lordosis (Dankaerts et al. 2006b). One recent study (O’Keeffe et al. 2013) took into account the existence of distinct movement subgroups among people with LBP, and investigated an ergonomics intervention among those with flexion-related LBP. This study examined a forward-inclined seatpan (‘Back App’) that also features a dynamic component and does not have a backrest, and investigated its effect on sitting discomfort among subjects with flexion-related LBP. Results demonstrated a significant reduction in LBD while sitting on the forward-inclined seatpan when compared to a flat seatpan with a backrest. As the forward-inclined seatpan encourages lumbar lordosis (O’Sullivan et al. 2012b), it was therefore proposed that LBD reduced as a result of the directional nature of the disorder. However, it is also possible that there may have been an enhanced placebo effect due to the novel appearance of the forward-inclined seatpan and the dynamic component. Additionally, this study involved patients with low levels of functional disability, no dominant psychosocial factors and a primarily mechanical behaviour of their LBP (aggravated/eased by spinal postures and movements). This study raised a number of issues to be investigated. These include examining whether seated LBD would also be reduced in a different LBP population, e.g. those with extension-related LBP. This would clarify if the effects seen in the previous study (O’Keeffe et al. 2013) were primarily related to the directional nature of the disorder (flexion-related LBP) and/or the fact that subjects had low disability levels, or simply reflected the placebo effect. Additionally, the forward-inclined seatpan may have reduced sitting discomfort as a result of an increase in lumbar lordosis and altering trunk muscle activation (O’Keeffe et al. 2013). As a backrest is proposed to reduce trunk muscle activation in pain-free subjects, it is therefore beneficial to determine whether a backrest could be as effective in reducing sitting discomfort and altering trunk muscle activation, among those with LBP. Therefore, in order to address these varying factors, another study is warranted, which includes subjects who are possibly less likely to benefit from a forward-inclined seatpan which increases lumbar lordosis, i.e. people with extensionrelated LBP. Additionally, as previously highlighted, there is limited evidence conducted on the effect of backrests among LBP patients; therefore, it needs to be investigated to determine any effect backrests may have on muscle activation and sitting discomfort. The initial hypothesis of this study was that LBD and overall body discomfort (OBD) among this subgroup would be increased rather than decreased on the forward-inclined seatpan. It was further hypothesised that trunk muscle activity would be reduced on the forward-inclined seatpan and the presence of the backrest may reduce LBD, OBD and trunk muscle activity. Therefore, the aims of this study were to compare LBD, OBD and trunk muscle activity when sitting on a forwardinclined seatpan with sitting on a flat seatpan during a controlled typing task in subjects with extension-related LBP, and also to determine the effect of a backrest on these parameters. 2. 2.1
Methods Study design
A single session, repeated measures, crossover design was conducted, corresponding to previous, similar studies (O’Keeffe et al. 2013). The dependent variables were trunk muscle activation and sitting discomfort, and the independent variables were backrest (present or absent) and seatpan inclination (flat or forward inclined). All subjects completed the same protocol on a single day, with the order of testing randomised using sealed opaque envelopes. Ethical approval was obtained from a local Research Ethics Committee. 2.2
Subjects
Subjects who were referred to a local pain medicine centre with LBP were recruited. Subjects were given an information sheet and written informed consent was obtained from all subjects prior to testing. Subjects were informed that they were free to withdraw from the study at any time. Subjects were included if they were aged over 18 years and had reported that their LBP was aggravated by extension-related activities (e.g. standing/walking/upright sitting) and eased by spinal flexion
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such as slump sitting. This is consistent with an active extension pattern classification (Dankaerts et al. 2006b). Subjects were excluded if they were pregnant, had LBP for less than three months, had any red flags, had a specific spine disorder (e. g. fracture, tumor stenosis) or had previous spinal surgery. The primary pain location was between L1 and the buttocks, to eliminate those whose extension-related LBP might have been related to spinal stenosis. Subjects whose pain was aggravated by flexion were also excluded. Based on these criteria, 12 (7 female, 5 male) subjects were recruited. This was a single-group study, with no pain-free control group. Subject demographics were obtained prior to testing (Table 1). The Numerical Rating Scale (NRS) was used to measure average LBP over the previous week (Childs, Piva, and Fritz 2005); fear of physical activity was determined using the physical activity subscale of the fear avoidance beliefs questionnaire (Waddell et al. 1993); functional disability was measured using the Oswestry Disability Index (ODI) (Fairbank and Pynsent 2000) and depression, anxiety and stress were measured using the Depression, Anxiety and Stress Scale (DASS-21) (Lovibond and Lovibond 1995).
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2.3
Trunk muscle activation
Surface electromyography (sEMG) was used to analyse the activation of three trunk muscles. The Motion Lab Systems MA-300 multi-channel EMG system (Motion Lab Systems Inc., Baton Rouge, LA, USA) was used to collect the sEMG data. Self-adhesive disposable Ag/AgCl disc surface electrodes were used which have an electrical contact surface of 1 cm2 and a centre-to-centre spacing of 2.5 cm. The sampling rate was 2000 Hz per channel, with a gain of 2000 and a common mode rejection ratio of . 115 dB at 60 Hz (O’Sullivan et al. 2006a). Prior to electrode placement, the skin was prepared to reduce skin impedance. Each electrode site was lightly abraded with fine sandpaper, body hair was shaved if necessary and the skin was cleansed with isopropyl alcohol solution, consistent with recommendations (Hermens et al. 2000). The muscles studied were superficial lumbar multifidus (level of L5, parallel to a line connecting the posterior superior iliac spine and L1 –L2 interspinous space), iliocostalis lumborum pars thoracis (ICLT) (level of L1, midway between the midline and lateral aspect of the subject’s body) and external oblique (EO) (below the rib cage, along a line connecting the most inferior costal margin and the contralateral pubic tubercle). These three muscles were selected based on pilot testing and previous research demonstrating that these muscles are most affected by changes in seatpan inclination (O’Sullivan et al. 2012a, 2012b). The surface electrodes were positioned parallel to the muscle fibre direction of each muscle unilaterally (O’Sullivan et al. 2006a). The side of dominant pain was chosen for the application of electrodes among subjects with bilateral and central LBP. For eight subjects, this was the right-hand side and for four subjects this was the left-hand side. A common ground electrode was placed at the ulnar styloid. One researcher applied the electrodes for all subjects. Since normalisation to maximal voluntary contraction has been reported as unreliable in LBP patients (Dankaerts et al. 2004), sEMG data were amplitude normalised to two standardised activities to elicit a sub-maximal voluntary isometric contraction (sub-MVIC) (O’Sullivan et al. 2002). For the abdominal muscles, the crook lying double leg raise was used. For this, the subject was in crook lying with the hips flexed to 458 and the knees flexed to 908. The subject raised both legs 1 cm off the supporting surface for 3 seconds. For the paraspinal muscles, the prone lying ‘double leg raise’ was used. For this, the subject was in a prone lying position with the knees bent to 908 and both knees lifted 5 cm off the ground for 3 seconds. The highest generated contraction from three repetitions was taken as sub-MVIC for each specific muscle (Dankaerts et al. 2006a). Each test was recorded for 5 seconds duration with a one-minute rest period between tests to reduce fatigue (O’Sullivan et al. 2002). The middle 3 seconds were analysed (Soderberg and Knutson 2000).
Table 1.
Mean (^ SD) subject demographics.
Measure
Mean(^ SD)
Age (years) Height (m) Mass (kg) Body mass index (kg/m2) Low back pain duration (years) Average weekly pain (Numerical Rating Scale) Fear-avoidance beliefs Oswestry Disability Index (%) Depression Anxiety Stress
41.7 (^ 9.3) 1.7 (^ .1) 84.8 (^ 23.4) 30.2 (^ 9) 9.2 (^ 6.4) 5.7 (^ 2) 19.8 (^ 6.7) 42.8 (^ 13.3) 20.5 (^ 15.1) 15.5 (^ 14.8) 22.7 (^ 15.1)
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2.4 Sitting conditions: forward-inclined seatpan versus flat seatpan A flat seatpan and a forward-inclined seatpan (‘Back App’) were compared for the purpose of this study (Figures 1 and 2). The ‘Back App’ was adjusted to allow a hip flexion angle of 558 with subjects placing their feet on the footplate (Figure 2), in line with previous studies (O’Keeffe et al. 2013; O’Sullivan et al. 2012a, 2012b). The ‘Back App’ can operate as a stable or dynamic design through adjusting the stability component which is located at the base of the seat. There are three different stability levels which comprise of the ‘green zone’ which is stable, the ‘black zone’ which is slightly unstable and the ‘red zone’ which involves greater instability. The stability level was maintained at the ‘black zone’ for this study which allows a mild degree of motion in three planes, in line with previous, similar studies (O’Keeffe et al. 2013; O’Sullivan et al. 2012a, 2012b). The flat seatpan was height adjustable and had wheels. The seatpan was controlled at a 908 hip and knee angle for each subject ensuring both feet were placed firmly on the floor (Gregory, Dunk, and Callaghan 2006). The flat seatpan differed slightly in size and material texture from that of the forward-inclined seatpan. Goniometry was used to measure both hip and knee angles as this has been reported to have high intra-rater and inter-rater reliability (Ekstrand et al. 1982). 2.5 Backrest A standard backrest was used for all subjects. The height and distance of the backrest was standardised (Figures 1a and 2a). During the backrest condition, the backrest was secured to a plinth, which was located 22 cm behind the subjects’ greater trochanter at the level of posterior superior iliac spine. 2.6
Discomfort
Subjects rated their discomfort at the end of each sitting condition using the Body Part Discomfort Scale (Corlett and Bishop 1976), as this has been extensively used for seat evaluation (Fenety, Putnam, and Walker 2000). This scale involves subjects subjectively rating their discomfort in each of 12 body areas on a six-point verbal numerical rating scale, where 0 ¼ ‘no discomfort’ and 5 ¼ ‘extreme discomfort’. LBD was rated using a single LBD score. OBD was examined separately as previous studies have demonstrated a reduction in LBD can be associated with increased discomfort in other body regions
Figure 1. Layout of the simulated workstation with subject sitting on the flat seatpan, with backrest (left) and without backrest (right).
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Figure 2. Layout of the simulated workstation with subject sitting on the forward-inclined seatpan with backrest (left) and without backrest (right).
when sitting on forward-inclined seatpans (Gadge and Innes 2007). OBD consisted of the mean discomfort of the 12 body areas to give a single total body perceived discomfort value (Fenety, Putnam, and Walker 2000). 2.7
Procedure
A simulated workstation was devised using an adjustable plinth as a desk (Figures 1 and 2). As self-selecting workstation set-ups can be linked with poor sitting postures (Gadge and Innes 2007), the height and distance from the workstations was standardised. Subjects were positioned by the same researcher for each sitting condition to enhance consistency. Subjects’ elbows were at 908 and kept in line with the trunk during the typing task (Kingma and van Dieen 2009). The distance between the subject and the plinth was standardised so that subjects’ greater trochanter was 30 cm from the edge of the plinth (O’Keeffe et al. 2013). A laptop was placed 10 cm from the edge of the plinth, directly in front of subjects. A paperweight was used to position a standardised document to be typed on the right-hand side of the laptop screen. The set-up and testing protocol were piloted prior to the study to enhance consistency and accuracy. Subjects were instructed to ‘sit as you normally would’ on the flat seatpan (O’Sullivan et al. 2006b), to ‘maintain your balance’ on the forward-inclined seatpan (O’Keeffe et al. 2013; O’Sullivan et al. 2012b) and for both seats to ‘rest against the backrest’ when the backrest was available. Subjects typed for 10 minutes in each sitting condition with sEMG data recorded for 5 seconds every 3 minutes, similar to previous research (McGill, Kavcic, and Harvey 2006; O’Sullivan et al. 2012b). Pilot testing demonstrated that longer periods of sitting in this population resulted in unacceptable levels of discomfort. Subjects were blinded as to when muscle activity was being recorded. Subjects rated their discomfort for each body region at the end of the 10-minute testing periods. A one-minute rest period between sitting conditions was provided to minimise the confounding effects of fatigue (O’Sullivan et al. 2012b). 2.8
Data analysis
Minimal filtering of sEMG data was undertaken. A high-pass RC Butterworth filter (–3 dB/octave at 10 Hz) and a linear phase Bessel low-pass filter (– 24 dB/octave at 500 Hz) were applied to the data. The middle 3 seconds of raw sEMG data from each five-second testing period (O’Sullivan 2006) were analysed using a root-mean-square (RMS) algorithm (using
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Motion Lab Systems Analysis software). These RMS sEMG values were normalised to sub-MVIC for the three muscles and the raw data was expressed as percentage of muscle activity. The mean, and variation (SD), from the three time intervals was assessed for all three muscles and was used for statistical analysis. There was little variance between each time interval, corroborating previous studies which demonstrated that time had no significant effect on muscle activity in short sitting tasks (McGill, Kavcic, and Harvey 2006). All raw sEMG files were visually inspected and data from channels with signal interference was removed. Data were analysed using the Statistical Package for Social Science (SPSS version 20.0). Data were tested for normality using the Shapiro – Wilks test, and homogeneity of variances using Levene’s test. All dependent variables (EO, ICLT, LM, LBD and OBD) were either normally distributed (Shapiro – Wilks, p . 0.05), or their plots indicated minimal variation from normality of distribution, allowing for parametric data analysis. A two-way ANOVA (with post-hoc Tukey) was conducted to compare each of the dependent variables (EO, ICLT, LM, LBD and OBD) between two independent variables (presence of backrest and seatpan inclination). Statistical significance was set at p , 0.05. 3. Results 3.1 Subjects All 12 subjects completed the protocol. Subjects reported at least moderate pain (mean ^ SD NRS ¼ 5.7 ^ 2), disability (mean ^ SD ODI ¼ 42.8 ^ 13.3) and psychological distress (Table 1). Surface EMG data was missing for one subject due to signal interference. 3.2
Low back discomfort
There was no significant interaction (F(1, 44) ¼ 0.000, p ¼ 1.000) between the effects of a backrest and seatpan inclination on LBD. Simple main effects analysis showed that LBD did not differ significantly (F(1, 44) ¼ 0.076, p ¼ 0.785) according to the presence or absence of a backrest. However, LBD was significantly greater with the forward-inclined seatpan when compared to the flat seatpan (F(1, 44) ¼ 19.354, p ¼ 0.000) (Figure 3). 3.3 Overall body discomfort There was no significant interaction (F(1, 44) ¼ 0.477, p ¼ 0.493) between the effects of a backrest and seatpan inclination on OBD. Simple main effects analysis showed that OBD did not differ significantly (F(1, 44) ¼ 0.068, p ¼ 0.795) according to the presence or absence of a backrest but did differ significantly depending on seatpan inclination, with OBD significantly greater on the forward-inclined seatpan (F(1, 44) ¼ 6.24, p ¼ 0.016) (Figure 4).
Figure 3. Change in the mean (^SD) of low back discomfort (LBD) for all sitting conditions. Note: Negative values indicate that discomfort reduced in that sitting condition.
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Figure 4. Change in the mean (^SD) of overall body discomfort (OBD) for all sitting conditions. Note: Negative values indicate that discomfort reduced in that sitting condition.
3.4 Trunk muscle activation There was no significant interaction between the effects of a backrest and seatpan inclination for mean activation of any of the three muscles analysed ( p . 0.05). Additionally, simple main effects analysis showed no significant difference ( p . 0.05) in mean trunk muscle activity according to the presence or absence of a backrest, or depending on seatpan inclination. There was also no significant difference in the variation of trunk muscle activity between any of the sitting conditions ( p . 0.05). 4. Discussion 4.1 Sitting discomfort The findings of this study demonstrate that in a subgroup of people with extension-related LBP, consistent with an active extension pain disorder (Dankaerts et al. 2006b), LBD and OBD were significantly increased on a forward-inclined seatpan in comparison to a flat seatpan. A number of studies have previously investigated the effect of forward-inclined seatpans on sitting discomfort. However contrasting results have been obtained with both decreased (Bridger 1988; Gadge and Innes 2007) and increased (Bendix, Jensen, and Bendix 1988; Bishu et al. 1991) LBD reported with a forward-inclined seatpan. However, most studies investigating seatpan inclination included pain-free populations, and those that did include LBP patients did not consider their individual clinical presentation. The importance of investigating subgroups of LBP patients has been previously highlighted (Dankaerts et al. 2006b). Therefore, this current study deliberately selected subjects with extension-related LBP. Previous research demonstrates that this subgroup tend to sit in a hyperextended posture which is correlated with their symptoms (Dankaerts et al. 2009). In other words, rather than assuming the most comfortable sitting posture available to them, their sitting posture seems to exacerbate their discomfort, and this direction-specific behaviour appears to be related to deficits in repositioning sense and body schema (Luomajoki and Moseley 2011; O’Sullivan et al. 2003, 2013c; Sheeran et al. 2012). Based on previous research using the same forward-inclined seatpan, the most likely explanation for an increase in LBD in this study is an increase in lumbar lordosis (O’Sullivan et al. 2012b). Although this previous study was conducted among pain-free subjects, it is in line with other research which has demonstrated that similar forward-inclined seatpans can increase lumbar lordosis (Gadge and Innes 2007; Gale et al. 1989; Koskelo, Vuorikari, and Ha¨nninen 2007). Therefore, the directional nature of the disorder among the active extension pattern group included resulted in an increase in LBD, as hypothesised. In contrast to this, it was recently found that LBD was reduced on the same forward-inclined seatpan (O’Keeffe et al. 2013). However, that study examined those with flexion-related LBP. Therefore, matching the forward-inclined seatpan according to the flexion-pattern of their LBP most likely reduced LBD as a result of the directional nature of the disorder (O’Keeffe et al. 2013). This highlights the potential importance of matching ergonomics interventions to the clinical presentation of
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individual patients rather than generic prescription, as it is evident that different subgroups can respond with either significantly increased or reduced LBD, based on their individual LBP disorder. The increase in OBD in this study corroborates the results of a previous study which demonstrated that forward-inclined seatpans increased discomfort levels in other body areas, such as the hips and buttocks (Gadge and Innes 2007). It is also somewhat consistent with the lack of a decrease in OBD in the recent study using the same forward-inclined chair among people with flexion-related LBP (O’Keeffe et al. 2013). Another possible reason for the differences in OBD between sitting conditions was the foot position. The forward-inclined seatpan required subjects to distribute more weight through the front of their feet, with increased ankle plantar flexion. While research is limited on the effect of foot position, it has been shown to affect lower leg muscle activation which could contribute to lower leg discomfort (Carlsoo 1961). Therefore, this may have led to an increase in OBD observed on the forward-inclined seatpan, although since foot position does not significantly increase trunk muscle activation (Carlsoo 1961) it most likely had no effect on LBD. Contrary to the hypothesis, the addition of a backrest to both of the seatpan conditions did not significantly reduce LBD or OBD. Previous research has demonstrated very slight reductions in sitting discomfort when using a backrest (Gregory, Dunk, and Callaghan 2006; Kingma and van Dieen 2009; Lander et al. 1987). However, these other studies were all among pain-free subjects, providing limited evidence for backrests having any effect on sitting discomfort in LBP patients. The lack of effect may also reflect an inability among this group of patients to relax their paraspinal muscles (Dankaerts et al. 2006a). 4.2
Trunk muscle activation
In contrast to our hypothesis, there was no significant difference in trunk muscle activity for any of the sitting conditions in this study. Previous research indicated that trunk muscle activity may be reduced among pain-free subjects with a backrest (Gregory, Dunk, and Callaghan 2006; Kingma and van Dieen 2009). However, very little research has been conducted on the effect of a backrest on muscle activation among people with LBP, and data using participants with LBP are confounded by differences in the degree of hip flexion between the comparison chairs (Cram and Vinitzky 1995). There is also considerable inconsistency in research examining the effect of inclined seatpans on trunk muscle activation, with both increased (Cram and Vinitzky 1995; Lander et al. 1987) and decreased (Koskelo, Vuorikari, and Ha¨nninen 2007) muscle activation reported. However, one recent study (O’Sullivan et al. 2012b) investigating the effect of the same forwardinclined seatpan used in the current study among pain-free subjects demonstrated reduced paraspinal muscle activity. There are a number of likely explanations as to why muscle activity was not altered between sitting conditions in the current study. As highlighted, most previous studies have been done using pain-free populations. A meta-analysis (Geisser et al. 2005) identified higher muscle activity levels during static postures in LBP subjects when compared to pain-free subjects, and a reduced ability to relax the paraspinal muscles. Elevated levels of trunk muscle activation and an inability to relax the back muscles in sitting has been demonstrated in extension-related LBP patients previously (Dankaerts et al. 2009). Therefore, this group may be less able to relax into anterior pelvic tilt and lumbar lordosis on a forward-inclined seatpan than pain-free subjects (O’Sullivan et al. 2012b). Unfortunately, the lack of kinematic data does not allow us to confirm this hypothesis, though it appears reasonable based on data from people without LBP (O’Sullivan et al. 2012b). Similarly, this group may be less able to relax their lordosis and use a backrest in the manner that pain-free participants can. Another possible reason for no change in muscle activation in the current study is the greater levels of functional disability (mean ODI ¼ 42.8%) than in the previous study using this forward-inclined seatpan (O’Keeffe et al. 2013). Subjects with more disabling LBP, such as those included in this study, are reported to have altered patterns of muscle activity during sitting when compared to control subjects and those with non-disabling LBP (DeGood et al. 1994). Therefore, this may explain why trunk muscle activation was reduced amongst pain-free subjects on the forward-inclined seatpan (O’Sullivan et al. 2012b) and not in this group. Finally, the dynamic component of the forward-inclined seatpan is another factor which must be considered in this study. This may increase spinal movement in sitting (Kingma and van Dieen 2009; O’Sullivan et al. 2006b), yet most research suggests dynamic sitting in isolation has a negligible effect on LBD (O’Sullivan et al. 2012c), spinal posture (O’Sullivan et al. 2006b) and trunk muscle activation (O’Sullivan et al. 2013b). Therefore, while we cannot separate the dynamic element from the forward-inclined seatpan, it seems unlikely that the dynamic element contributed to the results obtained in this study. 4.3
Implications for clinical practice and research
The results of this study may have important implications for the management of people with LBP and research in this area. It has been reported that seating modifications are ineffective in the management of musculoskeletal disorders (Driessen et al. 2010). Despite the fact that sitting discomfort actually increased among the specific LBP population included in this study, there are many aspects worth noting towards the role of seating modifications.
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Subjects in this study were actively seeking medical treatment for their LBP. They were specifically selected as they subjectively reported pain with extension-related postures such as walking/standing and upright sitting. This was conducted so that subjects who were sensitive to lumbar extension were matched to a seating device that increased lumbar extension. A particular subgroup of LBP subjects were selected, rather than prescribing it to all people with LBP. Although this approach is clinically relevant, the consideration of matching interventions to subgroups of LBP patients is not frequently investigated (Vibe Fersum et al. 2010). This is important as the response of LBP patients to seating modifications may vary according to their individual clinical presentation (Dankaerts et al. 2006b). This has been highlighted in the current study, with an increase in LBD, which is in direct contrast to previous results (O’Keeffe et al. 2013) which found reduced LBD when using the same forward-inclined seatpan among those with flexion-related LBP. Although lordotic sitting is commonly considered as optimal for the lumbar spine (O’Sullivan et al. 2013a, 2012d), this study provides evidence that seating modifications may need to vary between subgroups of LBP populations to prevent the ‘washout’ effect (Dankaerts et al. 2009). In particular, the results of this study suggest that forward-inclined seatpans should not be recommended for patients with extension-related LBP. Additionally, subjects in this study had at least moderately disabling LBP, with high levels of fear avoidance. As such, there was likely to be a greater degree of central nervous system sensitisation involved in their pain (Smart et al. 2010). Therefore, while their pain did demonstrate at least some mechanical pain behaviour, they may be less likely to benefit from a physical ergonomics intervention in isolation. For example, the level of pain and fear among participants in the current study was twice that of the subjects with flexion-related LBP who reported a positive response to sitting on the same forward-inclined seatpan in a previous study (O’Keeffe et al. 2013). It is likely that a biopsychosocial disorder such as chronic LBP requires some matching of interventions (Fersum et al., 2013; Hill et al. 2011; O’Sullivan 2005), both for psychosocial components as well as physical components. The current study demonstrates that if ergonomics interventions are to be effective, they should be selected with the individual patient presentation in mind. 4.4
Limitations
This study investigated a small sample of a particular subgroup of people with LBP, and therefore results may not be generalised for other LBP populations. The sitting task was of short duration as subjects only typed for 10 minutes in each sitting condition. However, pilot testing revealed that 10 minutes of testing was sufficient to observe increases in LBD and OBD in this subgroup of LBP patients. Ideally, these ergonomics studies should be investigated over a much longer time period and in typical occupational settings, as the standardised nature of the office task in this study does not reflect the true office environment. The evaluation of additional outcome measures such as spinal kinematics could have enhanced interpretation of the likely mechanism of effect. Future studies should further attempt to identify the mechanism(s) through which LBD was increased on the forward-inclined seatpan by examining sEMG, spinal kinematics and contact with the backrest. Based on previous research, facilitation of lumbar lordosis was the likely cause of effect for increased LBD (O’Sullivan et al. 2012b). There were a limited number of muscles analysed, with only three muscles, on one side of the trunk, investigated in this study. Investigating more trunk muscles on both sides could allow for more detailed evaluation of the degree of muscle co-contraction (McGill, Kavcic, and Harvey 2006), and whether this changed between seating conditions. However, the muscles tested were selected based on previous research (O’Sullivan et al. 2012a, 2012b) demonstrating that they are most affected by changes in seatpan inclination. Furthermore, the assessor was not blinded. The size, and texture of the material, differed slightly between the two seatpans, and since this could cause differences in local buttock discomfort, it should be considered when interpreting the results. However, the main difference between the seatpans was LBD, suggesting this was not a major issue. 5. Conclusion In a specific subgroup of people with extension-related LBP, there was a significant increase in LBD and OBD while sitting on a forward-inclined seatpan when compared to a flat seatpan. The lack of investigations into subgroups of people with LBP may explain the inability to detect effective ergonomics interventions, as the individual clinical presentation of people with LBP has not been taken into account. Therefore, future research should target interventions to the clinical presentations of subgroups of LBP patients. LBP is a complex multidimensional disorder and future studies should investigate seating modifications as part of a biopsychosocial management plan. Funding BackApp provided the chair, but did not fund, this study. One author (KOS) was funded by a research fellowship from the Health Research Board of Ireland. One author (MC) was supported by a bursary from the Occupational Health and Ergonomics clinical interest group of the Irish Society of Chartered Physiotherapists.
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