1

Introduction

The concept of an optimal or ideal sitting posture is often used in practice but lacks a basis in evidence. When moving from standing to sitting, a posterior pelvic tilt is associated with reduced lordosis. The lumbosacral joint can even reach maximum flexion in slumped sitting . Some authors consider that correct sitting is achieved when the lordosis is similar to that in a standing position . However, no study has demonstrated that such sitting postures prevent disc degeneration or low back pain (LBP) .

Preferred postures in sitting appear to differ between healthy people and patients with chronic LBP (cLBP). Patients could be classified into groups with less or more lordosis than healthy subjects and patients with less-modulated lumbar curvature when instructed to move from upright to slumped sitting .

The angle of the acetabulo-femoral joint affects pelvic rotation and hence lumbar curvature during sitting . Thus, ergonomic chairs, such as the kneeling chair, aim to limit hip flexion and promote lumbar lordosis . Whether these chairs can be recommended to prevent or alleviate LBP is unclear. Moreover, non-specific LBP can represent several sources of pain that may even affect sitting behavior in various ways. However, in most cases, the source of pain is not actually known. For a start, whether lordosis is indeed affected by the chair, throughout a range of postures, from upright to slumped sitting, needs to be determined as does whether this effect is present in CLBP patients and healthy people alike. Numerous studies have explored the effect of different sitting positions on lumbar curvature with external measures (inclinometers or external markers) , but we have no data from radiologic measures including pelvic parameters.

Sitting on a sloping (or kneeling) chair may promote an anterior pelvic tilt and enhance lumbar lordosis as compared with sitting on a flat chair, probably for both slumped (relaxed) and upright (active) sitting. Moreover, CLBP patients may modulate lumbar lordosis less between sitting and standing than healthy controls. In this study, we aimed to study pelvis orientation and lumbar curvature with radiographs from patients with non-specific CLBP and matched controls when sitting upright and slumped on 2 types of chairs: a usual chair (“flat chair”), with a horizontal seat, and a kneeling chair (“sloping chair”). The radiography protocol we used did not allow for investigating large cohorts without formal indications of the usefulness of this method for this goal (pelvic variables in a sitting position for LBP). We therefore studied a relatively small cohort as a pilot study to determine whether CLBP patients responded differently from healthy subjects.

2

Methods

2

Methods

3

Radiography

A digital radiography system (Definium 6000, GE Healthcare) was used. The effective radiation dose for the 4 radiographs was 1.8 mSv. Radiographs were captured directly onto a computer. Lateral static computed radiographs of the lumbar spine and pelvis were obtained at a set distance of 180 cm from the X-ray source and centered on L4. One radiograph was taken with the subject standing up ( Figs. 1A, 2A ) and 4 radiographs were taken with the subject seated in 2 postures in each of 2 chairs: flat chair (height 45 cm, 0° seat inclination; Fig. 1B1, 2 , Fig. 2B1, 2 ) and sloping chair (height 40 cm, 20° forward sloping seat inclination, 20° backward sloping knee support inclination; Fig. 1C1, 2 ; Fig. 2C1, 2 ) and upright posture ( Fig. 1B1, C1 ; Fig. 2B1, C1 ) and slumped posture ( Fig. 1B2, C2 ; Fig. 2B2, C2 ). Before the experiments, subjects were instructed to maintain postures without touching the back of the chair. Furthermore, they were instructed to obtain the slumped posture by rounding the spine and relaxing the back muscles and put their hands on their thighs.

Standing position and the 4 sitting conditions analysed (B1: flat chair, upright; B2: flat chair, slumped; C1: sloping chair, upright; C2: sloping chair, slumped).

Radiographs taken under all conditions and variables measured described in Fig. 2 A for conditions described in Fig. 1 .

4

Measurements

From each radiograph, pelvic incidence, sacral slope, and lumbar lordosis were measured by use of the Keops sagittal balance analyzer (SMAIO, Lyon, France). The following markers were digitized: one point on each of the femoral heads, 2 corners of the sacrum, 4 corners of L5 and the top corners of L1. Data obtained in the sitting position were referenced to data obtained in the standing position (sitting value – standing value) to reduce inter-subject variability and analysed as absolute change and percentage change from standing values. Four sets of 100 blinded and randomized radiographs (20 subjects × 5 conditions) were created and analyzed twice (on 2 separate days) by 2 independent experienced physicians (MV, AD). Before unblinding, a preliminary analysis of intra- and interrater reliability was performed, then one of the 4 series was chosen at random for statistical analysis.

5

Statistical analysis

Statistical analysis involved use of SAS 9.3 (SAS Inst., Cary, NC) and R 2.9.2 ( http://www.R-project.org , the R Foundation for Statistical Computing, Vienna, Austria). Inter- and intrarater reliability was measured by the intraclass correlation coefficient (ICC) with the individual effect considered as a random effect. The ICC was considered good at > 0.9 and acceptable at > 0.8. Under the condition that inter- and intrarater reliability was acceptable, a global ICC combining inter- and intrarater variability was computed by a mixed model including an individual random effect, a rater fixed effect and an order of measure fixed effect. Absolute and relative changes from standing to sitting in lumbar lordosis and sacral slope were analysed by a linear mixed-effects model with 2 repetition factors (posture and chair) and a group factor (CLBP versus controls). Interactions between factors were also tested. P < 0.05 was considered statistically significant.

6

Results

CLBP patients and controls were well-matched ( Table 1 ): mean age 42.5 ± 9 and 43 ± 9 years, body mass index 26 ± 5.4 and 25.6 ± 5 kg/m ² . The ICCs (2 measures and 2 raters) for pelvis variables during standing and sitting in 2 types of chairs and with 2 positions were > 0.9 for all conditions ( Table 2 ). Therefore, we randomly selected one measure for further analysis before ICCs were computed, and only the first measurement of rater 2 was analysed.

Table 1

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