Seat Design, Spine Curvature and Intradiscal Pressure

In nowdays digitalized and industrialized society, sitting is the most frequent posture at work, at school and also during the leisure time (surfing on the internet, gaming...). Because of gravity, the erect posture is associated with an increase of the lumbar intra-discal pressure, which has first been evidenced in the in vivo measurements of Nachemson [1-3]. In these studies, the authors depicted a rise of the intra discal pressure from lying supine to standing and to sitting. In addition, it has been shown that flexing forward while standing or sitting substantially increased intradiscal pressure [3], suggesting that its rise in the seated posture pertains to the flattening of the lumbar lordosis. Indeed, an extensive literature provides evidence that lumbar flexion and flattening of the lumbar curvature are associated with sitting. Ackerblom [4] reported an average lumbar flexion of 35° in people sitting upright, while Keegan [5] stated that it was impossible to seat in a standard chair without a considerable flattening of the lumbosacral curve. According to Schoberth [6], who used X-ray examination, sitting upright is associated with an average flexion of 60° in the hip joint and a 30° flexion in the lumbar region. Likewise, Lord [7] stated that lumbar lordosis while standing is nearly 50% greater on average than in sitting. For most authors, this phenomenon is the consequence of thigh flexion which drives the pelvis backward. In the study by Keegan [5], it has been shown that the flattening of the lumbar lordosis increases with thigh flexion from 0° to 130° in sidelying posture. The author presumed that it was related to the tension of the posterior thigh and gluteal muscles that tend to rotate the pelvis backward when the thighs are flexed. Eklund and Liew [8], who studied the effect of hip and knee flexion on the lumbar curvature in the lying and sitting posture, found similar results and stated that hip angle is a strong determinator of lumbar posture.