Chairs for Low Back Pain

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Andersson et al. (1979) looked into the spinal kinematics of sitting. They showied through radiographic analysis that lumbar lordosis significantly diminishes when sitting unsupported due to pelvic rotation. Lumbar support restores lordosis far more effectively than changes in backrest inclination. Their data confirm that even with varying lumbar support heights, the presence of support itself, especially when positioned to maintain or restore lumbar curvature, markedly improves spinal alignment and reduces disc pressure and back muscle activity.

A study by Snijders et al. (2004) showed that slouched sitting (with the pelvis tilted posteriorly and the spine flexed) can place substantial mechanical stress on the iliolumbar ligaments (IL) and the lumbosacral junction. This occurs via a "crowbar" mechanism, where L5 acts as a lever pivoting on the IL, leading to forward displacement of L5, backward sacral rotation (counternutation), and dorsal gapping of the L5–S1 disc. These changes occur even in the absence of high compressive load and are particularly pronounced when dorsal muscle activity is relaxed. The authors found that this passive loading, especially during prolonged sitting without lumbar support, can lead to significant strain and possibly microtrauma in the IL.

The study also demonstrated that the use of lumbar support positioned at L5 markedly reduces these rotational and translational movements. However, this beneficial effect is negated if a high backrest applies force to the scapulae, thereby inducing slouching and shifting the center of gravity posterior to the ischial tuberosities. This observation is reinforced by the findings of Van Veelen et al. (2000), who emphasize that a minimum of 6 cm of "free shoulder space" between the lumbar and scapular contact points is necessary to prevent upper-back loading from overriding the lumbar support. Their work also supports a seat-backrest angle of around 90° as biomechanically favorable, showing reduced shear forces on the seat and lower EMG activity in back muscles with increasing free shoulder space.