Prompted by the rapidly increasing number of chronic back pain cases (back pain is now the most widely reported chronic work related injury in U.S.), a recent study using MRI imagery revealed that adhering to the recommended right angle position when seated may put excess strain on spinal disks. The straight-backed posture creates an unnecessary weight load on the spine, which can actually be more detrimental than previously expected.
The most severe damage, however, was seen in those participants that spent long hours slouched toward their knees. The forward lean dramatically elevates muscle tension, and creates dangerous friction across the spine and lower back in particular.
The same study examined subjects who sat with a slightly backward lean, forming a (roughly) 135 degree angle. Those participants who worked in the relaxed position significantly decreased tendon, muscle and spinal strain, yielding the best results. Considering the results, researchers recommend the use of seating that features a shoulder-height back rest, specifically angled to promote the more beneficial open angled posture.