Sitting is a common aggravating factor in low back pain (LBP) and of the most common strategies used by physiotherapists in the management of LBP is providing advice on spinal postures (Poitras et al., 2005). However, there is debate regarding what is an optimal sitting posture.
Is it erect, stiff and 90 degrees posture?
In fact, stiffening the back only arrived in the 19th century. Before then, aristocratic fashion favored languid slouching.
No
Posture should really be considered as the sum total of the positions and movements of the body throughout the day and throughout life. It should include not only the fundamental static positions in lying, sitting and standing and the variations of these positions but also the dynamic postures of the body in motion or in action, for it is here that posture becomes most important and most effective. Posture has a direct relation to the comfort, mechanical efficiency and physiologic functioning of the individual.
What can bad posture (Static/uncomfortable) do and why it should be considered?
There is an increased risk of injury when trying to execute complex movements with insufficient technical control or with bad posture.
Prolonged lordotic sitting postures can be associated with increased fatigue and discomfort.
Bad postures are common aggravating for musculoskeletal pain.
While it is desirable to maintain a good position of the body most of the time, it is not harmful to assume relaxed positions at times.

In this study a total of 296 physiotherapists who attended continuing professional development workshops on LBP in four countries (Ireland; n ¼ 111, England; n ¼ 88, Germany; n ¼ 41 and the Netherlands; n ¼ 56) participated in this study.Their comments indicated that sitting postures which matched the natural shape of the spine, and appeared comfortable and/or relaxed without excessive muscle tone were often deemed advantageous.
Role of dynamic sitting approaches to facilitate spinal micro-movement as a means of reducing LBP while sitting (van Dieen et al. 2001).
Approaches such as dynamic sitting may only be effective as a stand-alone management tool in LBP subjects with low levels of disability who are exposed to long periods of sitting, and whose pain is specifically aggravated during static sitting and is clearly relieved with movement. Another potential role for dynamic sitting devices is facilitation of non-provocative spinal postures with less requirement for paraspinal muscle activation, such that the effort of sitting is reduced (O’Sullivan et al. 2006b, Claus et al. 2009). Prolonged sitting periods, for example periods exceeding 30 min, are a common aggravating factor for many subjects with LBP (Williams et al., 1991; O’Sullivan, 2005).
Your good posture is your next posture
https://www.youtube.com/watch?v=CZFuGd6Z1aQ
Neutral sitting postures encouraging lumbar lordosis have been recommended in the management of sitting-related low back pain (LBP). Training with Pilates-based exercises improves dynamic body control.
Takeaway messages:
Set an example by practicing good posture.
There is no best sitting posture, all postures were acceptable options.
Dynamic sitting has been proposed to reduce low back pain (LBP) and/or low back discomfort (LBD) while sitting.
Avoid exaggeration of postural positions that cause distortion of good alignment.
References:
Neutral lumbar spine sitting posture in pain-free subjects. K O'Sullivan, P O'Dea, W Dankaerts, P O’Sullivan, A Clifford, L O’Sullivan. Manual Therapy 15 (6), 557-561
Can we reduce the effort of maintaining a neutral sitting posture? A pilot study. K O’Sullivan, R McCarthy, A White, L O’Sullivan, W Dankaerts. Manual therapy 17 (6), 566-571
The effect of dynamic sitting on the prevention and management of low back pain and low back discomfort: a systematic review. K O'Sullivan, M O'Keeffe, L O'Sullivan, P O'Sullivan, W Dankaerts. Ergonomics 55 (8), 898-908
What do physiotherapists consider to be the best sitting spinal posture?. K O'sullivan, P O'sullivan, L O'sullivan, W Dankaerts. Manual therapy 17 (5), 432-437
McMillan, A., Proteau, L. and Lèbe, R.M., 1998. The effect of Pilates-based training on dancers' dynamic posture. Journal of Dance Medicine & Science, 2(3), pp.101-107.
Howorth, B., 1946. Dynamic posture. Journal of the American Medical Association, 131(17), pp.1398-1404.
Kendall, H.O. and Kendall, F.P., 1968. Developing and maintaining good posture. Physical therapy, 48(4), pp.319-336.