Since spinal biomachanics is in my neck of the woods, I will try to explain for those that may be interested.
It's all related to the lumbar lordotic saggital curve, it is a physiological curve that allows the lowerback to adequately distribute the load evenly along its intervertebral discs decreasing overall intervertebral disc pressure in the lower segments, it distributes the proper 1/3 to 2/3 loadbearing weight disyribution between the posterior facet joints and the disc joints respectively, it gives the lunbar area flexibility to absorb correctly the vibrations and axial loads while sitting, it avoids post sheer force of the disc opposing disc bulging, and I could go on deeper.
When you sit down on a seat that is lower to the ground, your lower extremities are then in an elevated position closer to your hip's height than on a higher seat, this is the case of a car seat or low sofa, as opposed to a chair. This position causes both more hip flexion and knee extension therefore stretching the hamstrings, the hamstring then pull on their insertion in the post inferior pelvis and cause the hip to tilt backwards into a less vertical position, this in turn forces the lower back to flex accordingly, and this flexion reduces the lumbar curve, and that is were the problem is as described in the first paragraph.
The same thing happens when you tilt your backrest too much away from the vertical, the lumbar lordosis is reduced from lack of support.
This is the reason for the lumbar support car seats have, to push the lowback forward and maintain the lordorsis. This is also why the higher the seat the better for the lowback. Also why the more the knees are flexed the better (less hamstring tension, and in the case of sciatica, the less sciatic tension too) as long as its not at the expense of excessive hip flexion because then there is a point where you start to decrease the lordosis again.
Prolongued sitting with inadequate lumbar lordosis can lead to mechanical lowback pain through muscle strain, facet syndrome, etc as well as risk of eventually developing disc lesions such as sciatica, lumbar stenosis, etc.
Bottom line, all the focus needs to be on the lumbar lordosis, all the adjustments to the seating position need be with that as the priority, all else will follow (except in the case of sitting in a car where you also need to take into account the driving position to operate the controls efficiently).
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⇒ Carlos - Porsche 991 Carrera GTS