This is a common misconception in the ergonomic community.
From my experience and current research the OLD 90 degree torso to leg angle has been debunked as it causes the occupant to lose the lower back curvature.
Try this exercise…lie flat on the ground and bring your legs up bending them at the knee to a 90 degree angle with the ground. This will place the lower portion of the leg parallel with the floor. Now get a sense/feel of your lower back and notice that most of, if not all, of your lower back is touching the floor. The lower back has lost its curve!
Now just lower the legs so that your feet are flat on the floor and your knees are bent. You’ll most likely notice that there now has developed a space between your back and the floor. This is why most personal trainers have you adopt the previous posture when working out your core muscles because it flattens your back and isolates your core muscles. This is very bad for the lower back in the seated posture. The most optimal height of a chair and the most optimal posture for the seated occupant is to have the knees below the hips 90+10 or 15 degrees, depending on the individual, thus maintaining the curvature of the back and maintaining the closest posture to that of standing which is the most physiologically efficient.
Remembering that is the 90 degree posture seated there is about 300 pounds per square inch on the discs and the joints of the lower back. When standing it goes to 100 pounds and when lying down it goes to 25 pounds.
The Bionomic seating solution is the only system to incorporate this understanding and is now recognized by the Australian Government Tax office as a therapeutic device.