Final answer:
Proper sitting posture requires aligning the body's center of gravity over the pivot at the hips, minimizing muscle strain. Good posture reduces the need for excessive muscle exertion, while poor posture can lead to back pain and muscle strain. Special chairs can aid in achieving proper sitting posture to reduce discomfort.
Step-by-step explanation:
Proper posture while sitting in a chair includes positioning oneself so that the body's center of gravity (cg) is directly above the pivot point in the hips. This alignment ensures that the back muscles do not have to exert unnecessary force to maintain balance, as seen in good posture. In contrast, poor posture creates a clockwise torque around the hips because the upper body's cg is not aligned, causing tension and strain on the back muscles. Using special chairs designed to help position the body's cg above the seat can also contribute to reducing back pain, especially when sitting for prolonged periods such as at a desk with a computer.