Your body sags
Poor posture in sitting is by far the most common cause of pain and stiffness in the back and, for that matter, in the neck. Most everyone spends a greatdeal of time sitting: even if you do not spend most of your day behind a desk, you probably spend much of your evening in an easy chair or on a couch. Most people who sit for long periods will eventually develop poor posture in sitting. This is because when you sit in a particular position for even a few minutes, the muscles that support your lower back and your neck become tired. When they become tired, you relax, placing yourself in a position of poor posture.
Your body sags. You slouch in a chair, adopting a position that is still sitting, but is somewhere between sitting straight up and lying down. As a result, you typically lose the lordosis in the back and the lordosis in the neck.
If you stay in a slouched sitting posture long enough, your ligaments will become overstretched and pain and stiffness will result. Worse, once the slouched sitting posture has become a habit and is maintained most of the time, it may also cause distortion of the nucleus of the discs in the vertebral joints. (A distorted disc is one that has lost its normal shape; typically, it bulges in one direction or another.) Once this occurs, pain will come not only from certain positions but from certain movements.
Poor sitting posture may itself produce lower back pain. And, regardless of the original cause, once lower back pain or stiffness has developed, poor sitting posture will perpetuate or worsen the problem.
