Why is sitting with bad posture a contributing factor to aches and pains?
Back ache
Is your sitting posture like this picture? If so this means you are allowing your lumbar spine to slouch (curve outwards). This puts uneven pressure on your discs, causing them to bulge backwards, which with time will distort and weaken them making them more liable to strain or herniate (slipped disc). It will also stretch and weaken the ligaments at the back of the spine, increasing the likelihood of ligament strain. Any of this happening will give you back ache as the muscles try to protect the damaged or weakened areas.
Leg and buttock pain
As the slumping causes weakness and damage to the lumbar ligaments and discs, the pain in your legs or buttocks could be "referred pain" originating in the lumbar area. If you are sitting with your pelvis tilting backwards, you may be putting pressure on a part of your buttocks that is not "designed" to take it, and this can cause buttock pain. Sitting on the “wrong” part of the buttocks can also put pressure on the nerves and blood vessels and cause numbness, tingling or pins and needles in the legs and feet.
Headache and neck pain
Is your sitting posture like this picture? If you slump, with your upper spine curved forwards, you usually need to lift your head up so that you can look straight ahead (at the computer screen?). This is hard work for the muscles that hold the head up, and also means you are bending your neck joints too far backwards. Either of these factors can contribute to headaches and neck pain. Slouching puts pressure on the diaphragm and restricts “abdominal” breathing (breathing by moving the diaphragm up and down which pushes/pulls air in and out of the lungs) and forces you breath by raising and lowering the rib cage. This requires tightening of the muscles that support the rib cage (the scalene and sterno-cleido-mastoid muscles) that connect the top of the rib cage to the neck. This will be a contributing factor to more neck tension problems and will also tend to pull the head and neck further forward.
Shoulder pain
Your shoulder blades are meant to function in a “vertical” position. If you slump, with your upper spine curved forwards, your shoulder blades rest against your rib-cage, which makes it a little harder for them to move freely, and after a period of time this makes the shoulder muscles ache and stiffen up, as a result of the extra work they are having to do. (This stiffness in the shoulders causes tension in a muscle that links the shoulder to the upper neck, so that when you turn your head you may feel sharp pain in the neck as well as a feeling of neck stiffness and headaches).
Repetitive Strain Injury(RSI)/Tennis Elbow
If the shoulders have stiffened up, you then compensate by overworking the wrist and elbow joints and muscles. This causes fatigue and tension in these muscles, which then can be a contributing factor to RSI or tennis elbow.
