A little more knowledge about the muscles involved when sitting will help to understand why and how they are causing low back pain. The primary muscles that become involved in this situation are the iliacus and psoas major muscles. These muscles run from inside your hip bone (upper femur) and run upwards to the anterior (front) of the vertebrae of the lumbar spine (for psoas major) and to the inside of the pelvic bone (for iliacus). These muscles essentially come together to form the iliopsoas, which are commonly referred to as your hip flexors, and as such is the primary muscle that lifts your femur upwards to cause hip flexion. But because these muscles also attach to your spine, they essentially are “lumbar flexors”, as contraction of the muscle will also cause the spine to pull forward and down.
As you can imagine, when you are sitting down, the hip flexors are in the shortened position. Over time, tissues that stays in a shortened position will no longer want to lengthen to its original length. This shortened muscle will then pull your spine and hip bones forward causing excessive compression through the spine and hips, decreased extension, joint dysfunction, and ultimately pain. The pain and discomfort are usually felt during other activities such as standing or walking for extended periods, going to and from a sitting position, or even walking up steps.
The key to reducing this pain is to keep the muscle tissues at a healthy resting length where they can allow for full, unrestricted mobility in the hips and spine. This can be achieved by simple stretches. Stretching a muscle lengthens and separates individual muscle fibers, allowing them to return to their natural length thus decreasing the tension pulling on the low back.