Hamstring Muscle Shortening
When we sit down, the hamstrings are pulled and maintained in that position for as long
as we are sitting, which could be hours. Although the hamstring muscles are inactive,
they are held at a shortened length. This is a probable cause of tight hamstrings. Tight
hamstrings usually start at the age of 5 or 6 when children start their seated school
careers and are associated with back pain. The reason is that tight hamstrings stop the
hips from flexing during forward bending. Remember the hamstrings connect to the hip
bone above and to the thigh and leg bones below. If these muscles remain tight and do
not relax, it forces the lower back to bend beyond its strong middle range. Rounded
shoulders and a hunched (forward curved) back due to shortened hamstrings are
typically seen in those who sit for long hours.
Overworking of the erector spinae muscle group
Chair sitting is unique in that the gluteus maximi (buttock muscles) are totally relaxed at
the same time as having an upright torso, and they are therefore not able to contribute
to extension of the lower back and stabilization as they usually do. It is the erector
spinae muscles alone that are holding up the back without the help of the gluteal
muscles. Prolonged sitting tires out the erector spinae muscles which give up the
struggle to maintain the correct "hollow" in the lumbar spine. This again contributes to
the forward curved lower back and a stooping posture when the person stands up.
Iliopsoas Muscle Strain
Assuming you are using a chair whose backrest slopes backwards or is too far behind,
your iliopsoas muscles must continuously pull your body forward to stop you falling
backward. If you are sitting for a long time, the iliopsoas muscle has to remain
contracted for that period of time. That is a sure-fire recipe for iliopsoas muscle
shortening. Ten to fifteen percent of back pain is due to a tight iliopsoas muscle.