Check that your chair and desk are well adjusted:

Adjust the seat height so that your feet rest flat on the floor. Make sure that there is no pressure on the back of the knees. Adjust the desk height so that your wrists can rest comfortably on it. Your forearms should be level with the desk top and your upper and forearms should form a 90 degree angle or slightly more.

Tip: If you do not have an adjustable chair or desk, try using a lumbar support cushion, a cushion on the seat or a footrest.