Take a step back and think about where you spend most of your day. If you're a young athlete, you probably spend most of your time at school or maybe work or practice and even a little time at home, if you're lucky. Now think about what position your body is in during those periods. I would bet that you spend most of your day sitting down. You may walk to class or run in practice, but the majority of your day is spent in a seated position.
Spondylolisthesis. This condition occurs when one vertebra slips over the adjacent one. There are 5 types of spondylolisthesis but the most common are secondary to a defect or fracture of the pars (between the facet joints) or mechanical instability of the facet joints (degenerative). The pain can be caused by instability (back) or compression of the nerves (leg).
Avascular necrosis (also called osteonecrosis). This condition happens when blood flow to the hip bone slows and the bone tissue dies. Although it can affect other bones, avascular necrosis most often happens in the hip. It can be caused by a hip fracture or dislocation, or from the long-term use of high-dose steroids (such as prednisone), among other causes.