Spondylolysis is the most common cause of lower back pain in adolescent athletes. It is a vertebral stress fracture, most commonly occurring in the L4 or L5 vertebrae. If the stress fracture weakens the bone so much that it becomes unstable and the vertebra starts to slip out of place, then the condition is known as spondylolisthesis. In addition to sports injuries, spondylolisthesis occurs in women over the age of 40 as a result of degenerative disk disease. While sports injuries often cause the injury, it is thought that there may be a genetic predisposition to being susceptible to the stress fracture. The pain associated with these conditions usually feels like muscle strain of the lower back. In addition, spondylolisthesis can also cause back and hamstring muscle spasms that could affect posture. The doctor may need to perform X-rays, MRI, or CT scans to make a definitive diagnosis.