There are two main causes of hip impingement:
A problem with the ball at the top of the femur. If the head isn't shaped normally, the abnormal part of the head can jam in the socket when the hip is bent. This may happen during activities such as riding a bicycle or tying your shoes.
A problem with the socket (pincer impingement). If the front rim of the socket (called the acetabulum) sticks out too far, the area of the thigh bone (femur) just below the ball may bump into the rim of the socket during normal movement.