Joints form the meeting point between two or more bones. At the hip joint, the femur meets the pelvis. In a standing position, the femur hangs directly down from the pelvis. This position is extension of the hip. When you take a step, you lift your thigh and move your leg in front of your body, decreasing the angle between the femur and the pelvis. This action flexes the hip joint. Hyperextension works in the opposite way, with the femur moving past the straight, extended position to move behind the body.
Muscles That Extend the Hip
To move the thigh from a position of flexion to extension, you use your hip extensors. Located on the back side of the body, these muscles include the gluteus maximus as well as the upper fibers of the hamstrings. These same muscles move the thigh past extension into hyperextension. Weakness in these muscles will prevent full extension of the hip.
Muscles That Must Stretch to Extend the Hip
As the muscles on the back of the hip and upper thigh contract to extend the hip, the muscles on the front of the hip must stretch. The primary muscle group that must stretch is the iliopsoas, a powerful hip flexor. A tight iliopsoas prevents extension of the hip and pulls the top of the pelvis forward. A tipped pelvis places a large amount of strain on the lumbar vertebrae and can cause low back pain and inefficient movement.
Restrictions in the Structure of the Hip
In addition to weak hip extensors and tight hip flexors, there are several ligaments that restrict hip hyperextension. Your hip has three external ligaments: the iliofemoral ligament, the pubofemoral ligament and the ischiofemoral ligament. One of the primary functions of all three of these ligaments, but particularly the iliofemoral ligament, is to prevent excessive hyperextension of the hip.
Degrees of Hyperextension
Because of the ligaments of the hip, the normal range of hyperextension is from zero to 15 degrees. When the leg moves past this degree of hyperextension, most of the movement comes from the lumbar spine, not from the hip itself. To neutralize this tendency, when therapists assess the degree of hip hyperextension, they stabilize the lower back so that all of the movement comes from the hip.