The humeral-ulnar joint allowed for hyperextension and flexion of the forearm. In contrast, on the palmar side, a thick ligament prevents hyperextension. Genu recurvatum is also called knee hyperextension and back knee. The iliofemoral, or Y, ligament is located anteriorly and serves to prevent hip hyperextension. Hyperextension of the knee may be mild, moderate or severe. It is a spinal cord injury caused by hyperextension of the back. McCallum suffered a complete hyperextension of his left knee, almost to a right angle. Hyperextension of the spine should be avoided. They reduce the chance of hyperextension, and increase the agility and strength of the knee. Back injuries such as hyperextension are caused by attempting to re-surface in the opposite direction. Falling causes the weight of the body to force hyperextension. He also noted apparent variations on PD, such as Parkinson's disease with hyperextension. This phenomenon, also known as hyperextension, can give the illusion of throwing.