The leg will bend backwards when viewed from the side, there may be several conditions as follows: 1. It may be caused by too much laxity in the posterior cruciate ligament of the knee joint. This condition is often seen in women who have been practicing ballet for a long time, and when their lower limbs are straightened, they will see excessive backward displacement of the lower leg when viewed from the side. This is due to the laxity of the posterior cruciate ligament, which is not able to limit the backward displacement of the upper end of the lower leg, followed by this deformity. In addition, if there is an old posterior cruciate ligament injury, this condition may also occur.2. It may be due to congenital hereditary factors or acquired malnutrition, resulting in upper calf deformity.3. There may be bilateral hip dislocation. If the child has bilateral hip dislocation, when he or she walks upright, he or she will often find that the lower limbs are bent backwards excessively.