Dropped Foot, or "foot drop," describes the loss of the ability to raise the foot at the ankle, which leads to difficulty in walking. The cause is usually injury to the peroneal nerve, which supplies the muscle responsible for lifting the foot. Injury to the nerve can occur anywhere from the spine all the way down to the knee. Surgical treatment is often useful in improving dropped foot, depending on the circumstances.

Source: drnathfootdrop.com