Goosefoot (pes anserine) bursitis of the knee: Inflammation of the pes anserine bursa, a small lubricating sac located on the inside of the knee, about 2 inches below the joint, between the shinbone and the three tendons of the hamstrings muscle. Symptoms include pain and tenderness on the inside of the knee, below the joint, that increases with exercise or climbing stairs. Usually develops as a result of constant friction or stress on the bursa; can be a result of osteoarthritis of the knee, incorrect training techniques, tight hamstring muscles, obesity, or an out-turning of the knee or lower leg.

Source: American Association of Orthopaedic Surgeons