"Pes anserinus" describes the connection of three tendons: the sartorius, gracilis and semitendinosus.

These tendons combine and attach to the knee just below the knee cap. The name means "foot of the goose" and refers to the shape of these tendons, which resembles a webbed foot.

When the bursa (sac) beneath the pes anserinus becomes inflamed, it causes a painful condition called pes anserinus bursitis. This is common, affecting up to 10% of runners. It's more common in women and those who are obese or have diabetes.

People with anserinus bursitis feel pressure or soreness just below the knee cap, on the inner side of the leg. The pain tends to be worse during activity (especially actions with repetitive knee flexing or rising from a seated position) and also at night.

Pes anserinus bursitis is more common among people who:

• Are active in athletics, especially sports that require quick starts, stops or changes in direction, like basketball and tennis
• Have just started an exercise program or have recently increased the intensity of their exercise
• Are female
• Have osteoarthritis of the knee, diabetes or are obese
• Have pes planus (flat foot)

However, pes anserinus bursitis may develop for no apparent reason.

Treatment includes losing excess weight, rest, pain relievers and anti-inflammatory medications.

Physical therapy and/or a cortisone injection into the sore area may be helpful.

As a last resort, surgery to remove the bursa (or a bone spur) may be considered. This is rarely necessary and the overall effectiveness of surgery for pes anserinus bursitis is uncertain.