An old running friend said it was most likely a "Baker's cyst"?

He first described the condition now known as Baker's cyst.

This usually makes the pain and swelling of a Baker's cyst go away.

Baker's cysts in children do not point to underlying joint disease.

In rare cases, a Baker's cyst is removed by surgery.

Baker's cysts usually require no treatment unless they are symptomatic.

Many activities can put strain on the knee, and cause pain in the case of Baker's cyst.

Baker's cysts can also be rarely associated with Lyme disease.

Knee pain during running could be caused by a 'Baker's cyst'

Ice pack therapy may sometimes be effective way of controlling the pain caused by Baker's cyst.

A Baker's cyst is a pocket of fluid that forms a lump behind the knee.

Arthritis is the most common cause in adults, but a Baker's cyst also can be caused by a knee injury.

Baker's cyst: Collection of fluid in the back of the knee.

If arthritis or another problem is causing the Baker's cyst, your doctor may treat that problem.

A baker's cyst is easier to see from behind with the patient standing with knees fully extended.

Often a Baker's cyst causes no pain.

Baker's cysts usually develop from a persistent effusion as in conditions such as arthritis.

Baker's cysts in children often involve the bursa, a small sac of fluid behind the knee.

This bulge full of synovial fluid is what is known as the 'Baker's cyst'.

Baker's cysts in children, unlike in older people, nearly always disappear with time, and rarely require excision.

How is a Baker's cyst diagnosed?

So I would advise you to first get a diagnosis to make sure that you are dealing with a Baker's cyst.

Recently, prolotherapy has shown encouraging results as an effective way to treat Baker's cysts and other types of musculoskeletal conditions.

Baker's cyst: formation of abnormal synovial cysts in connection with joints.

Baker's cysts may form and rupture.