The membrane of the bursa is semi-permeable, allowing some materials to flow across the membrane into and out of the sac. An injury to your bursa may cause it to fill with blood or white blood cells.

The Running Doc notes that all Achilles injuries, such as the one suffered in 2010 by soccer star David Beckham, aren’t the same. What looks like Achilles tendonitis, for instance, may actually ...

The retrocalcaneal bursa lies between the Achilles tendon and the calcaneus (heel bone). With repeated trauma the bursa can become inflammed. This injury is often mistaken for Achilles tendinitis.

And that seems to be working. “We were treating the Achilles and the Achilles actually is fine,” O’Hara said today. “It’s actually more like a bursitis that we’re dealing with.