What is bursitis?
A bursa is a potential space located between two anatomical structures, such as bone and tendon. Bursa act like a small cushion designed to reduce compression and help provide friction-free motion between adjacent structures.
The heel has two bursae (see images below); one located superficial to the Achilles, known as the subcutaneous bursa and one located deep to the bursa, the retro-calcaneal bursa. The subcutaneous bursa is located between the skin and the Achilles tendon. The retro-calcaneal bursa is located between the Achilles tendon and the calcaneum (heel bone).
Either bursa can become inflamed due to excessive friction and compression. This is known as a bursitis and is often due to overuse or unaccustomed activity. For example, while training for a marathon, increasing your walking distance too quickly or wearing new shoes. The bursa is a very pain-sensitive structure, and when it becomes inflamed can be very debilitating.