A bursa is a small, fluid-filled sac that acts as a cushion between a bone and other moving parts, such as muscles, tendons, or skin. Bursitis occurs when a bursa becomes inflamed. People get bursitis by overusing a joint. It can also be caused by an injury. It usually occurs at the knee or elbow. Kneeling or leaning your elbows on a hard surface for a long time can make bursitis start. Doing the same kinds of movements every day or putting stress on joints increases your risk.