Tendons are tough bands of connective tissue made up mostly of a rigid protein called collagen. Tendons firmly attach each end of a muscle to a bone. They are often located within sheaths, which are lubricated to allow the tendons to move without friction.
Bursae are small fluid-filled sacs that can lie under a tendon, cushioning the tendon and protecting it from injury. Bursae also provide extra cushioning to adjacent structures that otherwise might rub against each other, causing wear and tear—for example, between a bone and a ligament or a bony prominence and overlying skin (such as in the elbow, kneecap, or shoulder area).