After carrying my 20-pound baby over my arm, I started having terrible shoulder pain. The doctor thinks I may have rubbed the tendon of my rotator cuff against the bone enough to tear the tendon. What is the rotator cuff, and what does it do?

The rotator cuff is made up of four muscles and their tendons that surround the top rim of the shoulder. These muscles are important for smooth and coordinated use of the upper arm. They help hold the upper arm bone in the socket while still allowing the arm to move overhead.