Lay back onto a tennis ball, position it under the knot and get to work. A chiropractor can manipulate (pop) the joint where the rib meets the spine. Frequently, one of the ribs has stopped moving normally and the muscles around it cramp up. Sometimes a less qualified person can pop your back by picking you up in a bear hug, pressing down to the side of the spine, or even walking lightly on it. The trick is not to hold your breath, but to let them force your breath out. For prevention do thoracic extension mobility exercises, such as on a foam roller, then add seated rows, face pulls, and external rotator exercises.