Whenever one muscle contracts to move a bone, another muscle MUST relax. Reciprocal action of muscles provides the ability to move the bone in opposite directions. It also makes the movement smooth, preventing jerky, or irregular action. If you see movement that does not look smooth, it is most likely an indication of a muscle that can not release. If you read an equine anatomy book, this concept is also discussed as agonist-antagonist muscles. Stress point therapy is specifically designed to release muscles that have failed to release themselves.