I've experienced extreme tightness at the sternum and just below. It is so tight that it restricts breathing. Like the diaphragm is paralyzed. Feels like if you were trying really hard to suck in your stomach except it is doing it on it's own. When it does stop my sternum hurts. The muscles were so tight they strained themselves. Only relief is to sit on the floor, feet on the ground with knees up. rest arms on knees with head on arms. If you were to panic you would probably hyperventilate and pass out. Had my heart checked out and according to stress test etc all is fine.