For some reason, over the past few months, I have noticed that I can actually voluntarily depress my sternum. Not like pressing with my hands, but actually moving it with my chest muscles. I have talked to several people in my family and friends and they claim they cannot do this. Sometimes there is a cracking sensation and a small sound, but it is never painful. What could cause this? Is this abnormal? I'm really curious. I have never, no my knowledge, experienced any chest trauma of any kind, so I am extremely curious what could be causing this