Hi, I've been doing the exercise in this video: http://www.youtube.com/watch?v=iuvFgyHILyY. Basically, it works on your core by raising your shoulders while in a plank position. The thing is, when I lift my right shoulder (I'm right handed) there is a pretty loud crack. The volume of the popping slowly diminishes as I continue, but whenever I start, the crack is pretty damn audible. I've read that the noise is generated from bubbles bursting at my joints. So would that mean this is a good thing, and is similar to releasing stress? Should aim to crack all the joints involved with tennis?