Anybody know where does this crack come from?

nckhrs

Hi,

When I put both of my hands behind my back, and then I push my head against my hands, I heard this crack sound (and of course, a relief). This doesn't happened every time, but only when I am tired (I can only do that 2 - 3 times a day).

I can also lay down on my bed and does that.

So, I would like to know:

Where the crack comes from? Is it my upper back? Neck/spine joint? Bottom neck?
Is it muscle sound or joint sound? Which muscles got stretched?

Thanks a lot!