Have you ever busted your buns?

I used to, but not anymore.

I'm always busting my buns and my hump. :P

I've never been what you might call "gifted" in the gluteus maximus area.   Sometimes I have to sit on pillows just so I won't bruise my tailbone.  One time at work I told my employees that we were going to have to "bust our buns" to get a project done before we opened.  One lady looked at me and said, "but you don't have any buns to bust".  All I could do was thank her for noticing.  

That part of me doesn't get as tired as my stamina.   I've busted my lungs I guess.

I've fallen on my a$$ many times. Is that what you mean?