Ass. My entire day has revolved around hauling, busting or just plain sitting on it. Did some agency work from 8:30 AM to 10:30 AM. Then did the newspaper tango from 11:30 AM to 8 PM. Then came home and did some more agency work, then came upstairs to put the finishing touches on my friend's site. And now it is 1:30 AM.
Note that all of this exhausting work was done entirely with my left hand. The ass itself was not actually involved in any real labor, unless you consider the act of slowly expanding 'labor.'