After a chicken has been killed and processed, you need to let it sit for 1 to 3 days in the fridge. Just like everything else that dies, it'll get rigor mortise which totally ruins the meat. Rigor passes over a few hours to a day, so you want to make sure you give your processed chicken plenty of time to rest and relax. When you pick it up to prepare it, the joints should be loose and limp, as should the muscles. If it's still stiff, you need to let it rest another day in the fridge.
Hope this helps.