I'm not sure what exactly it would be, but I can say this:
Champ has awful hips, just terrible. He's been on glucosamine supplements since he was just a year old because they're that bad. I did notice a difference when I put him on the commercial supplements, he seems less stiff, doesnt' walk awkward, and in general seems a little more comfortable.
I did some reading in search of a more natural supplement, and read about chicken feet having really high glucosamine content, and have since quit with the commercial supplement and I give him a few chicken feet through the week... I can honestly say it's working for him. Dare I say he's doing better than on the commercial tablets.
ETA: chicken feet really gross me out, and I hate watching him eat them, but he loves them.. which is good because I was tired of shoving pills down his throat all the time.