A medicine ball is weighted, and I don't think it would work for what you have in mind (because it's too firm and wouldn't give in to pressure much).
The benderball seems to be made out of vinyl or some other "rubbery" plastic, but a heavy foam ball would work much the same as it would have similar "give."
When I first saw these on tv, they were quite expensive and looked remarkably like the cheap, but sturdy brightly colored vinyl balls that Walmart, K-B Toys, and sometimes even dollar stores sell in huge bins as children's toys. I've looked at the construction, and it seems to me that they would be sturdy enough for most people to use, and if they would break, it would likely be at the seal, and air would leak out slowly enough that injury wouldn't be likely, but that's only my opinion. I have no proof of that, and the benderball is now fairly inexpensive but at the time I very much considered buying one of the $2 Walmart balls to try it at home (this was before I injured my back and floor workouts were feasible).
Last edited by kaplods; 02-26-2008 at 06:04 PM.