I have never used a stability ball with my clients as a foundation to exert force off of. I am saying not a great environment to build much of anything. It may make a great base to sit on and work at your computer, however to do a chest press, or plank on…not a fan!
Its challenging enough to maintain a perfect neutral posture in a plank. To put our forearms or elbows (that’s what I see) is a neural mess. When we have to many internal forces trying to overcome external application its a S____ storm of who’s going to hang in there the longest. The same time we feel it in our low backs, and shoulder blades don’t we? I mention the plank because I see so many trainers creating earth quakes with their clients as the say “Yea wow I feel it” The stability ball makes a great chair, it really does…that I am serious.