Forearms on the ball, hold the wobble, brace hard.
Fight the wobble by bracing harder, keep your hips from dropping or piking.
Putting your forearms on a ball changes the plank from a static hold into a constant negotiation, your core has to correct every tiny roll before it becomes a big one, so the deep trunk muscles never switch off. The straight-line demands are the same as a floor plank, but the unstable surface rewrites the difficulty completely. It is the natural next step once floor planks stretch past comfortable minutes without stretching your abs.
Shorter than your floor plank, and that is the point. Start with twenty to thirty second holds done well, and add time only while your hips stay level and the shaking stays small.
Some shake is normal, it is your stabilisers firing rapidly against the wobble. If the shake turns into your hips sagging or the ball drifting, end the set, quality holds beat long ugly ones.
One where your elbows sit roughly under your shoulders with your body in a straight line, for most people that is a 55 to 65 cm ball. Too big tips you upright, too small drops your chest and loads the shoulders.
