Forearms on the ball, feet on the dome, double the wobble.
Keep ribs down and hips level, the wobble exposes any sag instantly.
With forearms on a ball and feet on a balance trainer, both ends of your plank are unstable, so the core cannot lean on a single fixed anchor and has to co-ordinate corrections at the shoulders and hips simultaneously. Every ripple in one surface echoes through to the other, and your trunk is the only thing keeping the line straight. This is end-stage plank progression territory, for when a single unstable surface no longer bites.
Only if the standard stability ball plank has stopped challenging you. Doubling the instability is a progression tool, not a starting point, and a shaky version of this is worth less than a strict version of the simpler plank.
Set your forearms on the ball first, then walk your feet back onto the trainer one at a time. Take your time getting set, the entry is half the exercise the first few sessions.
Your whole trunk, ribs to hips, firing continuously. If you mostly feel your shoulders burning, the ball is too far forward, pull your elbows back under you and re-stack the position.
