Plank on a ball, then roll forward.
Keep your hips level, the ball exposes any sag so stay braced head to heel.
A plank with your forearms on a stability ball is a different animal to one on the floor, because the ball wobbles and your core has to cancel every twitch. Rolling the ball forward by reaching your arms out lengthens the lever, spiking the demand on your abs to resist your hips sagging, then pulling it back loads them again. It is a genuine progression for anyone whose floor planks have gone stale, building the stiff, braced trunk that carries over to heavy lifts.
Two reasons. The ball constantly shifts, so your core fires nonstop to keep you still, and rolling it forward extends your body like a mini rollout, which multiplies the load on your abs.
Only as far as you can keep a dead straight line from head to heels. The moment your hips sag or your lower back takes over, you have gone past your current range, shorten the reach and build up.
Almost always a lost brace, the hips drop and the lower back catches the load. Tuck your ribs down, squeeze your glutes, shrink the forward roll, and if it still aches go back to a standard plank first.
