Stability Ball Plank

The stability ball plank places the feet on an exercise ball while the forearms stay on the floor, and the instability underneath changes what the hold demands. A floor plank asks the abdominals to resist gravity pulling the hips down; add a ball and they must also resist the small, constant corrections the ball introduces in every direction. The obliques and the deep stabilisers work continuously rather than holding one static line, which is closer to how the trunk behaves in real movement. It is a genuine progression rather than a novelty, and it should only follow a solid floor plank. The set ends when the hips start to sag or hitch upward, not when the timer says so, because a plank held with a collapsed lower back trains the opposite of what it was meant to.