Walk your hands out from the dome into a plank and back.
Keep your hips level as you walk out, do not let them sag at full extension.
Starting from standing, walking your hands out along the floor stretches your body into an ever-longer lever, and your core has to hold the line harder with every hand step away from the dome. The pause at full plank is the peak, the position where sagging hips have the most leverage against you. It earns a place as core work that trains the hardest part of the plank, getting into and out of full extension under control.
Your hands start on the dome as you hinge down, which gives you a raised, slightly unstable launch point before you walk out along the floor to full plank and back.
Walk out only as far as your hips stay level, then walk back. Your distance is a moving target, extend it a hand-step at a time as your core gets stronger.
Some shoulder work is unavoidable since your hands carry you, but the core is the target. If your lower back aches instead, you are sagging at full extension, shorten the walk.
