Plank

The plank is the foundational core exercise: instead of moving, you hold your body in a straight line on your forearms and toes and simply refuse to let it sag. That makes it an anti-extension hold — the core braces hard to keep your spine from bending under gravity, while your shoulders and glutes chip in to keep the line rigid. It trains the deep, stabilizing job the core actually does in real life far better than crunches do, building the kind of endurance and bracing strength that protects your back and steadies every other lift. It needs no equipment at all and scales in both directions: drop to your knees to make it easier, or add time, elevation, or load to make it brutal. The things to watch are simple — wrists or elbows can complain on long holds, and letting your hips sag quietly cheats the whole point. Squeeze your glutes, brace your abs, and hold a true straight line rather than chasing the clock. As a core foundation, it's essential.