The plank is the most fundamental core exercise and trains the lower back and abs isometrically. Core strength transfers to every other lift and protects the spine.
Technique
1Elbows under your shoulders, forearms on the floor
2Body in a straight line — do not pike your hips or let them sag
3Brace everything: squeeze your glutes, tighten your abs, pull your elbows toward your toes
4Breathe normally — do not hold your breath
5Progressions: raise one arm or leg, or use an unstable surface