Get into a side plank, propped on your forearm with your elbow under your shoulder, hips lifted and body in a straight line. Extend your top arm straight up toward the ceiling, then rotate your chest toward the floor and thread that arm through the gap under your torso. Unwind back to the open position with the arm pointing up again. Your hips are the referee - they stay high through every rotation, and the moment they dip, the rep didn't count.

Works: obliques, abs, shoulders, glutes

Breathing: Exhale as you rotate and thread the arm under, inhale as you open back up.

Common Mistakes: Letting the hips drop as the arm threads through, rotating from the shoulders only while the waist stays frozen, and rushing the reps until the movement turns into a wobble.

Make Easier: Bend your knees and rotate from the knees-down side plank, or build up with Side Bridges until the straight-line hold feels solid.

Make Harder: Slow each rotation to a 3-count, hold the reach for a 2-count at the top, or stack your feet to shrink the base.