Side Plank

Lie on one side and prop up on the forearm directly under the shoulder, stacking the feet (or staggering them for stability). Lift the hips so the body forms a straight line from head to feet and hold, keeping the top hip from sagging. The down-side external-oblique, internal-oblique, and quadratus-lumborum work to resist lateral flexion (anti-lateral-flexion), while the gluteus-medius stabilizes the hip.

Regress by bending the knees (shins down); progress by lifting the top leg or arm, adding hold time, or raising the bottom hip up and down (side-plank dips). A key counterpart to the front plank in the plank-progression, addressing the frontal-plane core that planks miss.

• Side hold on one forearm; body straight; resist the top hip sagging (anti-lateral-flexion).
• Works the obliques, quadratus lumborum, and gluteus medius.
• Regress on the knees; progress by lifting a limb, adding time, or hip dips.
• Trains the frontal-plane core the front plank omits.

• plank-progression — companion hold.
• plank — the front-plane counterpart.
• external-oblique / quadratus-lumborum — prime movers.

See also: Side Plank — in the balance/core-stability track covers the same exercise as one of McGill's "big three," with fuller anti-lateral-flexion coaching cues and progression detail than the calisthenics-track treatment here.