Day 18 takes the single leg plank down onto the forearms. The circuit is a straight-arm plank hold, an elbow plank hold, and then a raised leg elbow plank on each side - 30 seconds each. The first minute is familiar; the second minute combines the two things that make a plank difficult and runs them at the same time.
Lifting a leg in an elbow plank stacks two challenges on top of each other. The forearms already lengthen the lever and drop the hips closer to the floor, which puts more of the load on the abs than a straight-arm plank does. Take one foot off the ground and you also remove half the base, so the pelvis immediately wants to rotate toward the empty corner - and the muscles that stop it rotating are working hard while the rest of the core is already fully committed to holding the line. The two opening planks are there to set your standard before the harder pair arrives, not to warm you up.
Elbows under the shoulders, forearms pressed flat, glutes squeezed and hips level in all four positions. In the raised leg holds, lift the leg to roughly hip height with the knee straight and the foot flexed, and keep both hip bones pointing at the floor - if one starts turning up toward the ceiling, lower the leg until it stops. Breathe through all four; bracing without breathing is how a 30-second hold turns into a 20-second one.
Three points of contact instead of four, and your core has to fund the difference.
Make it Harder: Point the toes of the lifted leg and hold it above hip height in both single leg panels without letting the hips tilt.
Make it Easier: Hold a standard elbow plank with both feet down for the final two panels.
