(A) Facing the stairs, place your hands on the second step with your arms pressed into your sides, legs extended behind you, and core engaged (essentially in a push-up position).
(B) Bring your left knee to your left shoulder, then return your foot to the floor. Alternate legs, bringing your right knee to your right shoulder, then returning your foot to the floor. That’s one rep. Do 12 without pausing. (You’ll feel as if you’re running in place.)