The 5-Minute Ultimate Core Workout
Upper body: Push-up alternating knee tap
Start in "up" part of push-up, hands directly under shoulders, body
straight. Engage core; bend elbows to lower body toward floor. Press
back up; bring left knee to outside of left elbow; pause for 2
seconds, then return to starting position. Do another push-up, bring
right knee to right elbow, return to start; that's 1 rep.
Make it easier: Drop knees to floor. Continue for 20 to 30 seconds.