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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.

Next: Upper body: Plank jack