The No-Weights Workout: Planks
Credit: Photograph by John Loomis

Squeeze your glutes and stomach. Press elbows to the floor to activate shoulder muscles.

Make it Hard
From push-up position, walk your hands until your arms are nearly straight. Plank on fingertips and toes.

Master Class
From push-up position, raise one arm and the opposite leg to parallel with your back. Hold for one minute.