The No-Weights Workout: Mountain Climbers
Credit: Photograph by John Loomis

From push-up position, bring one knee to your chest and back; repeat with the other knee. Try to do 25 on each side.

Make it Hard
Lift your knee up and across your chest, toward your opposite shoulder. This works more of your oblique muscles.

Master Class
From push-up position, with your feet flat against a wall, do mountain climbers.