Isometric Push-Up

Find soft grass or ground, and lie facedown, hands by your shoulders, palms flat on the ground. Push your body up in a diagonal line (arms and body straight, body weight balanced on your knees and hands). Bend your elbows slightly to lower an inch or two, and hold the position for 10 seconds; work your way up to 60 seconds as you gain strength. Keep your stomach muscles tightened throughout the exercise.  Make it harder: Try it on your toes instead of your knees.