Even if you don’t have an ounce of fat, you could be soft in the middle. Try this test of core strength, courtesy of Gray Cook, author of Movement: Functional Movement Systems.
1. Ask a friend to watch you. Lie face down on the floor and place your hands shoulder-width apart, with your thumbs in line with the top of your forehead.
2. Then lift your elbows off the floor and position your feet the way they would be in a push-up, with your ankles flexed.
3. Now push yourself off the floor while maintaining a stiff plank position. If your hips lag behind your torso, your core is weak.
4. Try again, this time with your hands at chin level. If your hips lag this time, it shows that your core needs even more work.