This isometric exercise doesn't look like much—which makes it great for when you want to do a little toning at your desk. "You'll also feel it in your lower abs," says Day from Figure 4, especially if you keep your back straight throughout the sequence.
1. Sit up straight in your chair with a neutral spine and extend your legs in front of you.
2. Place your palms on either side of your body, flat on the seat of your chair.
3. Squeeze your legs together, then lift them so your feet hover one inch off the ground and your thighs are lifting slightly off your chair.
4. Bring your legs 2 to 3 inches apart, then tap legs together 15 times.
5. Release and lower your legs.
6. Reps: 5.