If you want to concentrate on developing and strengthening your core, do this intense core workout three times a week from my new book, Strong Is the New Skinny.

You will do 4 rounds of the following five exercises in this pattern:

1st round: Do each exercise for 60 seconds.
2nd round: Do each exercise for 50 seconds.
3rd round: Do each exercise for 40 seconds.
4th round: Do each exercise for 30 seconds.

(Use your smartphone, a stopwatch, or an egg timer if you don’t have a partner to do the timing for you.)

1. Body Roll-Out

2. Side-to-Side Ball Crunch

3. V-Up

4. Russian Twist

5. Leg Drop

Body Roll-Out

Start with your chest on the stability ball and place your hands on the floor in front of the ball. Walk your hands and your body forward until your toes are the only part of your body on the ball. Then, use your hands to walk yourself back to the starting position. Repeat.

Side-to-Side Ball Crunch

Lie on your back with your knees bent and hold a medicine ball in your hands, extending it above your head toward the ceiling. Exhale as you sit up, contracting your abs and bringing the ball forward and down toward the floor; then, rotate your torso and the ball to the right. Bring your torso back to the center, then lie back on the floor. Next time, sit up and rotate your torso and the ball to the left side, return to center, then lie back on the floor. Repeat the pattern.

THE CRANK-IT-UP CHALLENGE: Perform this move on a stability ball (pictured).

Photo: Jonah Light Photography

V-Up

Lie on your back with your legs together and extended, and arms long and out to your sides at 90-degree angles. While keeping your back flat, use your abs to sit up while drawing your knees in toward your chest. As you sit up, wrap your arms around your knees. Hold for a count of one, then release your body from this position, extending your legs out straight and lowering your torso toward the floor—but without lowering either your legs or your torso completely to the ground. From this position, repeat.

THE CRANK-IT-UP CHALLENGE: Do this move while keeping your legs entirely straight and reach your fingers toward your toes.

Photo: Jonah Light Photography

Russian Twist

Sit with your knees bent and your feet a few inches off the ground. Your torso should be leaning back at a 45-degree angle to the floor. Hold one dumbbell with both hands by using each hand to grasp each end. From this position, rotate as far as you can to the right side and touch the dumbbell to the ground, then rotate to the left side of your body and touch down. Repeat back and forth.

VARIATION: If you are just beginning and need more stability, you can keep your feet on the floor while rotating your torso.

Photo: Jonah Light Photography

Leg Drop

Lie on your back with your legs together and your arms either at your sides with your palms down or under your lower back. Keep your legs straight and lift them to a 90-degree angle so the soles of your feet are facing the ceiling. Slowly lower your legs to the ground, keeping your knees slightly bent and letting your legs hover an inch or so off the ground for a count of one, then raise them back so they’re perpendicular to the floor again. As you repeat this move, keep your abs tight and pulled back so that your spine is flush against the floor.

Photo: Jonah Light Photography

From STRONG IS THE NEW SKINNY by Jennifer Cohen and Stacey Colino. Copyright © 2014 by Jennifer Cohen and Stacey Colino. Reprinted by permission of Harmony Books, an imprint of the Crown Publishing Group, a division of Random House LLC, a Penguin Random House Company.