Swiss Ball Jackknife
Start in a push-up position with your toes on a Swiss ball. Pull your knees towards your chest, while keeping your upper body parallel to the ground. Avoid lifting your hips up as you bring your knees in. Slowly return to the start position.
Do 10 to 15 reps.
Russian Twists
You can do this exercise holding a medicine ball or a weighted plate. Sit on the floor, bend your knees, and lean back about 45 degrees. Hold your arms out straight in front of you. Twist your torso to one side, hold for a second, return to the start position, and twist to the other side. Remember to turn your torso, not your arms, from side to side.
Do 15 to 20 reps.
Kettlebell Windmill
The kettlebell windmill is not only great for your abs, but also good to improve your flexibility—especially if you have tight hamstrings. To begin, hold the kettlebell in your right hand. Lift your right arm up so the kettlebell is over your head. Turn your feet 45 degrees to the left. Bend down and touch the ground with your left hand while you keep the right arm straight overhead. Then return to the start position.
Note: If you can't lower all the way to the ground, go as far down as you can. With practice you'll gain flexibility to reach the ground.
Do about 3 to 5 reps on one side and then switch sides. The goal is to use heavier kettlebells, not do the maximum amount of reps.
More: Your 30-Day Ab Challenge
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