To do it: Lie on your back with your hands interlaced behind head, knees bent ,and feet hip-width apart on the floor. Inhale and lift your chest towards your knees, bringing your shoulders and head off the floor, maintaining a neutral pelvis (keeping it parallel to the floor). Exhale and rotate to the right side, and then exhale again rotating even further, lifting a little higher. Next, inhale and lift as you return to the center and repeat to the other side. Do 8-10 reps per side.

We put this on the list because of how easy it is to manipulate the degree of difficulty. If a regular plank is too easy for you, lift an arm, or a leg—or an arm and a leg. Put your feet into a TRX and give that a whirl. Still too easy? Take your feet out, and put your forearms in. Each one of these progressions leads to a greater training stimulus to the abs.
To do it: Lie on your stomach with your upper body propped up on your elbows and a dumbbell upright on the floor about six inches in front of your chest. Tuck your toes under and lift your body into a full elbow plank, making a straight line from your head to your heels, drawing in your abdominals. To start the move, lift your right arm off the floor and use your best freestyle stroke to reach over and past the dumbbell—as far as you can—allowing your hips to rotate into the stroke and turning through the balls of your feet. Finish the full stroke before returning to elbow plank position. Repeat with the left arm. That's one rep. Do 10 reps total, alternating sides each time.
To do it: Kneel with your elbows bent under your shoulders on top of a stability ball. Draw your abs in tight, keep your weight in your arms (chest lifted off the ball), and extend both legs out straight behind you, feet about hip-width apart. Maintain a straight line from your head, shoulders, hips, knees, and feet. Once you are stabilized on the ball, slowly roll the ball away from your body to increase the lever length and add stress on the abdominal region. For safety, go slowly and start with short movements in and out for 10-15 reps. When you feel ready, you can progress how far away you reach and your number of reps, Richey says.

We all want a solid core for different reasons: sports performance, pain prevention, that finish line photo—but strengthening your midsection is particularly important for runners. That’s because your core is the stabilizing center of your body (it keeps you standing, least of all sprinting), and it can make or break your speed goals, prevent (or contribute to!) an injury, and yes, make you feel pretty badass in a sports bra.
As the kettlebell shifts from hand to hand in front of your body, your core has to stabilize to fight against the movement of your arms, and your biceps, shoulders, and back muscles need to work together to control the pace at which you are catching and releasing the kettlebell. Your lower-body will get a workout, too. You can perform this abs exercise with a bent waist to hammer your hamstrings and glutes or try it in a squat position to target your quadriceps. No matter which way you try it, you'll see bigger strength gains and a smaller waistline in less time. 

To do it: Stand with your feet hip-width apart and pressed firmly into the floor and hold a medicine ball (or other similar weighted object). Brace your abs in tight (as if preparing for someone to punch your stomach) and use your lower body to start the movement by bending your knees, sitting back into your hips, and reaching the ball down across the outside of your left leg. Stand up, swinging your arms across your body and up to the right while pressing your hips forward. Do 10-12 reps going from left hip to right shoulder, and then repeat on the other side.
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