Bent-Elbow Plank: This exercise works the whole trunk, particularly the transversus abdominis. Start by lying on your belly and then lift yourself up onto your toes and forearms (elbows in line with shoulders) while contracting your abdominals and keeping your back neutral. Hold that position for five seconds, then rest and repeat. Ultimately, strive to hold the pose for 90 seconds without any rest -- for one set. If you're more experienced, you can also do this exercise on your hands and toes. (As a beginner, start on your hands and knees with a neutral spine and simply contract the abdominals on an exhale without moving your back.)

Why it made the list: It turns out the ball is good for more than just sitting and waiting for your partner to finish his set! A research team from California State, Sacramento demonstrated that the pike movement is one of the most effective total-ab workouts.[3] It topped their EMG list for upper abs, lower abs, and obliques. This movement may be the heavy hitter that's been missing from your daily routine. While not exactly the same, the pike can also be done using a TRX system with similar results expected.

A regular pushup works your core. A reverse pushup will work it so hard your abs will want to form a labor union. “When your legs fire your body forward, your core has to work extremely hard to decelerate your body,” says B.J. Gaddour, C.S.C.S., director of Men's Health StreamFIT. So this ab exercise not only works your shoulders and arms, but gives you a killer core workout at the same time.
"This exercise is so effective because it teaches you not to rely on your bigger muscle groups (like your thighs) and focuses on using your abdominals correctly in a controlled manner without using momentum. The use of the ball gives you natural feedback of your weaker side so that you can adjust and work on symmetry of your musculature, preventing future injuries."
The inspiration for this exercise might come from skiing, but it's also an effective way to prepare your midsection for many sports, including tennis, softball, and golf. The reason: "It trains your abs, lower back, and hips to work together to rotate your body from side to side," says Durkin. And the more powerfully you can rotate, the faster you can swing and the harder you can throw.
For training, you need to set a serious pace for when you hit the gym. Standing around your overloaded squat bar that you were going to do quarter reps on every five minutes won’t cut it. Start serious volume short-rest training by laying a smackdown on your muscles. Building mass comes at the price of getting lean, so maintenance and permanent pump will be the strategy—the results will be worth it. For training you’re going to do 4 exercises at 4 sets and 12 reps minimum per body part approach. It’s encouraged to do 5 or even 6 sets, and if you’re not struggling with those, then go further. When it comes to abs, slow and steady wins the race. I know it sounds cliché, but large range-of-motion reps with added weight for your 12 reps will produce far deeper cuts than doing 50 crappy situps. Give yourself at least a 4-count per rep on your abs.
A University of Southern Maine study found that a single set of a weight-training exercise torches as many calories as running at a 6-minute-mile pace for the same amount of time. So for every second you spend lifting weights, your body is expending high amounts of energy. Add high intensity interval training (HIIT) principles to your workout, and you could see even more gains.
To do it: Stand with your feet planted 3-4 feet apart, toes slightly turned out, hands on your hips. Lower into a plie by bending your knees out over your toes and lowering your hips directly underneath your shoulders. Then, as your straighten back up, slowly lift the right knee up towards the right shoulder. As you go back into the plie, slowly return the foot to the floor. Be sure to move at a very controlled pace to really engage the obliques the entire time—on the way up and on the way down. Do 10 repetitions on the right side, then another 10 repetitions on the left side for a total of 20. 

best ways to build core