Why it made the list: In our opinion, ab exercises with added resistance don't get enough love! They spur growth in the fast-twitch fibers like almost nothing else, and they can really build up the "bricks" of your six-pack. By adjusting the load, you can also train to failure at just about any rep target you want. A pin-loaded machine also works well when doing dropsets.
How to use this list: Clayton suggests doing each exercise below for 30 to 45 seconds before every run. “This will fire up your muscles so they’re active when you need them most,” Clayton says. Clayton even demonstrates them herself, so you can nail the perfect form. You will need a mat and a set of sliders. Two hand towels or paper plates will work. too.

If the basic plank is too easy, after 60 seconds, add a few arm and leg lifts to the next 60 seconds. Every 15 seconds, alternate lifting one arm out in front of you while maintaining your posture for 10 seconds and repeating on the other side, and then switch to the legs. Lift the toes 5-10 inches off the floor and hold it for 15 seconds, and repeat with the other leg.
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