Sit-ups from a suspended position are difficult, but will build a strong core.
Strengthening your core isn't just about building a rock-hard six pack. A strong core is important because those muscles in your midriff -- from your hip flexors up through your abs and including your back -- stabilize your body and are the transfer station that moves energy between your lower and upper body. Sit-ups and crunches work well for beginners, but those who are more physically fit will get an additional challenge from more difficult exercises.
A Drastically Distinctive Sit-Up
The same thing that makes the sit-up and press exercise one of the most difficult core exercises is also the feature that makes it so effective: it's done on a Roman chair, so you don't have support from the floor to release tension at the bottom of the move. Sit on a Roman chair with your ankles secured under the top padded bar. Hold a barbell or dumbbells at chest-height and lean back, lowering your torso until it's almost in a straight line with your legs. Sit up in a controlled move then press the weight up overhead, military-press-style. Bring the weight back to chest-height, lean back and repeat. Do three sets of eight to 10 reps each. While some exercises focus on one or two core muscle groups, this exercise will work your ab muscles, hip flexors, shoulders and back.
Balance Exercise Blasts Your Core
One of the most difficult core exercises works your back while helping to improve balance, another important function of a strong core. To perform single leg dumbbell rows, start in a standing, upright position holding a dumbbell in your right hand. Bend both your knees slightly and lift your right foot off the floor, then bend forward at your hips to about 45 degrees, keeping your back straight and allowing the dumbbell to hang down just below knee-height. Bend your elbow to pull the dumbbell up to bring it next to your chest, then lower it back to the starting position. Perform three sets of eight to 10 reps on each side.
Slide Into Your Pushups
Stack.com borrowed the slide board pushups exercise from NBA's Dwight Howard to put on their list of the most hard core exercises. To perform this challenging version of a pushup get in a plank position as if you were going to do a typical pushup, but your hands should each be resting on a sweat towel and there should be a slick board beneath the towels. Instead of bending both elbows to lower your body, you'll slide your right hand forward as far as you can while keeping your left hand in place. Your left elbow should bend to lower your body and allow your right hand to slide. Bring your right hand back to the starting position as you extend your left elbow to raise your body back up, then slide your left hand forward while bending your right elbow. Continue to alternate sliding your hands forward, counting a right and left slide as one rep. Complete three sets of eight to 12 reps.
Split squats are typically done to work your quadriceps, but when you hold onto the sides of a cable machine you can get your core muscles in on the workout, too. The 2011 book, "Surf Survival" by Andrew Nathanson, Clayton Everline and Mark Renneker lists it as one of the most difficult core exercises because it works on your legs, your core and balance all in one shot. Stand on the outside of a cable machine, with a bench behind you, and hold onto the frame to help keep your balance. Your arms should be extended so that you're standing arm's-length away from the machine. Bend your right knee and place the top of your right foot on the bench behind you. Bend your left knee, coming down into a squat until your upper left leg is almost parallel with the ground. At the same time, exert pressure on the cable machine frame as if you were trying to drive it down and to the right. This is the part of the exercise that recruits your core. Keep your back straight and hold your abs taut throughout the exercise. Extend your left knee to come back to the starting position. Do three sets of 12 to 15 reps before switching legs and repeating the exercise with your right leg, but driving down and to the left when you apply pressure to the machine frame.
Basic planks are difficult enough, as are one-arm pushups. Combining the two exercises for one core-punishing move makes the pushup superman with alternating arms one of the most difficult core exercises. To perform them, start off in a basic plank position with your arms extended. Lift your left arm and right leg off the ground and hold for two seconds, then return to the staring position. For the next rep you'll lift your right arm and left leg, holding for another two seconds. Continue alternating lifting your opposite arm and leg, completing a total of three sets of 10 to 12 reps for each side.