Start in a side plank with left forearm on floor, elbow under shoulder, feet stacked, and hips lifted so body forms one long, straight line. Stretch right arm up to ceiling. This is your starting position. Draw the right hand down and reach it below left underarm as you curl upper body forward so shoulders are parallel to floor. Return to starting position. Repeat for 30 seconds on each side.
Why it made the list: Decline-bench crunches amp up the challenge by increasing the range of motion over standard crunches, and you can dial up (or down) the degree of difficulty by adjusting the angle of the bench. Adding a medicine ball or weight plate against your chest adds a further level of customizable resistance. This also allows you to manipulate where you want to fail: low, medium, or high reps.
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.
When it comes to working the core, many people focus on abdominal muscles, and neglect their obliques (or what you may know as “side abs”). But, says Barrett, these clandestine muscles are just as important: they “keep everything tucked in.” To get a good oblique workout in, head to the pulley machine and start doing some Paloff presses. If you don’t know how to perform the exercise, read our comprehensive guide on mastering the move.
Eat carbs. "There's this notion that carbs are bad and that you need crazy amounts of protein to be lean and fit," says Alcantara, who disagrees, and eats just as much carbs as protein, although the ideal ratio varies based on your goals. "Whatever you eat to get the results you want has to be sustainable, otherwise you're going to end up right back where you started with the same habits that got you there." 

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