Rotational crunches are not the best exercise for your obliques, a Westfield State University study found. Classic crunches work the muscles on the sides of your torso twice as hard as bringing your elbow to your opposite knee.

"The rotational crunch doesn't produce enough force or speed to maximise muscle activation," says study author Christopher Paul, DC, CSCS. But don't do crunches on your back. Instead, target your obliques with the medicine-ball slam. Hold a medicine ball overhead, with your knees slightly bent. Reach back and then slam the ball to the floor in front of you without rounding your lower back. Complete three sets of 15 repetitions