Why it's on the list: To recruit the obliques, you need to do one of three motions: 

  • Bend to the side  
  • Rotate your trunk  
  • Suck in your belly

By adding a medicine ball to the twist, you're hitting your obliques with a double whammy by requiring the upper abs to contract isometrically to stabilize against the weight as you move. Try to get a little crunch on either side after the rotation to up the ante.

Dumbbell Russian twist gif

Twist Variations for Ab Development:  

In your workout: Do this exercise toward the middle of your routine for 3 sets of around 15 reps per side. Increase the degree of difficulty by extending your arms out or using heavier weight.

Ab Workout Abs