Medicine ball knee drops are a great way to target the obliques as well as the rectus abdominis and the back. The key is to use your abs to control your legs as you lower them down and then contract them to pull them back up.
The range of motion of this exercise is small. Only lower your legs as far as you comfortably can — your hips should keep in contact with the mat.
You can also try this move without a medicine ball or with a rolled up towel under the hips for more support.
Target Muscles: internal and external obliques, rectus abdominis
Set Up: Lie faceup on a mat with your legs bent and lifted as shown. Squeeze a medicine ball between your knees and extend your arms out to the sides.
Action: Slowly lower both knees to one side, pause, then return to the center and repeat on the other side; that’s one rep. Do three sets of 10 to 12 reps on each side.
Tip: If you have back issues, do it without the ball, or skip this move altogether.