Jackknife
 

Side jackknife

Lie on your side with your right leg on top of your left. Put your right hand behind your head, elbow flared out, and the other on the floor in front of you for balance. Contract your obliques to bring your right leg and elbow together, then slowly lower. Perform all your reps on one side before swapping.

Top tip: If you’re new to this move it could fatigue your hip flexors before you feel any benefit in your obliques. Foam roll the problem area after every session to loosen the tendons and help you up the reps.