Exercise

Obliques
Lie on your back with your arms at a 45° angle to the floor and your hands behind your head. Bring your right elbow towards your left knee, flexing your leg and keeping your right shoulder firmy on the floor. 

Breathe out to do the exercise and breathe in to return to your original position.

Variation
Keeping your left leg pointed upwards to the ceiling, bring your left hand towards your right foot.

How many?
Beginner: 3 sets of 4 reps with 30 second recovery 
Intermediate: 4 sets of 12 reps with 30 second recovery
Advanced: 4 sets of 15 reps with 30 second recovery