Boat pose is one of the best yoga poses you can carry out to strengthen your core,and here's a variation to form it even harder. Begin sitting on your mat. Lift your knees up to your chest, balancing with your shins parallel to the floor.
Keep your spine long and your chest tall, and holding for one inhale and one exhale.
Straighten your legs,and hold for one inhale and one exhale.
This counts as one rep.
Repeat for a total of 20 reps.
Source: popsugar.com