With a Pilates ball
Stand on top of those big balls that you see in the gym so that you look towards the floor, as if you wanted to crush the ball with your abdomen and hip.
Stretch both legs back, with your feet together and place your hands behind the nape of the neck.
Now raise your chest as much as you can, take it back, without taking your feet off the floor, so that your body forms a diagonal line. Come back slowly so that your head is again in the direction of the ground.
Do 5 series.