100's are a standard move in Pilates, allowing you to develop strength and endurance in the core.
Do it right: Lie face up and bring legs up, keeping them straight and together, ensuring you have the abs scoop. For a modification, keep the knees bent or leave the feet on the floor. Exhale and lift the head and shoulders off the ground, tucking the chin, and lower the legs as much as you can without arching the back. Begin pumping the hands up and down, inhaling for 5, exhaling for 5 until you hit 100.