Hi Katia! What specific ab work are you feeling in your lower back? Are you an advanced exerciser or fairly new to ab training? Many times it is advanced ab work where the legs are extended away from the torso (and/or suspended in air too) that creates greater stresses to the lower back. If your low back bothers you during these type of movements keep your knees bent on the floor rather than lifting and extending them out. Let me know if you have further questions. Take Care!