These ab machines can create lower back problems because they rotate the area beyond a safe range of motion. Your lumbar spine can only twist 13 degrees—that’s less than one hour on a clock. Turning beyond that puts massive strain on your vertebrae, and, combined with the resistance of the machine, can quickly become more than your back can bear. Your lower back is actually designed to resist twisting, not generate it.
To boost your rotational strength, pick core exercises that keep your spine stable and your core rigid.
Use these instead: Side plank Pallof press Anti-rotational cable chop