3. Roman chair sit-ups belong to the group of abdominal exercises that are completely worthless. During this exercise, your thighs should be parallel with the ground and your upper body should lower itself down backwards at a 90-degree angle. However, doing roman chair sit-ups can seriously damage your body. Instead, you should do regular sit-ups and they will help you achieve your goals faster. According to many fitness experts, regular sit-ups are still one of the best ways to work on your core and abs.