Whether you're looking to burn fat or build muscle, doing sit ups really isn't the smartest idea. I know a fair few people who are in great shape, men and women, and they rarely, if at all do sit ups.You see, sit ups burn hardly any calories, they might help to 'tone' muscle, but the days of sit ups are gone. Athletes and fitness enthusiasts much prefer exercises like leg raises, hanging knee raises, hanging leg raises, planks etc. Most people don't know that other exercises that are not predominantly aimed at the core, actually build up the core. Squatting actually hits your core a lot, but people just think it hits your legs.
If you want to burn fat, just stop doing sit ups. As mentioned, they burn hardly any calories, so why bother? To get great abs, you need to work your core but get rid of any excess body fat, and sit ups are not the way to achieve that. I'm sure you have seen people in the gym who are overweight and performing sit ups, thinking they are doing an awesome workout, I know I have.
To build up your core muscles, you need to place them under tension (like any other muscle). That's why doing 20 leg raises are a lot harder than doing 20 sit ups, and that's because sit ups just don't really do that much. So stop wasting your time doing sit ups and start performing leg raises, hanging leg raises, hanging knee raises etc.
