The sit-up is a core exercise that strengthens the core muscles on the anterior (front) side of your body, mainly the rectus abdominis muscle. This is one of the most basic and effective exercises for developing a 6 pack abs. To perform this exercise correctly, simply lay flat on the ground with your knees bent. Lock your arms across your chest to prevent them from swinging and creating momentum. Bring your chest all the way to your thighs. As you do this, exhale and squeeze your stomach muscles. Unless you are doing a timed workout you want to go slow and focus on the contractions.