The Sit Up is one of the most basic body weight exercises that will help in your quest for a strong core. When performing the sit up, you can either brace your feet or perform them free standing. Either way are a challenge, but when you brace your feet your quadriceps and hip flexors come into play much more as they aid in pulling you up. Make sure to focus the work on the abdominal as much as possible. Go slow with this exercise as it is much more effective than a set of sloppy high reps.
Step 1: Brace your feet under something heavy or fixed to the ground.
Step 2: Place your hands either behind your head or on your chest.
Step 3: Slowly come up and hold for a second, return to start and repeat.
Tips and Safety: Imagine folding one vertebrae at a time as you rise. This will help you isolate your abs as much as possible. To make this even more of a challenge try slow and controlled sit ups. Count from 5-10 seconds on the way up, hold, and then the same on the way down.