The most basic exercise is the abdominal crunch performed on the floor. The exercise is performed on a flat, stable surface without additional resistance. With less than a full range of motion, the exercise requires minimal flexibility and is a great exercise for someone just starting out.
Once you have become more advanced throughout your training, the next step in your progression is to increase range of motion and decrease stability. Bending forward as you perform a crunch is an example of spinal flexion. The spine also bends backward in what is commonly referred to as rear extension. To return from full rear extension, you need to again call on the abdominals to contract from a stretched position. This requires more mechanical work than on a flat surface, due to the greater range of motion. It also activates the stretch flex, eliciting a stronger contraction. This phenomenon is similar to firing a rubber band across the room. The farther you stretch the rubber band, the farther it will fly through the air. The unstable surface forces greater activation of core muscles to maintain proper positioning. By involving more muscles, you will burn a greater number of calories.
