Decline Crunches vs. Decline Sit-Ups

Your abdominal muscles are very resistant to fatigue, according to Dr. Len Kravitz of the University of New Mexico. They are used constantly throughout the day to get up out of bed, sit up straight and carry bags of groceries.

Some people train abs just two or three times per week, while others train them everyday. You can increase the challenge by doing sit-ups and crunches on a decline bench, but you need to know what you're working so you can make the best choice.

Both decline sit-ups and decline crunches are performed on a bench with your head lower than your hips and legs. The deeper the incline, the harder the exercise.

Set the bench at your desired angle, starting with just a slight decline if you are new to the exercises. Secure your legs against the pad, holding your body in place by hooking your feet under the pad. For the crunch, cross your arms over your chest. Exhale and curl your head, shoulders and upper back away from the bench, contracting your abs. Inhale and release for one full rep.

To do a full sit-up, start in the same position with your arms crossed over your chest. Exhale and curl away from the bench all the way up, bringing your torso completely off the bench. Try to touch your elbows to your thighs. Inhale and lower slowly down for one complete rep.

Read more: The 41 Hardest Ab Exercises

These two exercises target different muscles in your midsection. The primary mover in the decline crunch is the rectus abdominus, or the six pack muscle. This muscle runs from the bottom portion of your rib cage down to your pubic bone. It's job is to flex, or forward bend, the spine.

In the full sit-up the target muscle is the iliopsoas. This muscle lies deeper than the rectus abdominus. It runs from your hip and lower portion of the spine down past your hip onto the femur. It's job is hip flexion and spinal rotation.

Read more: 21 Sit-Up Variations You Won't Totally Hate

No one muscle acts alone in the body, so although decline sit-ups and crunches have a primary mover, they also have assisting muscles. When you are performing a decline crunch your obliques act as a synergist, helping the rectus abdominus curl the spine.

In a sit-up the synergists are hip and thigh muscles like the rectus femoris and adductor muscles. The rectus abdominus and obliques act as spinal stabilizers in a sit-up. This means that they are contracted and held tight so that you can perform the motion at your hip joints.

If your goal is to build a six pack and define your abs, then the decline crunch is the better choice. However, with abdominal training you should include flexion, extension, side bending, rotation and stabilization. The decline sit-ups aid in spinal stabilization so you can add these as well to a full abdominal training program.

If you are new to ab training, start with sessions two or three times per week, and as you get stronger you can do them every day. Choose at least five to eight exercises and do them back to back in a circuit one to two times through. Do eight to 12 reps of each exercise, or enough to fatigue the muscle.