Hold hands across your chest to make crunches slightly easier.
When an exercise is considered single-joint, or isolation, it means that it involves movement only at one joint. During biceps curls, for example, only your elbow joints are moving. However, lat pulldowns require movement at the shoulders and elbows and is thus not considered a single-joint activity. When you're performing an abdominal crunch, the only movement occurs at your spine and therefore it is considered a single-joint, or isolation, exercise.
The abdominal crunch is most commonly performed on an exercise mat and ideally with your knees bent and feet elevated onto a bench or chair. With your hands behind your head or arms across your chest, flex your spine forward by lifting up your shoulder blades a couple inches off the floor. Lower your shoulders back to the mat and then go into the next rep. You always want to do crunches when your muscles are warm, so either fit them in at the end of your workout or do light cardio for five to 10-minutes before beginning.
Because abdominal crunches are an isolation exercise, they don't require assistance from many muscle groups. The primary muscle, which handles the greatest percentage of the load, is your rectus abdominis, which is the largest muscle in your abs. The rectus abdominis runs over the front of your torso, from your pelvis to your sternum, so that when it contracts, it's able to flex your spine forward against resistance. Also helping out are your obliques, which are most commonly known for their torso twisting and side-bending capabilities, but they also assist with spinal flexion.
Difference Between Sit-ups
Although sit-ups are similar to crunches, they require movement at multiple joints and thus are considered a compound exercise. Not only is the spine flexing forward, requiring work from the rectus abdominis and obliques, but to rise up all the way until your torso reaches your thighs requires your hip joint to perform flexion. As a result, a collection of hip flexor muscles, including the iliopsoas, tensor fasciae latae, rectus femoris and sartorius, are recruited to handle the hip flexion element of the exercise.
Benefits of Isolation Exercises
Isolation exercises are ideal for more specifically targeting development in certain muscle groups. If you're interested in building strength in your abs because you'd like to improve your posture, or are trying to build size and tone in your abs, crunches would be more effective to incorporate into your workouts than sit-ups. Because crunches will more quickly fatigue your abs and obliques, they should be scheduled at the end of your workout so the muscles aren't tired when they're needed to assist during compound exercises.