The crunch has dozens of variations, such as the reverse crunch, twist crunch, incline crunch and weighted crunch. Which muscles activate during the crunch depends on the style you choose to perform. When creating your ab routine, choose abdominal exercises that work the abs from different angles to ensure you activate all the abdominal muscles.
The rectus abdominis, which is responsible for spinal flexion -- pulling your ribs toward your pelvic bone -- is the primary mover during a standard crunch. This is one solid muscle sheath separated vertically and horizontally by tendinous tissue, which gives the muscle its "six-pack" look, which can be seen on lean individuals. The rectus abdominis originates on the crest of the pubic bone, attaching to the ribs and lower part of the sternum.
Lower Vs. Upper Abs
The rectus abdominis acts as one muscle. Whether you perform a crunch or a reverse crunch, the entire muscle activates, not just the top or bottom half. However, during a standard crunch, when you pull your torso off the floor, the upper portion of the rectus abdominis contracts slightly more than the lower portion. During a reverse crunch, when you pull your knees in toward your chest and leave your upper body stationary, the lower portion activates slightly more than the upper portion.
The obliques, the muscles on each side of the rectus abdominis, act as synergists during the standard crunch. These muscles assist in the movement, but are not primary movers. The obliques flex and rotate the spine. If you add a twist to the crunch, pointing your shoulder toward the opposite knee as you curl up, the obliques are activated more effectively. As you twist to the right, your left external oblique and your right internal oblique activate. As you twist to the left, your right external oblique and your left internal oblique activate.
To activate the proper muscles, you must perform the crunch correctly. The crunch has a short range of motion. The abdominal muscles are primarily active during the first 30 to 45 degrees of spinal flexion. A full situp involves other muscles as the primary movers, mainly the hip flexors. For a crunch, lift your head and shoulder blades off the floor but keep your low back in contact with the floor. Keep your neck in neutral alignment with your spine. Look at the ceiling and don't pull your neck forward. Perform the crunch in a slow and controlled manner, exhaling as you curl up and inhaling as you lower back to the floor. Train your abs two or three times per week. Choose three to five exercises, completing one or two sets of eight to 12 repetitions per exercise.
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