The correct formula for building a strong core is not always the same for every person. While you may want to train your abdominals twice a day, every day, more may not always give you the results you seek. Abdominal muscles need rest just like the other muscles of your body, so proceed with caution.
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Four muscles are located around your midsection. The rectus abdominis is long and flat and extends from your rib cage down the middle of your abdomen to your pelvis. If you have a small amount of body fat, you see the RA has a washboard appearance. You have a pair of obliques: the internal and external. The obliques are located on the sides of your abdomen and aid in rotating or twisting movements. The deepest layer of abdominal muscles is the transversus abdominis which acts like a corset around your torso.
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According to Dr. Peter Francis at San Diego State University, abdominal exercises should be performed for five minutes a day. The core is exercised best when you use a variety of movements including sit-ups, crunches, stability ball crunches, captain's chair lifts and reverse crunches. The more variety you use on your abdominals, the better the muscles respond.
The reason you are able to exercise your abdominals on a daily basis is due to the abs never reaching complete muscular fatigue. The abdominal movements use the weight of your body and not heavy amounts of weight for training. The core is used to moving your body weight throughout the day and does not reach complete failure when you perform abdominal exercises. Therefore, it doesn't require as much rest to repair. You can exercise the abs twice a day if you feel the muscles have not reached complete failure. You know when they do if you are unable to perform another repetition.
Take a Break
Abdominal muscle tissue does break down when you train it. The muscle fibers separate which require rest to repair and rebuild. Your training goals are important, but if you see your exercise technique suffering because of weak fibers, take a day off from training. You will return stronger and able to complete your next abdominal training session.