Bruce Lee took advantage of his time by training his incredible abs unnoticed while sitting in boring meetings. He isometrically contracted his abs by pressing his lower back into a chair. There was no apparent movement, but his rectus abdominis muscles were receiving a secret, awesome workout.
Your torso is a vital area for you to tone and strengthen if you want to increase your power. Torso exercises stabilize the spine, protecting you from injury. Abdominal muscles allow your torso to turn, twist, and bend. Your waist connects your upper and lower body to generate the tremendous torque necessary for dynamic punching and kicking. Solid abdominals are essential to make this connection. Strong core muscles also enhance your balance, protecting your internal organs.
The set of abdominal muscles referred to as the "6-pack" is the rectus abdominis, and in reality it is a "10-pack" - go ahead, count them. This muscle group originates on the pubic bone. The insertion (end of the muscle that is attached to the body part to be moved) is on the cartilage of ribs 5 through 7 and the xiphoid process (the lower tip of the breastbone). The rectus abdominis is a straplike muscle designed for smooth, long movement. Its main purpose is to raise your body from bed each morning. The simple crunch trains this muscle group by flexing the trunk forward.
For perfect crunches, begin each repetition as if you were in slow motion. Contract your rectus abdominis, exhaling as you let your muscles pull your shoulder blades up off the floor. Exhaling on each repetition allows you to squeeze your abs without arching your back. Inhale on the return to prepare you for the next contraction; expand into the neutral spine position without relaxing fully, keeping the abs active.
Beware of infomercial abdominal machines. Many of these products make false claims, use only a one-dimensial approach to ab training, are cheaply made, and are not really properly aligned, tested, or designed for safe, repeated use. To firm up your abs free of charge (well, we hope you paid for the book), lie on your back with your knees bent, your chin resting forward toward your chest. Curl your head and shoulders off the floor (without momentum or the use of a crane) upward and forward until your shoulder blades leave the floor. Follow this progression: Tilt, curl, flex for two seconds, then uncurl, untilt. Focus on flexing your rectus abdominis. The range of motion is only a few inches. It should feel as if you are working your upper abs more because the top of your rectus abdominis is thinner than the layer toward the pubis. Perform 10 repetitions. Use your rectus abdominis muscles to raise your body, not your head and neck. If crunches are too difficult, raise yourself off the floor with your arms and perform just the down phase of the crunch.
You can modify your arm position to change the degree of difficulty of crunches. The least resistance occurs when your arms are straight and outstretched along the sides of your body. Level 2 of difficulty is to cross your arms over your chest. Level 3 is elbows bent, fingertips to your ears, or arms extended up; either arm variation adds more resistance to the crunch.
To perform a reverse crunch, lie on your back with your knees flexed to your chest. Place your hands under your hips. Keep your knees together as you bring your feet toward the floor without touching it. Hold for three seconds, then slowly draw your knees back to your chest. It should feel as if you are working the lower part of your rectus abdominis because your hip flexor muscles (iliopsoas) are assisting.
When you attempt to train your abs, the hip flexors (iliopsoas), which are more powerful muscles, do most of the work. This is the main reason that, when done incorrectly, crunches (or anything for that matter) can be a disappointing waste of time. Even when you perform a crunch correctly, your rectus abdominis begins the movement but your hip flexors cannot help but become involved, especially if you attempt to perform crunches quickly with nonmuscular momentum. By raising the torso slowly and coming up only part of the way (full sit-ups have been known to be a big no-no for some time now), you can target your rectus abdominis instead of your hip flexors.
If you anchor your feet under a sofa, table, or gym bar or have a partner hold your feet, you work mostly the hip flexors because you naturally pull against the anchor with your legs. This diminishes the role of the abs in the activity. With your feet anchored, your back may arch, which strains the quadratus lumborum (lower-back muscles).
This is an excerpt from Athletic Abs.