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On p. 284, the book "Body Sculpting Bible for Men" talks about a form of crunches that can be used to train the abdominal muscles. The first step for the instructions for these crunches says:

Start by isometrically contracting the abdominal muscles.

What does it mean to "isometrically contract" a muscle or set of muscles?

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There are two major types of muscle contractions - Isometric and Isotonic.

Isotonic - This is the "typical" type of muscle contraction, in that it produces movement. The muscle contracts and shortens, and causes movement, such as bringing the hand to the shoulder for a bicep curl.

Isometric - This is a contraction where the muscle does not shorten, usually because both ends are fixed. If you think of the abdominals, this is what you would do if someone was about to hit you in the stomach, you would just clench up the abdomen.

  • Is exhaling an isometric action? – BKE Jan 20 '15 at 16:55
  • No, exhalation is the relaxation phase of the diaphragm. Nothing is contracting (Although you can control the rate of relaxation, or it will be handled automatically). Generally, inhalation is an isotonic contraction, the diaphragm contracts causing a vacuum which in turn expands the lungs and causes inspiration. – JohnP Jan 20 '15 at 17:10
  • Sorry, I did not mean resting exhalation, but forced exhalation. Does it count as isometric? Note, there are at least three ways the abdominals can be tensed: 1. forced exhalation using rectus abdominis, obliques, transversus abdominis 2. forced exhalation 'as if' exhaling, but holding the air in (opposing with the diaphragm) 3. stabilizing the spine as in the plank, when the abdominal muscles are tense, but breating in and out is still possible. I am not sure which counts as isometric (and which is meant in the book mentioned by the OP)? – BKE Jan 20 '15 at 22:29
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    @BKE - Forced exhalation would, but not because of the diaphragm. In that case, you are contracting intracostal (rib) muscles, abs, etc. causing the ribcage and abdomen to compress. The diaphragm just relaxes. I'm not familiar with the book of the OP, but isometric contractions have been recommended to help build both strength and muscle density in many different regimens. – JohnP Jan 20 '15 at 22:46
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In the context of your question, it means flexing the abs so that the muscles contract without actually changing the angle or the length of the muscle. By starting the movement with an isometric contraction, you ensure that that the abs will get the maximum benefit from the exercise. Isometric contractions are typically used in rehab settings, but, can also be used for general fitness. In fact, bodybuilders use isometrics when they practice posing from a competition. It helps them to hold their poses for extend periods of time without relaxing the muscles.

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