The human body contains more than 600 muscles. Skeletal muscle contains contractile units that have the ability to convert chemical energy into mechanical energy, thus enabling the muscle to contract or shorten. Muscles cannot independently lengthen. They can lengthen only by contracting the opposing muscles. When one muscle (the agonists) contracts, the opposite muscle (the antagonist) lengthens.

In order for muscles to contract at all, they must be attached to the bones. Strong, fibrous tissues located at each end of the muscle, called tendons, accomplish this. The attachment of the muscle at the proximal end of the bone (the end closer to the body) is considered the muscles' origin. The attachment at the distal end of the bone (the end farther from the body) is referred to as the muscle's insertion. The origin of the trunk muscles are always at the upper or superior attachment, while the insertion is found at the lower or inferior attachment.

Muscles have different contraction capacities and therefore can play different roles depending on the desired movement. A muscle can contract concentrically while it shortens or pulls. This typically results in a movement in which the muscles acts as a primary mover.

A muscle can also contract concentrically in cooperation with other muscles. The synergistic effort results in a movement that the muscle would not have been able to perform completely on its own. In this situation, the muscle would be considered a synergist or an assistor. On the other hand, a muscle may also contract eccentrically while it lengthens, Even though it is elongating, the muscle can still be contracting enough to help slow the movement. In this case, the muscle acts like a brake to control the body as it moves.

Yet another possibility is that a muscle could be contracting isometrically while it neither shortens nor lengthens. A muscle may utilize varying degrees of these isometric contractions in order to stabilize the body and certain joints during an exercise. The muscle would be working as a stabilizer in this situation.

A muscle may also contract to prevent an undesired effect of another contracting muscle. It would then be acting as a neutralizer. For example, the abdominal muscles neutralize part of the effects of the hip extensors and erector spinae as we walk. Neutralization prevents the spine from reaching large degrees of hyperextension.

As you learn the various movements the human body is capable of performing, the roles and responsibilities of each muscle during each joint motion will become clearer. If you are a personal trainer, fitness instructor, or coach, it is extremely important that you know this information in order to more competently teach and train your clients and athletes.

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