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 agonist) contracts, the opposite muscle (the
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 muscle's
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 muscle 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.
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 spine as we walk.
Neutralization prevents the spine from reaching large degrees of
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