Types Of Muscle Contraction

Muscle does two types of work: dynamic and static. In dynamic work, the origin and insertion of a muscle are affected by changes in muscle length. If the muscular force causes the origin and insertion to move toward each other, or if the muscle is shortened and contracted, the dynamic work is concentric. If the muscle force is exerted while the

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Figure 5-2 Different shapes of the skeletal muscles.

origin and insertion are receding from each other, the muscle is working eccentrically; that is, it tries to shorten but is actually lengthened by external forces when it tries to halt a movement in a joint. In static (isometric) work, a muscle contracts without any movement taking place in the joint. The muscle can achieve its maximal speed or movement when working in the best possible conditions. Muscular performance is greatly reduced in the presence of any pathologic condition, including those that the person may not consciously realize, such as symptoms of overtraining.

Whenever muscles contract, whether the contraction is concentric, eccentric, or isometric, the force generated must be graded to meet the needs of the activity. The amount of force produced depends on the number and type of motor units activated, the frequency of stimulation of each motor unit, the size of the muscle, the muscle fiber length, and the muscle's speed of contraction.

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