Page 8, Cardiac Muscle, Dr. D. Penney

Muscle Mechanics (Contraction):

The muscle contraction cycle consists of several parts: a latent period, then contraction, relaxation, and finally the resting period.

Contractions may either be isotonic (of several sorts) or isometric. They can occur as twitches resulting from single stimuli, or as sustained contractions resulting from multiple stimuli. The two types of contractions can be compared in several respects.

In an isometric contraction, tension is generated but no shortening occurs and no load (afterload) is lifted, thus no external work is done (Figure 6). In an isotonic contraction, shortening occurs and a load (afterload) is lifted, and external work is done.

During the latent period there is a slight relaxation, Ca++ is released, and heat is generated.

During an isotonic contraction, the initial contraction is isometric prior to the muscle lifting the load. Following this, the load is moved and the muscle shortens, followed by lengthening during relaxation. Thus the latent period is longer during an isotonic contraction than during an isometric contraction. Because tension is being generated through out and that is the parameter being measured, an isometric twitch is longer than an isotonic twitch.

By increasing the load on a muscle, shortening progressively decreases until eventually no load is lifted and the contraction becomes isometric.

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