The Mechanics of Contraction

When the nerve controlling a muscle is stimulated, the resulting action potentials in the muscle fibers set up the sliding interaction between the filaments of the individual myofibrils in the muscle. This sliding generates a force that tends to make the muscle fibers, and therefore the muscle as a whole, shorten. Whether or not the muscle actually shortens, however, depends on the load attached to the muscle. While we might attempt to order the muscles in our arms to lift an automobile, it is unlikely that the muscles would be able to shorten against such a load. The force developed in an activated muscle is called the muscle tension, and only if the tension is great enough to equal the weight of the load will the muscle shorten and lift the load.

We can distinguish between two kinds of response to activation of a muscle. If the muscle tension is less than the load, the contraction is said to be isometric ("same length") because the length of the muscle does not change even though the tension increases. That is, the force exerted on the load by the muscle is not sufficient to move the load, so the muscle cannot shorten. An isometric contraction is diagrammed in Figure 11-2a. In the figure, an isolated muscle

Figure 11-1 Schematic illustration of the innervation of a small number of muscle fibers in a muscle. The shaded muscle fibers form part of the motor unit of motor neuron I and the unshaded fibers form part of the motor unit of motor neuron 2.

To other muscle fibers

To other muscle fibers

To other muscle fibers

Figure 11-2

Measurements of muscle length and muscle tension during (a) isometric and (b) isotonic contractions. At the upward arrow, the nerve innervating the muscle is stimulated, causing activation of the muscle fibers.

Figure 11-2

Measurements of muscle length and muscle tension during (a) isometric and (b) isotonic contractions. At the upward arrow, the nerve innervating the muscle is stimulated, causing activation of the muscle fibers.

is attached to a load it cannot lift. When the muscle is activated, the resulting tension is registered by a strain gauge that measures the miniscule flexing of the rigid strut to which the muscle is attached. A single activation of the muscle triggers a transient increase in tension lasting typically about 0.1 sec. You can easily feel the tension developed in an isometric contraction by placing your palms together with your arms flexed in front of your chest and pushing with both hands, one against the other.

If the tension is great enough to overcome the weight of the load, the contraction is said to be isotonic ("same tension") because the tension remains constant once it reaches the level necessary to move the load. This situation is diagrammed in Figure 11-2B. The strain gauge again records the increase in tension, as with the isometric contraction. When the tension reaches the level necessary to lift the load, it levels off and the muscle begins to shorten as the load is lifted. During the change in muscle length, the tension remains constant and equal to the weight of the load. This is because it is this weight hanging from the muscle and support strut that determines the flexing measured by the strain gauge. Thus, while the muscle is changing length the contraction is isotonic. In an isotonic contraction, the force developed by the sliding filaments in the myofibrils making up the muscle produces work in the form of moving the load through space.

One difference between isometric and isotonic contractions can be seen in the different delays between muscle activation and the occurrence of a measurable change in either muscle tension (isometric) or muscle length (isotonic). The tension begins to rise within a few milliseconds, the time required for the effect of the excitation-contraction process discussed in Chapter 10 to take hold. However, if muscle length is measured instead there is a pronounced delay between activation of the muscle and beginning ofshortening. This delay is the time required for the tension to rise to the point where the load is lifted, which will depend on the size of the load. Thus, with light loads the shortening begins quickly, but with heavier loads the onset of shortening is progressively delayed. Finally, with sufficiently heavy loads there is no shortening at all and the contraction becomes isometric. In addition, with heavier loads the duration of shortening will be less and the maximum speed of shortening will be slower. In a sense, the measurement of tension during an isometric contraction gives a more direct view of the contractile state of the muscle; for this reason, subsequent examples in this chapter will be of isometric contractions.

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