Isotonic contractions

Fig. 9.3 shows a common arrangement for recording isotonic contractions. The after-load stop serves to support the load when the muscle is relaxed and to determine the initial length of the muscle. If it were not there the muscle would take up longer initial lengths with heavier loads, which would make it more difficult to interpret the results of experiments with different loads.

Fig. 9.7a shows what happens when the muscle has to lift a moderate load while being stimulated tetanically. The tension in the muscle starts to rise soon after the first stimulus, but it takes some time to reach a value sufficient to lift the load, so that there is no shortening at first and the muscle is contracting isometrically. Eventually the tension becomes equal to the load and so the muscle begins to shorten; the tension remains constant during this time and the muscle contracts isotonically. It is noticeable that initially the velocity of shortening during the isotonic phase is constant, provided that the muscle was initially at a length near to its maximum length in the body. As the muscle shortens further, however, its velocity of shortening falls until eventually it can shorten no further and shortening ceases. When the period of stimulation a

Length

Tension _

Stimulus

J -High load

Fig. 9.7. After-loaded isotonic tetanic contractions. a shows the length and tension changes during a single contraction, with shortening as an upward deflection of the length trace. b shows the initial length changes in contractions against different loads.

ends, the muscle is extended by the load as it relaxes until the lever meets the after-load stop, after which relaxation becomes isometric and the tension in the muscle falls to its resting level.

If we repeat this procedure with different loads (as in Fig. 9.7b), we find that the contractions are affected in three ways.

1 The delay between the stimulus and the onset of shortening is longer with heavier loads. This is because the muscle takes longer to reach the tension required to lift the load.

2 The total amount of shortening decreases with increasing load. This is because the isometric tension falls at shorter lengths (Fig. 9.6) and so the more heavily loaded muscle can only shorten by a smaller amount before its isometric tension becomes equal to the load. Fig. 9.8 illustrates this point.

Fig. 9.8. Diagram showing why it is that a lightly loaded muscle can shorten further than a heavily loaded one. Starting from point a on the length axis, the muscle contracts isometrically until its tension is equal to the load it has to lift, and then it shortens until it meets the isometric length-tension curve. With a heavy load (P1) this occurs at x, with a lighter load (P2) it occurs at y. Notice that point x can also be reached by an isometric contraction from point b. (When starting from a much extended length, a muscle may in practice stop short of point x when lifting load P1; this is probably caused by inequalities in the muscle.).

Fig. 9.8. Diagram showing why it is that a lightly loaded muscle can shorten further than a heavily loaded one. Starting from point a on the length axis, the muscle contracts isometrically until its tension is equal to the load it has to lift, and then it shortens until it meets the isometric length-tension curve. With a heavy load (P1) this occurs at x, with a lighter load (P2) it occurs at y. Notice that point x can also be reached by an isometric contraction from point b. (When starting from a much extended length, a muscle may in practice stop short of point x when lifting load P1; this is probably caused by inequalities in the muscle.).

3 During the constant—velocity section of the isotonic contraction, the velocity of shortening decreases with increasing load. It becomes zero when the load equals the maximum tension which can be reached during an isometric contraction of the muscle at that length.

Notice that the first two of these observations are essentially predictable from what we already know about isometric contractions. We can quantify the third observation by plotting the initial velocity of shortening against the load to give a force—velocity curve, as in Fig. 9.9. The curve is more or less hyperbolic in shape, and is believed by many physiologists to be of fundamental importance in the understanding of muscle functioning.

In 1938 A. V. Hill produced an equation to describe the form of the force—velocity curve, as follows:

(P + a) (V+ b) = constant where Vis the velocity of shortening, Pis the force exerted by the muscle, P0 is the isometric tension, and a and b are constants.

Fig. 9.9. The force-velocity curve of a frog sartorius muscle at 0°C. From Hill (1938).
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