The Relationship Between Isometric Tension and Muscle Length

At this point, it is worth considering how the magnitude of the isometric tension developed by a muscle depends on the muscle length at which the tension is measured; this will allow us to relate the tension of the muscle as a whole to the microscopic contractile apparatus within each muscle fiber and to the sliding filament hypothesis discussed in Chapter 10. Suppose the experiment diagrammed in Figure 11-2a were repeated at a variety of lengths of the muscle, as set by varying the distance between the upper support bar and the weight. We would find that as the muscle is stretched beyond its normal resting length, the tension developed upon stimulating the muscle would fall off rapidly, falling to zero at about 175% ofresting length. This is shown in Figure 11-3a, in which peak isometric tension is plotted against muscle length (expressed as a percentage of the resting length of the unstimulated muscle). Such behavior can be easily understood in terms of the underlying state of the thick and

Physiological range I*-rt.

Physiological range I*-rt.

50 100 150

Muscle length (% of resting length)

Thick filament Thin filament

Z line

Thick filament Thin filament

Z line

Figure 11-3 The relationship between muscle length and strength of isometric tension. (a) The graph shows the relation between the isometric tension produced when a muscle is stimulated, expressed as a percentage of the maximum observed, and the length of the muscle at the time it is stimulated, expressed as a percentage of resting length. The shaded area shows the range of muscle length over which the muscle would actually operate in the body. (b) The diagrams show the states of the thick and thin filaments within a sarcomere at each of the three numbered positions marked in (a).

thin filaments making up each sarcomere. As the distance between Z lines increases, the degree of overlap between thick and thin filaments declines, and thus the number of myosin head groups available to form cross-bridges is reduced. Finally, with sufficient stretch, there is no overlap, as shown in Figure ll-3b (number 3), and there can be no tension developed.

If the muscle is artificially shortened, isometric tension is also reduced, falling to zero at about 50% of resting length. This effect occurs as the distance between successive Z lines becomes sufficiently short that there is overlap between the thin filaments attached to neighboring Z lines. This overlap distorts the necessary spatial relation between thin and thick filaments required for cross-bridge attachments to form, so that once again there are fewer cross-bridges available to develop tension, as shown in Figure ll-3b (number l). In addition to this geometric effect, other factors (such as reduced coupling between depolarization of the membrane and release of calcium from the sarcoplasmic reticulum) might contribute to the reduced peak tension at short muscle lengths. The fall-off of tension with both increasing and decreasing length means that there is an optimal range of length for development of tension; in this optimal range, there is maximal overlap of thin filaments with the cross-bridges of the thick filament (Figure ll-3b, number 2). If a muscle is to operate at maximum efficiency, the range of length over which it is required to develop force when in actual use in the body should be close to this optimal range. This is indeed the case; as a skeletal muscle operates, its length remains within about 30% of the optimal length (the shaded region in Figure ll-3a). In order to ensure that this range is not exceeded, precise arrangements of muscle-fiber length, tendon length and attachment sites, and joint geometry have evolved that are appropriate for the functional task of each muscle.

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