Length Tension Relation

The springlike characteristics of the protein titin, which is attached to the Z line at one end and the thick filaments at the other, as described earlier, is responsible for most of the passive elastic properties of relaxed muscles. With increased stretch, the passive tension in a relaxed fiber increases, not from active cross-bridge movements but from elongation of the titin filaments. If the stretched fiber is released, its length will return to an equilibrium length, much like releasing a stretched rubber band. The critical point for this section is that, on top of this increased passive tension due to stretching, the amount of active tension developed by a muscle fiber during contraction, and thus its strength, can be altered by changing the length of the fiber before contraction. One can stretch a muscle fiber to various lengths and measure the magnitude of the active tension generated in response to stimulation at each length (Figure 11-25). The length at which the fiber develops the greatest isometric active tension is termed the optimal length, lo.

Muscle Fiber Length

40 60 80 100 120 140 160

Percent of muscle length

FIGURE 11-25

Variation in active isometric tetanic tension with muscle-fiber length. The blue band represents the range of length changes that can occur physiologically in the body while muscles are attached to bones.

40 60 80 100 120 140 160

Percent of muscle length

FIGURE 11-25

Variation in active isometric tetanic tension with muscle-fiber length. The blue band represents the range of length changes that can occur physiologically in the body while muscles are attached to bones.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

When a muscle fiber length is 60 percent of lo, the fiber develops no tension when stimulated. As length is increased from this point, the isometric tension at each length is increased up to a maximum at lo. Further lengthening leads to a drop in tension. At lengths of 175 percent lo or beyond, the fiber develops no tension when stimulated.

When all the skeletal muscles in the body are relaxed, the lengths of most fibers are near lo and thus near the optimal lengths for force generation. The length of a relaxed fiber can be altered by the load on the muscle or the contraction of other muscles that stretch the relaxed fibers, but the extent to which the relaxed length can be changed is limited by the muscle's attachments to bones. It rarely exceeds a 30 percent change from lo and is often much less. Over this range of lengths, the ability to develop tension never falls below about half of the tension that can be developed at lo (Figure 11-25).

The relationship between fiber length and the fiber's capacity to develop active tension during contraction can be partially explained in terms of the sliding-filament mechanism. Stretching a relaxed muscle fiber pulls the thin filaments past the thick filaments, changing the amount of overlap between them. Stretching a fiber to 1.75 lo pulls the filaments apart to the point where there is no overlap. At this point there can be no cross-bridge binding to actin and no development of tension. Between 1.75 lo and lo, more and more filaments overlap, and the tension developed upon stimulation increases in proportion to the increased number of cross bridges in the overlap region. Filament overlap is greatest at lo, allowing the maximal number of cross bridges to bind to the thin filaments, thereby producing maximal tension.

The tension decline at lengths less than lo is the result of several factors. For example, (1) the overlapping sets of thin filaments from opposite ends of the sar-comere may interfere with the cross bridges' ability to bind and exert force, and (2) for unknown reasons, the affinity of troponin for calcium decreases at short fiber lengths, resulting in fewer accessible sites on the thin filaments for cross-bridge binding.

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Essentials of Human Physiology

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