1.7.2 Length-Tension Relationship
The active length-tension relationship for a skeletal muscle fiber is shown in FIGURE 15A.
This figure shows the relative tension developed as a function of muscle length (relative to usual resting length). The resting length of a skeletal muscle fiber in the body is normally near its optimal length, i.e., the length at which maximum x is developed. The amount of tension that can be developed x as the muscle fiber's length is increased or decreased relative to this optimal length. Note that active tension falls to zero for lengths less than about x% of the resting length or more than about x% of the resting length. However, note that skeletal muscle fibers normally only operate in a range from about x% to x% of their resting length.
This figure also plots the passive tension developed when the fiber is stretched and the total tension (sum of passive and active tension).
The length-tension relationship can be understood in terms of the sliding filament mechanism of contraction.
The small range of lengths where tension development is maximum corresponds to the situation in which the x is optimal.
As the fiber length increases beyond the optimum length, developed tension x essentially linearly. This is because as the fiber is stretched the x are pulled out from the thick filaments and the region of overlap decreases. Thus, fewer x binding sites are accessible to bind with the cross-bridges of the thick filaments. With sufficient stretching of the fiber, there is no longer any overlap of the thick and thin filaments and no active x can be produced.
As the fiber length is decreased below the optimal length, tension is also reduced. There are several reasons for this decrease:
As initial length decreases, thin filaments from the opposite sides of the sarcomere begin to x. This results in a reduction of the number of actin binding sites that are exposed for cross-bridge binding. Note that if a cross-bridge from one side of a sarcomere binds to actin from a thin filament from the opposite side of the sarcomere, it will not x.
With further shortening, the ends of the thick filaments bump into the x lines.
Finally, it is also believed that when the muscle shortens to less than about 80% of its
optimal length x is reduced.
The length-tension relationship can also be plotted as tension versus sarcomere length as
shown in FIGURE 15B. Note that only active tension is shown.