Skeletal Muscle

Consider a living relaxed skeletal muscle fiber that is aligned vertically, with one end fixed the other free, as shown in Fig. 6.11. The length Lo of the fiber is measured in this force-free configuration. Then, a weight of W is attached to the free end. The fiber will elongate rapidly in response to the applied load and then will appear to reach a steady-state configuration. The length L of the fiber is measured at that instant. The experiment is continued in this fashion with the addition of extra load and al

sarcomere length (|a m)

Figure 6.11a-d. A muscle fiber under tension (a). The isometric stress-strain relation of the fiber during passive relaxed state and fully contracted isometric state is shown in (b). The fiber stress versus sarcomere length in the midregion of the fiber is illustrated in (c). The force-velocity relation of the muscle fiber during isotonic contraction is represented in (d).

Figure 6.11a-d. A muscle fiber under tension (a). The isometric stress-strain relation of the fiber during passive relaxed state and fully contracted isometric state is shown in (b). The fiber stress versus sarcomere length in the midregion of the fiber is illustrated in (c). The force-velocity relation of the muscle fiber during isotonic contraction is represented in (d).

lowing the fiber to reach a steady state corresponding to the new load. Two parameters are defined, the average stress and average strain:

a = F/A and e = A/Lo where F denotes the imposed weight W. Because the material is homogeneous and the fiber is a circular cylinder, the stress would be expected to be uniform on the cross section of the fiber. When the stress a is plotted against the strain e, the resulting curve takes the shape shown in Fig. 6.11b. Experiments indicate that a relaxed muscle fiber can be stretched with relative ease in the physiological range but that large increases in length require considerable tensile force exerted on the fiber and thus might lead to rupture of the fiber structure.

Next, let us maximally activate the fiber (by either electrical or chemical stimulation) and repeat the experiment. In this case, in the physiological range of muscle length, the fiber will typically shorten (rather than elongate) under the application of weight before reaching a steady-state length. The total fiber stress a that keeps the active fiber at a certain length is plotted against the strain e (Fig. 6.11b). This plot is called an isometric length tension curve. Three important features are evident from this plot. One is that the longer the fiber in the physiological range, the higher is the steady-state force it can produce. This is a reflection of the microstructure of the skeletal muscle (Fig. 6.11c). Under conditions of constant length, muscle force generated is proportional to the extent of overlap between the actin and myosin filaments. The degree of overlap decreases after sarcomere length approaches a certain value characteristic of the muscle.

In the active state, a muscle can produce tensile stress of the order of 10 to 40 N/cm2. The stress in a skeletal muscle in the active state is much higher than in the passive state at the same length. Also, when an active muscle is stretched further, the force generated by it begins to drop, signaling failure.

Another primary experiment on the contraction of muscle fibers concerns the rate of shortening of a contracting fiber against a resistance. Consider a fiber in isometric contraction. When the load applied on it is reduced, the fiber begins to shorten while still in tension. This is the so-called isotonic experiment often encountered in the muscle literature. A measure of the rate of shortening is the dimensionless parameter V:

in which V is called the dimensionless rate of shortening. The time derivative of fiber length L appearing in this equation is evaluated at 100 ms after the beginning of the shortening process. The parameter V is typically plotted against the ratio of the load carried by the fiber (F) to the load carried at isometric state at the same fiber length (Fo). Typical results on a skeletal muscle fiber are shown schematically in Fig. 6.11d.

The figure indicates that the smaller is the tensile force carried by a muscle, the higher is the rate at which it shortens; a muscle will contract fastest against zero load. A fast fiber contracts faster than a slow fiber. In the human, the maximal muscle fiber shortening rate ranges from about 1 to 10 fiber lengths per second. The rate at which a muscle can shorten during contraction determines how quickly one can flex arms and legs; it plays an important role in virtually all athletic events, from short-distance running to sports that require throwing and hitting skills.

Next, let us focus on parallel muscles where muscle fibers are parallel to the long axis of the fiber. One such muscle is schematically shown in Fig. 6.12. Such a muscle usually has a belly in the middle. Thus, the average tensile stress acting on a cross section that is perpendicular to the axis of the muscle decreases from one end of the fiber toward the mid-section. The muscle is much stiffer overall at its ends because it is in this

Figure 6.12a-c. Contraction of a parallel fiber muscle under the application of a weight (a). The stress distribution on a cross section close to the ends of the muscle and at midsection are illustrated in (b) and (c), respectively.

region that the fibers of the dense connective tissue of the muscle converge and become interwoven to each other to form tendon. Much of the force carried by the muscle is carried by this connective tissue in the end regions. In the belly region of the muscle, however, the cross-sectional area is large and the average tensile stress is small compared to the tensile stress carried by the adjacent tendons. Because the major muscles of the human body must produce forces comparable to the body weight, the cross-sectional area of a skeletal muscle in the belly region can be of the order of tens of centimeters squared. It is the belly part of the muscle that undergoes significant shortening during muscle contraction. The shape of the parallel muscle suggests that, in human body structure, the form (shape) may follow function.

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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