Special Mechanical Arrangements Allow a More Precise Analysis of Muscle Function

The types of contraction described above provide a basis for a better understanding of muscle function. The isometric and isotonic mechanical behavior of muscle can be described in terms of two important relationships:

• The length-tension curve, treating isometric contraction at different muscle lengths

• The force-velocity curve, concerned with muscle performance during isotonic contraction

Isometric Contraction and the Length-Tension Curve.

Because it is made of contractile proteins and connective tissue, an isolated muscle can resist being stretched at rest. When it is very short, it is slack and will not resist passive extension. As it is made longer and longer, however, its resisting force increases more and more. Normally a muscle is protected against overextension by attachments to the skeleton or by other anatomic structures. If the muscle has not been stimulated, this resisting force is called passive force or resting force.

The relationship between force and length is much different in a stimulated muscle. The amount of active force or active tension a muscle can produce during an isometric contraction depends on the length at which the muscle is held. At a length roughly corresponding to the natural length in the body, the resting length, the maximum force is produced. If the muscle is set to a shorter length and then stimulated, it produces less force. At an extremely short length, it produces no force at all. If the muscle is made longer than its optimal length, it produces less force when stimulated. This behavior is summarized in the length-tension curve (Fig. 9.10).

In Figure 9.10, the left side of the top graph shows the force produced by a series of twitches made over the range

Steps Skeletal Muscle Contraction

Ij^ A length-tension curve for skeletal muscle. Contractions are made at several resting lengths, and the resting (passive) and peak (total) forces for each twitch are transferred to the graph at the right. Subtraction of the passive curve from the total curve yields the active force curve. These curves are further illustrated in the lower right corner of the figure. (Force, length, and time units are arbitrary.) (See text for details.)

of muscle lengths indicated at the left side of the bottom graph. Information from these traces is plotted at the right. The total peak force from each twitch is related to each length (dotted lines). The muscle length is changed only when the muscle is not stimulated, and it is held constant (isometric) during contraction. The difference between the total force and the passive force is called the active force (see inset; Fig. 9.10). The active force results directly from the active contraction of the muscle.

The length-tension curve shows that when the muscle is either longer or shorter than optimal length, it produces less force. Myofilament overlap is a primary factor in determining the active length-tension curve (see Chapter 8). However, studies have demonstrated that at very short lengths, the effectiveness of some steps in the excitation-contraction coupling process is reduced—binding of calcium to troponin is less and there is some loss of action potential conduction in the T tubule system.

The functional significance of the length-tension curve varies among the different muscle types. Many skeletal muscles are confined by their skeletal attachments to a relatively short region of the curve that is near the optimal length. In these cases, the lever action of the skeletal system, not the length-tension relationship, is of primary importance in determining the maximal force the muscle can exert. Cardiac muscle, however, normally works at lengths significantly less than optimal for force production, but its passive length-tension curve is shifted to shorter lengths (see Chapter 10). The length-tension relationship is, therefore, very important when considering the ability of cardiac muscle to adjust to changes in length (related to the volume of blood contained in the heart) to meet the body's changing needs. The role of the length-tension curve in smooth muscle is less clearly understood because of the great diversity among smooth muscles and their physiological roles. For all muscle types, however, the length-tension curve has provided important information about the cellular and molecular mechanisms of contraction.

Isotonic Contraction and the Force-Velocity Curve.

Everyday experience shows that the speed at which a muscle can shorten depends on the load that must be moved. Simply stated, light loads are lifted faster than heavy ones. Detailed analysis of this observation can provide insight into how the force and shortening of muscles are matched to the external tasks they perform, as well as how muscles function internally to liberate mechanical energy from their metabolic stores. The analysis is performed by arranging a muscle so that it can be presented with a series of afterloads (see Fig. 9.9; Fig. 9.11). When the muscle is maximally stimulated, lighter loads are lifted quickly and heavier loads more slowly. If the applied load is greater than the maximal force capability of the muscle, known as Fmax, no shortening will result and the contraction will be isometric. If no load is applied, the muscle will shorten at its greatest possible speed, a velocity known as Vmax.

The initial velocity—the speed with which the muscle begins to shorten—is measured at various loads. Initial velocity is measured because the muscle soon begins to slow down; as it gets shorter, it moves down its length-tension curve and is capable of less force and speed of shortening. When all the initial velocity measurements are related to each corresponding afterload lifted, an inverse relationship known as the force-velocity curve is obtained. The curve is steeper at low forces. When the measurements are made on a fully activated muscle, the force-velocity curve defines the upper limits of the muscle's isotonic capability. In practice, a completely unloaded contraction is very difficult to arrange, but mathematical extrapolation provides an accurate Vmax value.

Figure 9.11 shows a force-velocity curve made from such a series of isotonic contractions. The initial velocity points (A-D) correspond to the contractions shown at the top. Factors that modify muscle performance, such as fatigue or incomplete stimulation (e.g., fewer motor units activated), result in operation below the limits defined by the force-velocity curve.

Musclefunction Graph
Time
The Muscle Function Steps

Afterload force

^FIGUREIRin^^ Force-velocity and power output curves for skeletal muscle. Contractions at four different afterloads (decreasing left to right) are shown in the top graphs. Note the differences in the amounts of shortening. The initial shortening velocity (slope) is measured (VB, VC, VD) and the corresponding force and velocity points plotted on the axes in the bottom graph. Also shown is power output, the product of force and velocity. Note that it reaches a maximum at an afterload of about one-third of the maximal force. (Force, length, and time units are arbitrary.)

Afterload force

^FIGUREIRin^^ Force-velocity and power output curves for skeletal muscle. Contractions at four different afterloads (decreasing left to right) are shown in the top graphs. Note the differences in the amounts of shortening. The initial shortening velocity (slope) is measured (VB, VC, VD) and the corresponding force and velocity points plotted on the axes in the bottom graph. Also shown is power output, the product of force and velocity. Note that it reaches a maximum at an afterload of about one-third of the maximal force. (Force, length, and time units are arbitrary.)

Consideration of the force-velocity relationship of muscle can provide insight into how it functions as a biological motor, its primary physiological role. For instance, Vmax represents the maximal rate of crossbridge cycling,- it is directly related to the biochemistry of the actin-myosin ATPase activity in a particular muscle type and can be used to compare the properties of different muscles.

Because isotonic contraction involves moving a force (the afterload) through a distance, the muscle does physical work. The rate at which it does this work is its power output (see Figure 9.11). The factors represented in the force-velocity curve are thus relevant to questions of muscle work and power. At the two extremes of the force-ve locity curve (zero force, maximal velocity and maximal force, zero velocity), no work is done because, by definition, work requires moving a force through a distance. Between these two extremes, work and power output pass through a maximum at a point where the force is approximately one-third of its maximal value. The peak of the curve represents the combination of force and velocity at which the greatest power output is produced, at any after-load force greater or smaller than this, less power can be produced. It also appears in skeletal muscle that the optimal power output occurs under nearly the same conditions at which muscle efficiency, the amount of power produced for a given metabolic energy input, is greatest.

In terms of mechanical work, the chemical reactions of muscle are about 20% efficient,- the energy from the remaining 80% of the fuel consumed (ATP) appears as heat. In some forms of locomotion, such as running, the measured efficiency is higher, approaching 40% in some cases. This apparent increase is probably due to the storage of mechanical energy (between strides) in elastic elements of the muscle and in the potential and kinetic energy of the moving body. This energy is then partly returned as work during the subsequent contraction. It has also been shown that stretching an active muscle (e.g., during running or descending stairs) can greatly reduce the breakdown of ATP, since the crossbridge cycle is disrupted when myofilaments are forced to slide in the lengthening direction.

These force-velocity and efficiency relationships are important when endurance is a significant concern. Athletes who are successful in long-term physical activity have learned to optimize their power output by "pacing" themselves and adjusting the velocity of contraction of their muscles to extend the duration of exercise. Such adjustments obviously involve compromises, as not all of the many muscles involved in a particular task can be used at optimal loading and rate and subjective factors, such as experience and training, enter into performance.

In rapid, short-term exercise, it is possible to work at an inefficient force-velocity combination to produce the most rapid or forceful movements possible. Such activity must necessarily be of more limited duration than that carried out under conditions of maximal efficiency. Examples of attempts at optimal matching of human muscles to varying loads can be found in the design of human-powered machinery, pedestrian ramps, and similar devices.

Interactions Between Isometric and Isotonic Contractions.

The length-tension curve represents the effect of length on the isometric contraction of skeletal muscle. During isotonic shortening, however, muscle length does change while the force is constant. The limit of this shortening is also described by the length-tension curve. For example, a lightly loaded muscle will shorten farther than one starting from the same length and bearing a heavier load. If the muscle begins its shortening from a reduced length, its subsequent shortening will be reduced. These relationships are diagrammed in Figure 9.12. In the case of day-to-day skeletal muscle activity, these limits are not usually encountered because voluntary adjustments of the contracting muscle are usually made to accomplish a specific task. In the case of cardiac muscle, however, such interrelationships between force

Skeletal Muscle Biceps

Triceps Biceps

The relationship between isotonic and isometric contractions. The top graphs show the contractions from Figure 9.11, with different amounts of shortening. The bottom graph shows, for contractions B, C, and D, the initial portion is isometric (the line moves upward at constant length) until the afterload force is reached. The muscle then shortens at the afterload force (the line moves to the left) until its length reaches a limit determined (at least approximately) by the isometric length-tension curve. The dotted lines show that the same final force/length point can be reached by several different approaches. Relaxation data, not shown on the graph, would trace out the same pathways in reverse. (Force, length, and time units are arbitrary.)

and length are of critical importance in functional adjustment of the beating heart (see Chapter 10).

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