Figure 6-8

Length-tension diagram for a single fully contracted sarcomere, showing maximum strength of contraction when the sarcomere is 2.0 to 2.2 micrometers in length. At the upper right are the relative positions of the actin and myosin filaments at different sarcomere lengths from point A to point D. (Modified from Gordon AM, Huxley AF, Julian FJ: The length-tension diagram of single vertebrate striated muscle fibers. J Physiol 171:28P, 1964.)

further shortening, the sarcomere maintains full tension until point B is reached, at a sarcomere length of about 2 micrometers. At this point, the ends of the two actin filaments begin to overlap each other in addition to overlapping the myosin filaments. As the sarcomere length falls from 2 micrometers down to about 1.65 micrometers, at point A, the strength of contraction decreases rapidly. At this point, the two Z discs of the sarcomere abut the ends of the myosin filaments. Then, as contraction proceeds to still shorter sarcomere lengths, the ends of the myosin filaments are crumpled and, as shown in the figure, the strength of contraction approaches zero, but the entire muscle has now contracted to its shortest length.

Effect of Muscle Length on Force of Contraction in the Whole Intact Muscle. The top curve of Figure 6-9 is similar to that in Figure 6-8, but the curve in Figure 6-9 depicts tension of the intact, whole muscle rather than of a single muscle fiber. The whole muscle has a large amount of connective tissue in it; also, the sarcomeres in different parts of the muscle do not always contract the same amount. Therefore, the curve has somewhat different dimensions from those shown for the individual muscle fiber, but it exhibits the same general form for the slope in the normal range of contraction, as noted in Figure 6-9.

Note in Figure 6-9 that when the muscle is at its normal resting length, which is at a sarcomere length of about 2 micrometers, it contracts upon activation with the approximate maximum force of contraction. However, the increase in tension that occurs during contraction, called active tension, decreases as the

Figure 6-9

Relation of muscle length to tension in the muscle both before and during muscle contraction.

muscle is stretched beyond its normal length—that is, to a sarcomere length greater than about 2.2 micrometers. This is demonstrated by the decreased length of the arrow in the figure at greater than normal muscle length.

Relation of Velocity of Contraction to Load

A skeletal muscle contracts extremely rapidly when it contracts against no load—to a state of full contraction in about 0.1 second for the average muscle. When loads are applied, the velocity of contraction becomes progressively less as the load increases, as shown in Figure 6-10. That is, when the load has been increased to equal the maximum force that the muscle can exert, the velocity of contraction becomes zero and no contraction results, despite activation of the muscle fiber.

This decreasing velocity of contraction with load is caused by the fact that a load on a contracting muscle is a reverse force that opposes the contractile force caused by muscle contraction. Therefore, the net force that is available to cause velocity of shortening is correspondingly reduced.

Energetics of Muscle Contraction

Work Output During Muscle Contraction

When a muscle contracts against a load, it performs work. This means that energy is transferred from the muscle to the external load to lift an object to a greater height or to overcome resistance to movement.

In mathematical terms, work is defined by the following equation:

Load-opposing contraction (kg)

Figure 6-10

Load-opposing contraction (kg)

Figure 6-10

Relation of load to velocity of contraction in a skeletal muscle with a cross section of 1 square centimeter and a length of 8 centimeters.

in which W is the work output, L is the load, and D is the distance of movement against the load. The energy required to perform the work is derived from the chemical reactions in the muscle cells during contraction, as described in the following sections.

Sources of Energy for Muscle Contraction

We have already seen that muscle contraction depends on energy supplied by ATP. Most of this energy is required to actuate the walk-along mechanism by which the cross-bridges pull the actin filaments, but small amounts are required for (1) pumping calcium ions from the sarcoplasm into the sarcoplasmic retic-ulum after the contraction is over, and (2) pumping sodium and potassium ions through the muscle fiber membrane to maintain appropriate ionic environment for propagation of muscle fiber action potentials.

The concentration of ATP in the muscle fiber, about 4 millimolar, is sufficient to maintain full contraction for only 1 to 2 seconds at most. The ATP is split to form ADP, which transfers energy from the ATP molecule to the contracting machinery of the muscle fiber. Then, as described in Chapter 2, the ADP is rephosphory-lated to form new ATP within another fraction of a second, which allows the muscle to continue its contraction. There are several sources of the energy for this rephosphorylation.

The first source of energy that is used to reconstitute the ATP is the substance phosphocreatine, which carries a high-energy phosphate bond similar to the bonds of ATP. The high-energy phosphate bond of phosphocreatine has a slightly higher amount of free energy than that of each ATP bond, as is discussed more fully in Chapters 67 and 72. Therefore, phospho-creatine is instantly cleaved, and its released energy causes bonding of a new phosphate ion to ADP to reconstitute the ATP. However, the total amount of phosphocreatine in the muscle fiber is also very little— only about five times as great as the ATP. Therefore, the combined energy of both the stored ATP and the phosphocreatine in the muscle is capable of causing maximal muscle contraction for only 5 to 8 seconds.

The second important source of energy, which is used to reconstitute both ATP and phosphocreatine, is "glycolysis" of glycogen previously stored in the muscle cells. Rapid enzymatic breakdown of the glyco-gen to pyruvic acid and lactic acid liberates energy that is used to convert ADP to ATP; the ATP can then be used directly to energize additional muscle contraction and also to re-form the stores of phosphocreatine.

The importance of this glycolysis mechanism is twofold. First, the glycolytic reactions can occur even in the absence of oxygen, so that muscle contraction can be sustained for many seconds and sometimes up to more than a minute, even when oxygen delivery from the blood is not available. Second, the rate of formation of ATP by the glycolytic process is about 2.5 times as rapid as ATP formation in response to cellular foodstuffs reacting with oxygen. However, so many end products of glycolysis accumulate in the muscle cells that glycolysis also loses its capability to sustain maximum muscle contraction after about 1 minute.

The third and final source of energy is oxidative metabolism. This means combining oxygen with the end products of glycolysis and with various other cellular foodstuffs to liberate ATP. More than 95 per cent of all energy used by the muscles for sustained, long-term contraction is derived from this source. The foodstuffs that are consumed are carbohydrates, fats, and protein. For extremely long-term maximal muscle activity—over a period of many hours—by far the greatest proportion of energy comes from fats, but for periods of 2 to 4 hours, as much as one half of the energy can come from stored carbohydrates.

The detailed mechanisms of these energetic processes are discussed in Chapters 67 through 72. In addition, the importance of the different mechanisms of energy release during performance of different sports is discussed in Chapter 84 on sports physiology.

Efficiency of Muscle Contraction. The efficiency of an engine or a motor is calculated as the percentage of energy input that is converted into work instead of heat. The percentage of the input energy to muscle (the chemical energy in nutrients) that can be converted into work, even under the best conditions, is less than 25 per cent, with the remainder becoming heat. The reason for this low efficiency is that about one half of the energy in foodstuffs is lost during the formation of ATP, and even then, only 40 to 45 per cent of the energy in the ATP itself can later be converted into work.

Maximum efficiency can be realized only when the muscle contracts at a moderate velocity. If the muscle contracts slowly or without any movement, small amounts of maintenance heat are released during contraction, even though little or no work is performed, thereby decreasing the conversion efficiency to as little as zero. Conversely, if contraction is too rapid, large proportions of the energy are used to overcome viscous friction within the muscle itself, and this, too, reduces the efficiency of contraction. Ordinarily, maximum efficiency is developed when the velocity of contraction is about 30 per cent of maximum.

Characteristics of Whole Muscle Contraction

Many features of muscle contraction can be demonstrated by eliciting single muscle twitches. This can be accomplished by instantaneous electrical excitation of the nerve to a muscle or by passing a short electrical stimulus through the muscle itself, giving rise to a single, sudden contraction lasting for a fraction of a second.

Isometric Versus Isotonic Contraction. Muscle contraction is said to be isometric when the muscle does not shorten during contraction and isotonic when it does shorten but the tension on the muscle remains constant throughout the contraction. Systems for recording the two types of muscle contraction are shown in Figure 6-11.

In the isometric system, the muscle contracts against a force transducer without decreasing the muscle length, as shown on the right in Figure 6-11. In the isotonic system, the muscle shortens against a fixed load; this is illustrated on the left in the figure, showing a muscle lifting a pan of weights. The characteristics of isotonic contraction depend on the load against which the muscle contracts, as well as the inertia of the load. However, the isometric system records strictly changes in force of muscle contraction itself. Therefore, the isometric system is most often used when comparing the functional characteristics of different muscle types.

quadriceps muscle, a half million times as large as the stapedius. Further, the fibers may be as small as 10 micrometers in diameter or as large as 80 micrometers. Finally, the energetics of muscle contraction vary considerably from one muscle to another. Therefore, it is no wonder that the mechanical characteristics of muscle contraction differ among muscles.

Figure 6-12 shows records of isometric contractions of three types of skeletal muscle: an ocular muscle, which has a duration of isometric contraction of less than 1/40 second; the gastrocnemius muscle, which has a duration of contraction of about 1/15 second; and the soleus muscle, which has a duration of contraction of about 1/3 second. It is interesting that these durations of contraction are adapted to the functions of the respective muscles. Ocular movements must be extremely rapid to maintain fixation of the eyes on specific objects to provide accuracy of vision. The gas-trocnemius muscle must contract moderately rapidly to provide sufficient velocity of limb movement for running and jumping, and the soleus muscle is concerned principally with slow contraction for continual, long-term support of the body against gravity.

Fast Versus Slow Muscle Fibers. As we discuss more fully in Chapter 84 on sports physiology, every muscle of the body is composed of a mixture of so-called fast and slow muscle fibers, with still other fibers gradated between these two extremes. The muscles that react rapidly are composed mainly of "fast" fibers with only small numbers of the slow variety. Conversely, the muscles that respond slowly but with prolonged contraction are composed mainly of "slow" fibers. The differences between these two types of fibers are as follows.

Fast Fibers. (1) Large fibers for great strength of contraction. (2) Extensive sarcoplasmic reticulum for rapid release of calcium ions to initiate contraction. (3) Large amounts of glycolytic enzymes for rapid release of energy by the glycolytic process. (4) Less extensive blood supply because oxidative metabolism is of

Characteristics of Isometric Twitches Recorded from Different Muscles. The human body has many sizes of skeletal muscles—from the very small stapedius muscle in the middle ear, measuring only a few millimeters long and a millimeter or so in diameter, up to the very large

Figure 6-12

Isotonic and isometric systems for recording muscle contractions.

Duration of depolarization

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Essentials of Human Physiology

Essentials of Human Physiology

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