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Muscle Physiology I

Page history last edited by PBworks 14 years, 6 months ago

Physiology 500A

Lecture # 11

Dr. H. Rasgado-Flores

Muscle Function I-- Contractile Mechanism of Muscle Cells

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Choice of two control systems:

 

1) Externally triggered mode. Versatile, general-purpose units. Digitally controlled by picojoule pulses. Energy amplification 106 approx.

 

2) Autonomous mode with integral oscillators. Especially suitable to pumping applications. Modules available with frequency and mechanical impedance appropriate for solids and slurries, liquids and gases.

 

Many optional extras e.g. built-in servo (length and velocity) where fine control is required. Direct piping of oxygen. Thermal generation. Etc.

 

GOOD TO EAT !

 

(Notice of a lecture on Muscle Presented by Professor D.R. Wilkie to the Institution of Electrical Engineers in London)

 

 

PURPOSE

 

1) To describe the cellular components of skeletal muscle

2) To describe the cellular and molecular processes of muscle contraction

3) To describe the basic mechanical variables in muscle contraction

4) To describe in the force-length relationship in isometric contractions

5) To describe the velocity-load relationship in isotonic contractions

 

I) INTRODUCTION

 

The ability to move is one of the fundamental characteristics of a living organism. Muscle contraction is an specialized example of this phenomenon. The main functions of skeletal muscle tissue are development of tension and shortening. The nervous system coordinates the activity of various muscles and of different parts of one or more muscles to produce useful movements and postures. The effect of muscle activity is transferred to the skeleton by means of tendons. The basis for movement is a biologic energy transformation called chemomechanical transduction. In this process most of the body's metabolic production of adenosine triphosphate (ATP) is converted into force or movement by muscle cells. For example, the musculature of an adult man in the resting state utilizes some 30 % of the total ATP energy generated by respiration. During very intense muscular activity, as in a sprint, the muscles consume 85% or more of the total ATP generated.

 

The performance of mechanical work is by no means limited to a few specialized tissues such as muscle. Actin and myosin are ubiquitous within eukaryotic cells. These proteins are involved in the movement of cells and the organelles within them. Indeed a striated muscle cell might be viewed as one end of a spectrum in which the myofibrils are relatively permanent structures, whereas in non-specialized cells the contractile components are assembled and dissolved as required. Evolution has led to specialization of muscle cells to minimize the ATP consumption required for specific functions.

 

It is constructive to consider why muscle, the striated variety in particular, has been such an appealing system for investigation. Firstly, a large proportion of the cell material is devoted to the contractile function. The two fundamental proteins involved, actin and myosin, comprise 80% of the structural proteins and are therefore available in large amounts for chemical characterization. Secondly, these proteins are arranged in a regular way which provides a clue to their mechanism of interaction. Thirdly, the contraction occurs on a macroscopic scale.

 

What you absolutely must understand from this section of the course, even if you get nothing else out of it….

*Follow the water:

-osmosis and volume

*Engergy sources of the cell:

-ion gradients and ATP

*Relationship of Vm, VT and Eion

*Understand, distinguish and interpret graphs:

-I vs. V

-I vs. time

-V vs. time

 

MUSCLE TYPES

 

Muscles have evolved to meet a variety of functions which demand gross differences in performance. Skeletal muscle may be required for short bursts of activity or prolonged contractions. Sustained activity is the hallmark of cardiac muscle which can function non-stop for over one hundred years. The flight muscles of a midge can contract one thousand times a second. A square centimeter of a molluscan adductor muscle can lift a 10 kg weight.

 

All muscles appear to involve interaction between actin- and myosin-containing filaments fuelled by ATP hydrolysis. However, the filaments may differ in their arrangement and in the protein isotypes they contain. Muscles also differ in the metabolic reactions they employ to generate ATP and in the way they are controlled by or respond to nerve impulses and chemical effectors. Traditionally muscles are classified in terms of their anatomy (striated vs. smooth). Both skeletal and cardiac muscle are called striated muscle because of a repetitive pattern of light and dark bands seen along the length of the muscle cells under the light microscope. Another distinction is made between voluntary muscles under conscious control and involuntary muscles in the internal organ system with autonomic innervation. However, the properties of different muscle cells are more readily understood in terms of their functional roles. A practical distinction between muscle cells is to separate them into attached to the skeleton and those in the walls of hollow organs.

 

Cells attached to a skeleton (these are all voluntary and striated) are often very long and bridge the attachment points of the muscle. The individual cells are anatomically and mechanically arranged in parallel. The cells function independently, and the total force produced by muscle equals the sum of the forces generated by its cells. Skeletal muscle cells are normally relaxed and are usually recruited to generate force and movement.

 

Muscle cells in the walls of hollow organs cannot function independently. In a continuous sheet of muscle, cells must be connected in series with each other as well as in parallel. Cells in hollow organs (typically involuntary and smooth) have two functional roles. They must be capable not only of generating force and movement, like skeletal muscle cells, but also of maintaining organ dimensions against applied loads. For example, vascular smooth muscle must bear the load imposed by the blood pressure to regulate blood flow.

 

II) STRUCTURE OF THE CONTRACTILE APPARATUS IN SKELETAL MUSCLE

 

Myofibers, Myofibrils, and Myofilaments

 

Skeletal muscle is composed of numerous parallel elongated cells referred to as muscle fibers or myofibers. These are about 10-100 m in diameter and vary with the length of the muscle, often extending its entire length. Under the electron microscope, the subcellular structure of the skeletal muscle fibers is composed of smaller fibrous structures 1 m in diameter, myofibrils which are separated by cytoplasm and arranged in parallel along the long axis of the cell (Figure 1). Each myofibril is further subdivided into thick and thin filaments. Thin filaments are about 7 nm wide and 1.0 m long, and thick filaments are about 10-14 nm wide (in mammals) and 1.6 m long. The arrangement of the thick and thin filaments produces the cross-striated appearance of the muscle, which results from a regular repetition of dense cross-bands (1.6 m in length) separated by less dense bands. The dense cross-bands, referred to as A bands because they are strongly anisotropic, contain the thick filaments arranged neatly in parallel. The less dense segments, the I bands, contain the thin filaments, which extend symmetrically in opposite directions from a dense thin line, the Z line. The term I band is based on the fact that this zone is highly isotropic.

 

The Z line structure contributes toward keeping the thin filaments arranged in register and with a regular spacing. The gap between the terminations of the thin filaments is called the H zone, and the darker area in the center of the H zone is called the M line (Figure 1).



Figure 1. Structure of skeletal muscle. a, Whole muscle. b, Muscle fibers. c, Schematic representation of the three-dimensional relationships between membrane elements of a skeletal muscle fiber.

 

Sarcomere

 

The sarcomere is the fundamental contractile unit of the muscle. It consists of the region between two consecutive Z lines; thus, this unit consists of one A band and two half I bands at each extreme of the sarcomere. Its length at rest varies between 2.0 and 2.6 m.

 

Table I summarizes the effect of muscle shortening and elongation on the size of the various muscle bands.

Sarcomere LengthI bandA bandH zone
SHORTENINGdecreasedecreaseconstantdecrease
ELONGATIONincreaseincreaseconstantincrease

 

In accord with the above considerations, the microscopic pattern seen in cross-sections of muscle fibers depend on the level of the section (Figure 2). Near the Z line, only thin filaments are observed, whereas at the site of overlap of thick and thin filaments, each filament is surrounded by six thin filaments and each thin filament by three thick filaments. A cross section through the M line shows the thin connections between adjoining thick filaments.



Figure 2. Longitudinal (top) and cross-sectional (bottom) diagrams showing the relationships between thick and thin filaments of a sarcomere.

 

Sarcolemma

 

The sarcolemma is the outer membrane surrounding each muscle fiber. The main function of the sarcolemma in muscle contraction is to conduct the wave of depolarization originating at the motor end-plate over the entire cell surface to initiate contraction.

Tubular extensions of the sarcolemma, called T (transverse)-tubules (Figure 1), extend deep into the fiber at the level of either the Z line or at the junction of the A-I bands, depending on the type of muscle. The T-tubules, about 0.03 m in diameter, allow a wave of depolarization traveling along the sarcolemma during muscle excitation to pass rapidly into the fiber so that deep lying myofibrils may be rapidly activated.

 

Sarcoplasm

 

The sarcoplasm of a muscle fiber consists of the contents of the sarcolemma, excluding the proteins of the contractile elements and nuclei. It contains the usual cytoplasmic organelles including mitochondria, sarcoplasmic reticulum, and Golgi apparatus.

 

Sarcoplasmic Reticulum

 

The sarcoplasmic reticulum (SR) is an elaborately anastomosing tubular network which surrounds the myofibrils and runs parallel to the myofilaments (Figure 1). The SR tubules may extend the full length of the sarcomere and end in dilated structures called terminal cisternae, which lie on opposite sites of the T-tubules. The SR can thus be divided into the longitudinal SR and the terminal cisternae. A group of one T-tubule and two terminal cisternae is called a triad. Densities called "feet" are located in the narrow space between the terminal cisternae and the T tubules (Figure 3).

 

The functions of the SR are the release of calcium (Ca2+) during muscle contraction and the sequestration and storage of Ca2+ during muscle relaxation.



Figure 3. Electron micrograph of a longitudinal section of a muscle fiber showing a triad at the level of the Z lie. Two "feet" are visible on each side of the T-tubule.

 

Nuclei

 

Skeletal muscle fibers are multinucleated. The nuclei are located in the periphery.

 

III) PROTEINS OF THE CONTRACTILE ELEMENTS

 

Thin Filaments

 

Thin filaments are composed primarily of three types of protein: actin, tropomyosin and troponin in a ratio of 7:1:1.

ACTIN

 

Actin is a monomeric globular protein with a molecular weight of 42,000 and a diameter of 4-5 nm. Each monomer contains binding sites for other actin monomers, myosin, tropomyosin, troponin, ATP, and cations. Under conditions existing in the cytoplasm it polymerizes to form twisted, two stranded filaments (Figure 4).



Figure 4. Composition and structure of thin filaments in muscle. Globular actin monomers (top) polymerize into a two-stranded helical filament. The thin filament structure is completed with the addition of stiff, rod-shaped tropomyosin molecules. Troponin (black rectangles) is a regulatory protein bound to the tropomyosin component of the thin filament in vertebrate striated muscles (bottom). For clarity the tropomyosin-troponin complexes associated with only one strand of the actin helix are illustrated. Thin filaments are anchored to Z disks in striated muscles. Filaments on each side of a Z disk have opposite polarities.

 

TROPOMYOSIN

 

Tropomyosin is a rod-shaped protein which consists of two -helical chains, each with a molecular weight of approximately 35,000, wound around each other to form a coiled coil. Each molecule of tropomyosin is associated with six or seven actins in one strand.

 

TROPONIN

 

Troponin consists of a complex of three separate proteins: Troponin-T, troponin-C, and troponin-I. Each troponin complex is bound to a tropomyosin molecule.

 

While only actin and myosin are directly involved in tension generation, the tropomyosin and troponin complex regulate the actin-myosin interaction, hence are called regulatory proteins.

 

Thick Filaments

 

MYOSIN

 

Myosin is a dimer with a molecular weight of 470,000. It consists of two globular heads, which hydrolyze ATP and interact with actin, and a rod-like region, which confers stability to the molecule. The myosin molecule contains six different polypeptides. These peptides are not covalently linked and can be dissociated by detergents or denaturating agents and are separated into three pairs: one set of large heavy chains and two sets of light chains. Most of the heavy chain has an -helical structure, and the two strands are twisted around each other in a supercoil that forms a long, rigid, insoluble "tail". Each end of the heavy chains has a globular tertiary structure (head). Thus each myosin molecule has two heads to one end of the long tail.

 

Myosin molecules will aggregate in a particular pattern to form filaments, similar to the thick filaments seen in intact muscle. The myosin molecules aggregate with their globular ends directed toward the ends of the filaments (Figure 5).



 

IV) SLIDING FILAMENT AND CROSSBRIDGE THEORIES

 

The sliding filament theory states that muscle contraction is the result of the overlapping of thin and thick filaments sliding past each other (Figure 6). Specifically, the thin filaments at each end of the sarcomere move in opposite directions toward the center and between the thick filaments to which they are linked by crossbridges. Polarity of thick filaments is thus a requisite for contraction.

 

The molecular basis for the sliding motion of the filaments is based in the fact that the globular heads of myosin form crossbridges with the actin monomers. Thus, the crossbridges move to and fro, first attaching to the thin filaments and pulling them toward the center of the A band during the cycle and then detaching again prior to their return stroke.



Figure 6. Effect of contraction on thin and thick filaments overlap. Muscle contraction (bottom figure)produces shortening of the sarcomere length as well as increase in the overlap of thin and thick filaments as compared when the muscle is either relaxed or stretched (upper figure).

 

Each crossbridge consists of the two globular heads of the myosin and an -helical tail by which the crossbridges attach to the backbone of the thick filament. Myosin molecules are arranged in the thick filaments with a definite structural polarity so that the heads of the molecules are always directed away from the midpoint of the filament. Thus, all crossbridges in one-half of an A band have the same orientation (polarity). This polarity is reversed in the opposite half of the A band. The actin monomers are also oriented oppositely on either side of their attachments to the Z line. During contraction the sets of thin filaments in each half sarcomere are drawn toward the center of the A band and subjected to sliding forces oriented in opposite directions.

Myosin can catalyze the hydrolysis of ATP producing ADP and Pi. However, the ATPase activity of myosin is inhibited by the high concentrations of Mg2+ that exist in cells. However, the complex formed by the binding of actin and myosin (actomyosin complex) constitutes a very active ATPase.

 

Chemomechanical transduction involves conformational changes in the crossbridges, which lead to filament movements with shortening and force development. The cycle of muscle contraction consists of the following steps:

 

1) In a resting muscle, in the presence of ATP, myosin has ADP and Pi bound to each head and, in this state exhibits a high affinity for actin. However, the complex troponin-tropomyosin sterically blocks the attachment of the myosin head to the actin; the crossbridge is oriented perpendicularly to the myosin filament.

 

2) during muscle stimulation, Ca2+ is released from the SR; Ca2+ binds to troponin inducing a conformational change in the troponin-tropomyosin complex allowing for myosin heads to interact with actin monomers. Crossbridges assume a conformation that minimizes their free energy. This preferred conformation corresponds to a 90o orientation with respect to the filaments.

 

3) Binding of the myosin-ADP-Pi complex to actin is followed by release of ADP and Pi. The resulting myosin-actin complex now has a lower free energy by changing its conformation from 90o to 45o. This change in angle generates force.

 

4) The myosin-actin complex at 45o has a high affinity for ATP. ATP binding to the complex induces crossbridge detachment.

 

5) Myosin produces ATPase activity. The products of the ATP hydrolysis, ADP and Pi remain attached to the myosin heads. Thus the myosin-ADP-Pi complex is regenerated. This complex is energized and now displays a renewed high affinity for actin and presents a lower free energy in the 90o conformation.

 

Each cycle can move the filaments about 10 nm relative to each other. The way in which enormous numbers of such cycles generate muscular contraction will be considered in the following class. The cycle will continue until interrupted in the detached state when the intracellular Ca2+ concentration is reduced to resting levels as a result of ATP-dependent sequestration in the SR.

 

The ATPase activity of acto-myosin complex present in different types of skeletal muscle correlates with the shortening velocity of the particular muscle type.

 

Death is accompanied by muscular rigidity (rigor mortis) because the concentration of intracellular Ca2+ increases in cells (promoting actin-myosin interaction) and because ATP is depleted leading to permanent crossbridge attachment.

 

Figure 7 shows a diagram describing the contraction-relaxation cycle including the mechanism of ATP hydrolysis by actomyosin and the major steps in the cross bridge cycle



Figure 7. Contraction-relaxation cycle in skeletal muscle. Actin in the thin filament (A) and the myosin cross bridge projecting from the thick filament (M) interact cyclically. This interaction involves four major steps during which ATP is hydrolyzed, and the energy released is harnessed to induce conformational changes in the crossbridge. Each cycle causes the thick and thin filaments to interdigitate by amount 10 nm. In the fist step the concentration of intracellular Ca2+ is increased promoting the binding of actin and myosin with a 90o angle. The second step consists in the release of ADP and inorganic phosphate (Pi) from the myosin head. This release induces the movement of the myosin head from a 90o angle to a 45o angle. This process generates force. During the third step ATP binds to the myosin head inducing the detachment of myosin from actin. During the fourth step ATP is hydrolyzed the actin-myosin complex. As a result of the ATP hydrolysis ADP and Pi remain attached to an energized myosin head which is able to interact with actin re-initiating the cycle again.

 

To summarize the kind of interactions that thin and thick filaments can have, an experiment can be performed with skeletal muscle cells exposed to glycerine. This procedure produces removal of the sarcolemma while maintaining intact the functionality of the contractile proteins. Therefore, under this experimental condition, it is possible to control the medium to which the contractile proteins are exposed while the thin and thick filaments are able to interact normally. In addition it is possible to register the state of contraction of the "glycerinated" muscle as well as the ATPase activity of the contractile proteins. Table II shows the three possible interactions that thin and thick filaments can have under various experimental conditions. In condition 1, the concentration of Ca2+ is very low (<10-7 M), ATP, Mg2+, and KCl are at their normal physiological values of 1 mM, 2 mM and 0.1 M, respectively. Under this condition the muscle is relaxed and the ATPase activity is very low. If we increase the concentration of Ca2+ to 10-5 M (condition 2), the muscle now contracts and the ATPase activity increases considerably. Finally, if we remove ATP from the medium while maintaining constant the concentration of Ca2+ (condition 3), the muscle will enter the state of rigor mortis; ATPase activity will be zero.



 

V) BASIC MECHANICAL VARIABLES IN MUSCLE CONTRACTION

 

Table III summarizes the most relevant quantitative estimates of the mechanical output of muscle contraction. These parameters are used to assess the normal behavior of muscles as well as to determine the effect of agents such as drugs or hormones on muscle physiology. The basic approach in muscle mechanics is to control all of these factors except the measured dependent variable.



 

VI) FORCE-LENGTH RELATIONSHIP IN ISOMETRIC CONTRACTIONS

 

To test the sliding filament theory of contraction it is possible to carry out a simple experiment using an isolated skeletal muscle. The experimental set-up consists of an experimental chamber in which an skeletal muscle can be mounted (see Figure 8). One tendon end is attached to a force transducer while the other end is attached to a micrometer which can be adjusted to either push or pull the muscle to attain various muscle lengths. A contraction of this type, when the muscle maintains a constant length during the contraction is termed isometric contraction. The hypothesis to be tested is the following:



Figure 8. Stress-length relationships in skeletal muscle. A, schematic diagram of the experimental apparatus for isometric contractions at various muscle lengths. Force is normalized as a stress, to allow comparisons of muscles of differing size (i.e., force/cross sectional area of the muscle cells). B, Three stress-length curves are shown for skeletal muscle: (1) the passive stress exerted as a function of the length the relaxed muscle, (2) the total stress exerted by the maximally stimulated muscle (passive + active), and (3) the active stress-length curve for the contractile machinery obtained as the difference between the total and passive stresses at any length. All muscles have an inherent maximal force-generating capacity, which is obtained at the optimal length (Lo). C, dependence of stress generation at various sarcomere lengths with the overlap of thick and thin filaments. Diagrams of four sarcomeres at lengths where the slope of the stress-length curve changes show how filament interactions and active stress depend on sarcomere length.

*If the sliding theory of contraction is correct, there should be an ideal length (i.e., sarcomere length) of the muscle at which there should be a maximal interaction of the thin and thick filaments. At this length, electrical stimulation of the muscle should produce a maximal contraction.

*Pushing the muscle to attain a smaller length than normal would result in overlap of the ends of the thin filaments facing the M line at the H zone. As a result of this juxtaposition the force of the electrically-induced contraction will be diminished.

*If the muscle is pulled instead of being pushed, two effects on force generation are expected to occur. First, as the muscle is being pulled, the connective tissue within the muscle will passively try to bring the muscle back to its normal length. This effect will result on generation of a passive force. This passive force will increase as the muscle is pulled further. Second, as the muscle is being pulled, the sarcomere length is increased resulting in a reduction in the overlap between thin and thick filaments. Consequently, the electrically-induced contraction will diminish as the muscle is being pulled. In fact, a linear reduction in force generation is expected to occur as the muscle is being pulled. To separate the active and passive contractions the experimenter will simply subtract the tension measured just by pulling (passive) from the one obtained as a result of the electrical stimulation (active).

Figure 8 shows that all the predictions of the sliding filament theory of contraction are met by the experimental results. Thus, the isometric contraction force-length relationship confirms the theory.

 

VII) VELOCITY-LOAD RELATIONSHIP IN ISOTONIC CONTRACTIONS

 

Another important experiment that can be performed to characterize the mechanical output of muscle contraction is to study the effect of load on the velocity of contraction. Based on every day experience it is well known that the heavier a load, the less quickly it can be lifted. In other words: the greater the force, the lower the velocity (speed of shortening). To quantify the effect of load on the velocity of muscle contraction an isolated muscle is placed in an appropriate experimental chamber. One tendon end is attached to a fixed force transducer while the other tendon is attached to a length transducer and to a lever which connects the tendon end with a weight (load) against which the muscle will contract. The muscle is stimulated electrically via electrodes. With this arrangement, electrically-induced contraction is accompanied by shortening against a constant load. This kind of contraction is termed isotonic contraction. The purpose of the experiment is to measure the velocity of muscle contraction against various loads. Figure 9 shows the results of such experiment. The Y axis at the left of the figure is the velocity of shortening or lengthening, the X-axis is the magnitude of the load against which the muscle is contracting. The figure shows the following:

 

*When the load is zero the muscle shortens at its fastest rate

*As the magnitude of the load increases, the velocity of shortening diminishes as well

*The rate of contraction does not decrease linearly with the load, it decays instead exponentially

*With heavy loads the muscle is unable to shorten

*With even heavier loads the muscle lengthens

 

The interpretation of this results is that when there is no load, the speed of movement is the greatest; when the load is so large that it cannot be lifted, the contraction is isometric (speed=0). Between these two extremes, speed varies with load in a way that is characteristic of the individual muscle; the general form of the curve is exponential as illustrated in Figure 9. The muscle can withstand (briefly) a heavier load when forcibly stretched than it can develop isometrically. The strength of the crossbridge attachment to the thin filament is greater than the force generated by its movement. Consequently, muscles can bear a load larger than the maximum active force, before the crossbridge attachment is mechanically broken and rapid lengthening occurs. This situation has physiological relevance when a muscle is contracted to decelerate the body when a person is walking or running downhill.



Figure 9. The dependence of shortening velocity on the load (stress) on a muscle. Shortening velocity is measured using a lever system which permits a muscle to shorten against a constant load. Velocity is measured using a length transducer (velocity = length/time). The velocity-load is constructed from points obtained in a series of contractions against different loads. If the load placed on the muscle is greater than the load that the active crossbridges can bear, the muscle will lengthen. Consequently, the velocity-load curve can be extended to describe this situation. Insets emphasize that the maximal stress (Fo) a muscle can develop depends on the number of interacting crossbridges, whereas the maximal shortening velocity (Vo) is limited by the rate at which a particular isoenzymatic form of myosin synthesized in a cell can interact with actin and release the energy stores in ATP. The power output of a muscle is the mechanical work (force times distance shortened) per unit of time and can be calculated as the product of load times shortening velocity.

 

WORK

 

Another important consideration to be made about muscle contraction is the work produced by the contraction. Work (W) is defined in your physics book as the force (F) applied to a mass times the distance (d) the mass or load has been moved with that force, W = F • d. If a muscle contracts isometrically, no shortening (d) is produced. Therefore, since d=0 from the physics point of view, no work is done. On the other extreme, in a pure isotonic contraction, no mass or load is lifted, and since force is equal to mass times acceleration (F = m • a) and there is no mass (m=0) to be lifted, there is no force (F=0) and work has a value of zero (W=F•d). In other words, the apparent work done under these circumstances is zero. We can say that a muscle activated at the two extremes (isometric or isotonic), apparently produces no "external work", but that, indeed work is done which appears as heat. Furthermore, there is a linear relationship between the heat produced during activation and the amount of ATP split during it. We can express the above by saying that the total work done by the muscle when it is activated consists of two components: external work, i.e., work that can be appreciated as "productive" plus the heat that is produced as ATP splits.

 

Work can be expressed in either joules (J): 1 Joule = 1 Newton • meter or in units of calories (cal) per gram (g) of muscle weigh. In fact 1 Joule = 0.239 cal.

 

POWER

 

It can take a short or long time to produce the same amount of work. From every day experience we know that a person is physically strong ("powerful") when he (she) can do a great amount of heavy physical work in a short period of time. In physical terms, power (P) is defined as the amount of work done per unit of time (t): P = W/t. Since work = F • d, and since velocity (v) is d/t, P can be defined as F • V. Power is related to load as shown by the dashed line in Figure 9. The Figure shows that when the value of the load to be lifted is zero, power is zero as well because even if the muscle contracts at its fastest it produces no work. On the other hand, if the load is too heavy, the muscle contraction will not be able to lift the load (d=0) in spite of all the effort invested and therefore, the values of the work and power will be zero. The values in between these two extremes follow a complex relationship. If the load is very light, the muscle will lift it very fast and will accomplish the work in a short period of time. However, the power of this action will be small because the force involved (and therefore the value of w) will be minimal. With small increments of load, the muscle contraction will still be able to accomplish the work in a short time and the power will increase. However, it will come to a point where the value of the load is high enough that it will begin to take longer and longer to accomplish the work. From this time up the value of power will diminish because it will take longer and longer to accomplish the work. Maximal power is obtained with a load of about 0.3 of the maximal force than can be developed.

 

EFFICIENCY

 

We all can produce a certain amount of work. However, it may require different amounts of energy to produce such amount of work. The work performed will be more efficient as less energy is utilized to perform it. Efficiency is defined as the external work done, divided by the total energy used or consumed to produce that amount of work. Like in a car, only a fraction of the total fuel consumed is used to produce external work and as you know, a car is more efficient when it gives more miles per gallon. The fuel or energy which is not used to move the car is consumed as friction, and it is dissipated as heat (you can think of it as necessary wasted energy). The muscle fuel is ATP and it is split into Pi and phosphocreatine converting chemical energy into mechanical energy. ATP consumption is measured in cal/g. Efficiency has no units and is a ratio of always less than 1. Therefore,

		    "external" work done in cal/g
Efficiency = ----------------------------------------------
		Total work performed as ATP split in cal/g

If a muscle produces an external work of 1 mcal/g but consumes 4 mcal/g muscle of ATP, then the efficiency of this muscle is:


		External Work: 1 mcal/g
Efficiency = ------------------------------ = 0.25 or 25%.
		Total work: 4 mcal/g


Figure 10 shows the relationship between force and efficiency. The Y-axis is the efficiency (0-25%); the X-axis is the relationship between any given force (P) and the maximal force (Po). When the load is zero the relationship is zero (isotonic contraction); when the load is too heavy the force generated is equal to the maximal one and P/Po=1.

 

Figure 10. Relationship between efficiency and load.

 

Clearly, efficiency is zero when the muscle contracts isometrically or when it has no load and contracts isotonically. Efficiency is greatest when the muscle is able to handle the load rather easily, which is when P/Po= 0.3, i.e., 30% of the maximal force that it can produce, and it drops when the load gets relatively heavier or too light. Notice that reducing the load from 80% (P/Po=0.8) to 60% (P/Po=0.6) of maximal force (Po) produced almost triples the efficiency. This is important when considering the performance of the heart as a pump, and the failing heart. The lesson is that when faced with a muscle struggling against a heavy load, an enormous gain in efficiency can be achieved by either modestly decreasing the load, or modestly increasing the contractile force of the muscle. Both methods are used in treating heart failure.

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