Description Introduction to Unit 3 - Lesson Introduction 3-4 Mechanisms for Regulating Active Cross-Sectional Area

Transcription

1 Content Display Unit 3 - Skeletal Muscle : Lesson 4 KINE xxxx Exercise Physiology 4 Unit 3 - Skeletal Muscle 5 Lesson 4 1 U3L4P1 - Introduction to Unit 3 - Lesson 4 Lessons 4 and 5 deal with factors that determine effectiveness of muscular force development. These lessons really go together as one lesson in terms of the material covered. I thought the material would cover too many pages for one lesson, however, and might seem overwhelming. So, I have broken the topic into two lessons. Hopefully this will facilitate your studying and learning. Please recognize, however, that from a topical perspective, Lessons 4 and 5 go together. It is important for you to see the content of these lessons as a package consisting of all the important factors that determine the amount of force a skeletal muscle can exert at maximum (i.e., strength), as well as the amount of force a muscle actually does exert at a given moment (maximal or submaximal). So, as you study the materials in Lessons 4 and 5, "step back" every now and then to recognize how the individual segments relate to each other and to the total package of skeletal muscle exercise physiology. Contents of Lesson 4: Description Page Introduction to Unit 3 - Lesson Introduction 3-4 Mechanisms for Regulating Active Cross-Sectional Area 5-17 Determinants of Effective Force Output - Length of the Muscle Determinants of Effective Force Output - Force-Velocity Relationship Review of Lesson 36 Outline of Content In the following outline I have included the topics addressed in both Lesson 4 and Lesson 5, since they all fall under the main topic, "Determinants of Effective Force Output of Muscle." Outline: VI. Determinants of Effective Force Output of Muscle Lesson 4 A. Introduction (Pages 3-17) 1 of 29 5/17/2001 3:44 PM

2 1. Overview of "Effective Force Output" and Determinants 2. Mechanisms for Regulating Active Cross-sectional Area a. Recruitment b. Rate Coding B. Length of the Muscle (Pages 18-21) C. Force-Velocity Relationship (Pages 22-35) Lesson 5 (Included here to complete topical outline) D. Stretch-Shorten Cycle E. Mechanics F. Fatigue 1. Definitions of Fatigue 2. Mechanisms of Fatigue 3. Miscellaneous Points G. Injury and Pathology 2 U3L4P2 2 of 29 5/17/2001 3:44 PM

3 - Introduction to Unit 3 - Lesson 4 (cont.) Learning Objectives After completion of Lesson 4, the student should be able to: 1. Define: motor unit recruitment, motor unit derecruitment, rate coding, size principle, LO, PO, Vmax. 2. Discuss the concept of "effective force output," and state three specific examples representing a wide range of actual force output values. 3. Differentiate between active force development and passive force development by muscle. 4. Discuss why active cross-sectional area is a fundamental determinant of the amount of force generated by a muscle at a given instant. List three basic factors that determine the potential active cross-sectional area of a muscle organ. 5. Discuss recruitment and rate coding as mechanisms for regulating muscle force output. What is the basic mechanism of each? As a general principle, how are these mechanisms used in different types of muscle contractions? Give several specific examples. 6. Discuss the effect of muscle length on its maximal ability to develop force. Include: How are active and passive force involved, and what is the mechanism of involvement of each of these? How big a factor is the length-tension relationship in normal human movements? Why? 7. Summarize the force-velocity relationship in skeletal muscle both in words and with a graph. State the primary determinants of Vmax and PO. 8. Discuss the force-velocity relationship as it applies to skeletal muscles in the intact human body. Include: What determines the exact points on the force-velocity curve for a given muscle? What parts of the curve actually apply and which (if any) do not? Does fiber type distribution affect this relationship? 9. Discuss the relationship between muscle force (or load moved) and power, from both the theoretical and the applied perspective. 10. Discuss how training can (or cannot) affect the load-velocity-power relationships. 3 U3L4P3 3 of 29 5/17/2001 3:44 PM

4 - Introduction I hope you have been able to see applications of the muscle physiology we have studied so far. But this lesson and the next in particular deal with application. I like to use the term effective force output to refer to the external manifestation of muscle function. In most athletic competition, we have indicators of effectiveness that are easily measured and precise: times in running, walking, swimming, and cycling; height and distance of jumping; distance an object is thrown or hit; etc. Direct measurements of effective muscle function are not always as precise or easily done. Nevertheless, the concept of effective force output is simple. Let me give several examples. The person who bench presses 250 pounds for the first time can be sure that his/her muscles generated force more effectively than ever before during a bench press. The sprinter who breaks 10.5 seconds for the first time in the 100 meters can be sure that his/her muscles generated force more effectively than ever before during a 100-meter run (unless the race conditions were much more favorable than ever before, such as presence of a 30-mph tailwind!). The bowler who bowls a perfect game can be sure that his/her muscles generated force extremely effectively in that game. The neurosurgeon, after successful removal of a brain tumor in a 4-hour operation, can be sure that his/her muscles generated force very effectively. In these examples, I intentionally selected a variety of activities, only one of which involved maximal force output by muscles. Sometimes muscles are most effective when they generate maximal forces. More often, effectiveness is not a matter of maximal force but rather a proper amount of submaximal force applied at just the right time and place, at just the right speed, etc. A surgeon using a scalpel is a good example of effectiveness of force. The total forces applied are very small fractions of maximums for the muscles involved, but they are just right for the task. Application of too much force by the surgeon could be disastrous. 4 U3L4P4 4 of 29 5/17/2001 3:44 PM

5 - Introduction (cont.) Following is a list of factors that influence the effectiveness of muscular force output: (a) Length of the muscle; (b) Velocity of contraction; (c) Eccentric contraction prior to a concentric contraction (i.e., the stretch-shorten cycle); (d) Mechanics; (e) Fatigue; and (f) Injury and pathology. I will address the first two of these factors in this lesson and the last four factors in Lesson 5. Some of these I will address only briefly, because details are outside the scope of this class. I will deal with others in more detail. The primary focus of both of these lessons will be force. This is not to minimize related aspects of force development, such as speed and power. Velocity of force development and power developed in contractions are often critical determinants of effectiveness of performance. And I will address these. But I will focus on force, because development of force is the basic function of muscle (remember the definition of contraction). That is what muscles do. Furthermore, force is closely related to speed and power, as we will discuss. 5 U3L4P5 - Mechanisms for Regulating Active Cross-Sectional Area A muscle has two mechanisms for developing force: (a) actively, via myosin attaching to actin and pulling (i.e., contraction), and (b) passively, via force stored in and returned by elastic elements, analogous to a rubber band. I will focus especially on active force development, again because this is what muscles DO. But I will not ignore the important role of passive force where applicable. 6 U3L4P6 5 of 29 5/17/2001 3:44 PM

6 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) A fundamental concept is that active force developed by a muscle at any given instant is determined by the number of myosin-actin attachments in parallel with each other. Let me give a simple analogy. Imagine that I have two identical strings. I can combine these strings in two ways to take advantage of their combined properties: (a) attached end-to-end, in series with each other, or (b) side-by-side, parallel to each other. Each has advantages and disadvantages. The advantage of the parallel arrangement is that I make use of the strength of the strings together; the strength of the two strings together is twice the force of one string by itself. (To make a very strong rope, many strings are combined in parallel.) The disadvantage of the parallel arrangement is that the overall length of the strings is the same as the length of one. Therefore, I haven t gained anything in terms of the distance over which the strings can work. This is the advantage of the series arrangement. The length of each string is added to the other, resulting in a doubling of the operating range (not counting the knot!). The disadvantage of the series arrangement is in force development. The strength of the two strings in series is the same as the strength of each by itself. A variation of this analogy is the person who may need to escape from an upper floor of a hotel by climbing down a rope of bed sheets tied together. Tying them end to end (in series) increases the chances of the rope being long enough to reach the ground. But, the strength will be no more than the strength of one, so the risk of the rope breaking during descent is higher than it would be if the sheets were tied with two or more in parallel. This arrangement, however, may mean the rope is not long enough to reach the ground. Hopefully, there will be enough sheets to use both series and parallel arrangements, so the rope is both strong enough and long enough. (Hopefully, also, there will be enough time to do all this tying of sheets!) 7 U3L4P7 6 of 29 5/17/2001 3:44 PM

7 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) Now consider a tiny, hypothetical muscle fiber with just two sarcomeres, and each sarcomere consists of just one myosin crossbridge. (See figure.) These sarcomeres could be end-to-end, in series with each other (as in Muscle A), or side-by-side, in parallel with each other (as in Muscle B). In parallel, the force of one crossbridge is added to the force of the other, so the strength of this muscle is twice the strength of one sarcomere by itself (like the strings in parallel). In series, the overall strength of the muscle is the same as the strength of one sarcomere (like the strings in series). This longer muscle with sarcomeres in series has advantages related to range of movement and speed of contraction, but it sacrifices ability to generate force. 8 U3L4P8 7 of 29 5/17/2001 3:44 PM

8 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) The number of parallel myosin-actin attachments in a muscle organ at a given instant is determined by: (a) the number of active muscle fibers in parallel with each other, (b) the size (cross-sectional area) of the active muscle fibers, and (c) the amount of myofibrillar material in the active fibers (nonmyofibrillar material takes up area without contributing force). This is often collectively referred to as active cross-sectional area. At least theoretically, the active cross-sectional area of a muscle, such as the biceps brachii, can range from 0% to 100% of the muscle s maximal potential area. Zero percent, of course, represents total relaxation of the muscle, and 100% represents the strength of the muscle (i.e., maximal force development). The effective force output of the biceps brachii muscle could be 5%, 37%, 66%, 100%, or any other percent of the muscle s maximum. Therefore, the appropriate amount of active cross-sectional area might be 5%, 37%, 66%, 100%, or any other percent of maximal potential area. How is the amount of active cross-sectional area controlled? The body uses two primary mechanisms for regulating active cross-sectional area to attempt to provide just the right amount of force for the task. One mechanism is referred to as recruitment and the other as rate-coding. 9 U3L4P9 8 of 29 5/17/2001 3:44 PM

9 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) Recruitment is the activation of previously inactive motor units. (Remember that an individual fiber cannot be activated. It can only be activated with all of the other fibers in the same motor unit, since they are all stimulated by the same alpha motor neuron from the central nervous system.) Often the term derecruitment is used to refer to the opposite of recruitment, that is, inactivating previously active motor units. Recruitment turns a motor unit on; derecruitment turns the motor unit off. Recruitment and derecruitment determine the number of active motor units, and thereby the amount of active cross-sectional area. Consider this overly simplistic example of recruitment: Imagine a muscle organ with only three motor units that are identical. Each motor unit has 100 muscle fibers, and each muscle fiber can generate 10 grams of force. At rest, none of the three motor units has been recruited; all are inactive, and the muscle s active force output = 0. If a small amount of force is needed from the muscle, the central nervous system will recruit one of the motor units. In this simplistic example, the active cross-sectional area will be one-third of the muscle s total area, and force developed by the muscle will be 1,000 grams (100 active muscle fibers x 10 grams of force per fiber), one-third of the muscle s maximal force. Recruiting the second motor unit to join the first will increase active cross-sectional area and force output to two-thirds of the total. And recruitment of all three motor units will result in the entire cross-sectional area of the muscle being activated and maximal force output (i.e., 3,000 g). To reduce force output from maximum, the central nervous system can derecruit one or more of the active motor units. Recruitment and derecruitment are fundamental to determining the force output of a muscle. Typical muscles have hundreds and even thousands of motor units, depending on the size of the muscle. So, many gradations of force can be developed simply by regulating the number of active motor units. But recruitment by itself would provide limited control of force output. This is because it can only add the force of entire motor units (i.e., a motor unit is either active or inactive), as in the simplistic example above. Note that in this example, the muscle had only four possible force outputs: 0 g, 1000 g, 2000 g, or 3000 g. What if the most effective force were 1500 g? Recruitment could not provide this. Rate coding could. 10 U3L4P10 9 of 29 5/17/2001 3:44 PM

10 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) Rate coding is the regulation of the frequency of stimulation of an already active motor unit. Here you must recall the concepts of twitch, summation and tetanus. Increasing the frequency of muscle fiber action potentials causes more frequent releases of calcium from the SR, and therefore higher myoplasmic [Ca ++ ]. This results in more myosin-actin attachments and more force (summation of force). In other words, the force developed by a single motor unit can vary over a range from its twitch force to its maximal tetanic force, depending on the frequency at which it is electrically stimulated. When a motor unit is initially recruited, usually it is stimulated at a frequency well below the frequency needed to elicit maximal tetanic force. Then, if more force is needed from this motor unit, its frequency of stimulation is increased, resulting in summation of force and more force output. If still more force is needed from this motor unit, frequency of stimulation will be increased until maximal tetanic force is elicited (i.e., the maximum number of myosin-actin attachments). Let s go back to the simplistic example of the muscle with three motor units to show how recruitment and rate coding can result in a force output of 1,500 g. I must add other assumptions: Assume that each motor unit can be stimulated at frequencies ranging from zero to 100 times per second (abbreviated hertz, Hz). Also, the force output of the motor unit is exactly proportional to the frequency of stimulation: 20 Hz elicits 20% of maximum, 50 Hz elicits 50%, 100 Hz elicits 100%, etc. With both recruitment and rate coding, the central nervous system now has a wide range of choices about how to make the muscle s total force be 1,500 g. One possibility is to recruit two motor units and stimulate one at 100 Hz and the other at 50 Hz. This results in 1,000 g of force from the first motor unit and 500 g from the second, and a total of 1,500 g. Another option is to recruit all three motor units and regulate stimulation frequency as follows: Motor Unit Hz; Motor Unit Hz; Motor Unit Hz. This results in 700 g of force from Motor Unit 1, 500 g from Unit 2, and 300 g from Unit 3, giving the total of 1,500 g needed. 11 U3L4P11 10 of 29 5/17/2001 3:44 PM

11 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) I hope the example I have used aids understanding of how recruitment and rate coding combined allow the central nervous system to regulate total force output of a muscle organ quite precisely, to increase the chances of it being most effective. Now let me point out how the example was overly simplistic. Some of this should be obvious. Whole muscles have a lot more than three motor units; typical muscles have hundreds to thousands of motor units, depending primarily on the size of the muscle organ. All motor units in a muscle are not identical. In fact, no two motor units are identical. Recall that all muscle fibers in a given motor unit are very similar. But motor units differ in (a) size (i.e., number of fibers), (b) types of muscle fibers (i.e., types of motor units), and (c) range of frequencies of stimulation. To generalize, on one extreme, small motor units have slow muscle fibers and low frequencies of stimulation. Stimulation at 20 Hz elicits maximal tetanic tension in many of these motor units. At the other extreme, large motor units have fast muscle fibers, and high frequencies of stimulation (e.g., 50 Hz) are needed to elicit maximal tetanic tension. 12 U3L4P12 IN-LINE QUIZ: TRUE-FALSE? A large motor unit with type IIb muscle fibers has a higher maximal tetanic tension than a small motor unit with type I muscle fibers. TRUE The key to force development is active cross-sectional area (i.e., number of myosin-actin attachments in parallel). The larger motor unit will have greater active cross-sectional area because (a) it has more fibers, (b) its type II fibers will have larger diameter than the type I fibers of the small motor unit, and (c) its type IIb fibers will have less mitochondrial mass and therefore more myofibrillar material per unit of cross-sectional area. 13 U3L4P13 11 of 29 5/17/2001 3:44 PM

12 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) Let s examine the related roles of recruitment and rate coding a little further, including applications to specific exercise examples. In most circumstances, the central nervous system recruits motor units in a muscle organ in a very regular order in relation to total force output. For example, if the regular order of recruitment of Motor Units A, B and C in a muscle is A first, then B, and C last, this A-B-C order will almost always occur in response to increasing force output. There may be an exception, however. If A and B are slow motor units and C is a fast motor unit, C may be recruited first IF the required movement is very rapid and does not require high force. Recruitment order normally follows a size principle, so that smaller motor units are recruited first and then larger units as more force is needed. (Actually, it s the size of the cell body of the motor neuron that relates to the size principle, but the result is as just stated.) Since small motor units usually are made up of type I muscle fibers and large motor units by type II muscle fibers, as a general principle, slow motor units are recruited when low muscle force is needed and fast motor units are added as more muscle force is needed. 14 U3L4P14 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) Can all of the motor units in a muscle be recruited? This is an important question related to maximal force output of a muscle. As a generalization, a strength-trained person can recruit all the motor units in most muscles. Apparently one effect of strength training is an enhanced ability of the central nervous system to recruit motor units. That is, before training, the central nervous system cannot recruit some motor units. But as a result of the heavy resistance overload of training, the central nervous system learns to recruit these motor units. This probably happens by way of altering inhibitory factors, as follows. In untrained persons, some motor neurons are greatly inhibited by various nervous input (perhaps as a mechanism of protecting relatively weak muscle). As a result, it is very difficult to activate (recruit) these motor neurons and motor units. Then, with strength training, these inhibitory inputs are reduced, making the motor units easier to recruit. 15 U3L4P15 12 of 29 5/17/2001 3:44 PM

13 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) What is the interrelationship between recruitment and rate coding in muscle force development over the entire range of force outputs? In some muscles, rate coding is a more important mechanism for regulating force than recruitment is, and in other muscles recruitment is more important. As a general principle, rate coding is the predominant mechanism of force regulation in muscles that are involved in very intricate movements with relatively low forces. Muscles of the eyes and fingers, for example, are regulated by rate coding far more than muscles such as the quadriceps femoris, gastrocnemius, and biceps femoris. Tiny changes in force output of the surgeon s hand and finger muscles are controlled predominantly by rate coding. Rate coding is very important when a golfer strokes the delicate 4-foot putt on a downhill and sideward slope. 16 U3L4P16 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) The relative roles of recruitment and rate coding in most skeletal muscles (with the exceptions noted in the previous paragraph) follow the S-shaped pattern illustrated in the figure. At both low levels and high levels of total force output (Categories I and III in the figure), force output is controlled more by rate coding than by recruitment. At intermediate levels of force output (Category II in the figure), control is more by recruitment than by rate coding. In fact, it is likely that all motor units are recruited in some muscles of some people at forces below the muscle s maximum force output. And then further increase in force to maximum is entirely the result of rate coding. 13 of 29 5/17/2001 3:44 PM

14 17 U3L4P17 - Mechanisms for Regulating Active Cross-Sectional Area (cont.) Let me give an example involving an ordinary exercise activity, as reported by Vollestad and Blom (Acta Physiologica Scandinavica 125: , 1985). During cycling on a bicycle ergometer with progressively increasing power output, about 40% of all motor units in the vastus lateralis muscle (one of the quadriceps muscles of the thigh) are recruited by 43% VO2max and 80-85% are recruited by 75% VO2max. Above 75% VO2max, the remaining motor units are recruited; this is true even though less than 50% of maximal voluntary force is developed during cycling at the lowest power that elicits VO2max. So, in this specific exercise, recruitment appears to be the major mechanism for increasing force development by this muscle. Since a person can cycle at power values much greater than that at which VO2max is first achieved, additional force output at those higher powers must be achieved via rate coding. It is also interesting to note the recruitment pattern of the different fiber types in response to the progressive cycling exercise studied by Vollestad and Blom. At 43% VO2max, all type I fibers and 20% of type IIa fibers had been recruited. By 75% VO2max, all type IIa fibers were recruited. The remaining unrecruited fibers (type IIb) were recruited only above 75% VO2max. 18 U3L4P18 14 of 29 5/17/2001 3:44 PM

15 - Determinants of Effective Force Output - Length of the Muscle (Ref. Fig in Powers and Howley.) Quite a long time ago, physiologists described the relationship between muscle length and maximal isometric force developed by the muscle. This is referred to as the length-tension relationship. The relationship is very simple: For active force development by a muscle, there is an intermediate length that is optimal; maximal active isometric force is less than maximum at shorter lengths and at longer lengths. This optimal length is abbreviated LO. There is a straightforward anatomical reason for this length-tension relationship: The amount of overlap of thick and thin filaments within sarcomeres determines the potential number of myosin and actin binding sites. If the sarcomere length is too short or too long, potential binding sites are lost. As a result, the potential number of parallel myosin-actin attachments is less than optimal, and the potential active cross-sectional area is reduced. 19 U3L4P19 - Length of the Muscle (cont.) How do we know what LO is in intact muscles in the body? This is rather straightforward too. LO is the normal resting length of a muscle, that is, the length that the muscle is at most of the time. You may recall that I mentioned earlier that muscle fibers change in length in response to growth, fixation at a new length, or the like by adding or subtracting sarcomeres at the ends of the fibers. It does this rather than changing the lengths of individual sarcomeres, so sarcomeres stay at or near lengths that provide optimal overlap of thick and thin filaments. A muscle will thus adapt to keep sarcomeres at optimal lengths when the muscle is at its normal resting length. 20 U3L4P20 15 of 29 5/17/2001 3:44 PM

16 - Length of the Muscle (cont.) So, maximal active tension in a given muscle is achieved at the muscle s normal resting length. What about movements that require us to contract muscles when they are at other lengths? Is strength reduced at those lengths, since the muscle is either shorter or longer that LO? Two considerations are important here. The first consideration is that most muscles in the body do not change in length very much even at the extremes. There are two anatomical reasons for this: (a) The skeleton and joint structures normally limit range of motion. The elbow flexor muscles, for example, can only lengthen so far because of the limits of the elbow joint for extension. (b) Most muscles in humans are attached very close to joint axes of rotation. Consider the elbow flexor muscles that attach just distal to the elbow joint. Over the complete range of elbow joint motion, the point of attachment does not move far. As a result of these anatomical limitations, the range of length changes in most muscles is no more than 15% in one direction or the other from LO. Therefore, most sarcomeres are always at or near lengths that provide optimal overlap of myofilaments. Even at an extreme length, the potential for myosin-actin attachments is near maximum, so maximal active isometric force is not greatly reduced. The second consideration related to the question about strength changes with changes in muscle length is that I have only dealt with active force development thus far (i.e., the force that is generated by myosin molecules pulling on actin molecules). There can also be passive force added to active force. By passive force I mean the elastic force that is generated by various molecules, membranes and tissues in the whole muscle. This elastic force is quite low at LO. But elastic force becomes greater in proportion to increases in a muscle s length beyond LO. So, at longer muscle lengths, there may be slight reduction of maximal active force, but this is offset by increased elastic force. In fact, total force (active + passive) may be highest when a muscle contracts at lengths longer than LO. 21 U3L4P21 16 of 29 5/17/2001 3:44 PM

17 - Length of the Muscle (cont.) Let me summarize the major points regarding effect of muscle length on effective force output: Maximal active force development is greatest at LO. LO is the normal resting length of a muscle. Human anatomy limits extremes of length changes in most muscles so that muscles are usually contracting at or near LO. At muscle lengths longer than LO, elastic force is added to the active force. As a result, total tension may be greatest at lengths longer than LO. Part of the effect of muscle length on force output relates to active cross-sectional area (parallel myosin-actin attachments) and part does not (elastic force). 22 U3L4P22 - Determinants of Effective Force Output - Force-Velocity Relationship (Ref. Fig and 8.20 in Powers & Howley) Another variable that often determines the effectiveness of a muscle s force output is the velocity or speed of the contraction. In physiology, this effect is described by what is referred to as the force-velocity relationship, or sometimes the load-velocity relationship. Anyone who has ever tried to lift a weight or move a load rapidly knows from experience that the heavier the weight or load, the more difficult it is to move it rapidly. In fact, the maximal rate of moving a resistive load decreases as the weight of the load increases. For example, if I gave you a 2-pound dumbbell and asked you to lift it as fast as you can with an arm curl, you could lift the weight very quickly. If I then asked you to lift a 10-pound weight as rapidly as possible, you could probably still lift it rapidly, but not as rapidly as you could curl the 2-pound weight. If we continued to do this (you lift and I tell you what to do!), I would eventually find a weight that you could just barely lift in an arm curl. You could lift it but only very slowly. Another common example is the choice of a baseball or softball bat. IF everything else is equal, the heavier the bat the better. But, everything else is not equal. Velocity of swinging the bat decreases as bat weight increases. So the challenge is to select the right combination of bat weight and velocity of swing. (Actually, this relates to power, which we will discuss later.) 17 of 29 5/17/2001 3:44 PM

18 (Actually, this relates to power, which we will discuss later.) Let s examine this relationship between force and velocity in more detail to understand the underlying physiology, and to expand on the application of this relationship. 23 U3L4P23 - Force-Velocity Relationship (cont.) The basic force-velocity relationship that applies to skeletal muscle is demonstrated in a series of experiments on isolated muscles. A muscle is externally and maximally stimulated so that it shortens (contracts concentrically) as rapidly as it possibly can with a known weight attached to it. Such maximal velocities of shortening are measured with different weights attached to the muscle. The findings of such a series of experiments are summarized in the figure. Let me emphasize that every point on the curve (with one exception) is a maximal velocity of shortening for the given load that the muscle must lift (or force that the muscle must develop). Of course, the muscle could lift any of the loads at a slower speed than maximum (i.e., there is an infinite number of points below the curve), but it could not lift any load faster than this maximum velocity. Note first the two extreme points on the curve. When the load is zero (i.e., no external weight lifted by the muscle), the muscle can shorten at its highest velocity. This highest velocity of shortening is termed Vmax. At the other extreme of the curve, velocity is zero and the force is the highest on the graph. 18 of 29 5/17/2001 3:44 PM

19 24 U3L4P24 IN-LINE QUIZ What kind of contraction is it when velocity is zero? (a) concentric; (b) isometric; (c) eccentric; (d) isokinetic. Correct answer: (b) isometric. Explain others 25 U3L4P25 - Force-Velocity Relationship (cont.) The point on the extreme right of the curve represents a maximal isometric contraction. This load, or the force developed by the muscle, is higher than the force during any concentric contraction. This maximal isometric force is abbreviated PO or FO. (Recall that the absolutely highest active force that a muscle can develop in an isometric contraction occurs at LO, the muscle length associated with maximal overlap of myofilaments.) It is very important to note that the force-velocity relationship described by the graph above ignores eccentric muscle contractions. I will address the relationship during eccentric contractions later, but for now I want to focus on concentric contractions plus the one point on the graph that represents isometric contractions. To reiterate, at one end of the force-velocity curve is Vmax, when load = 0, and at the other end is PO, when velocity = 0. In between, the relationship is curvilinear: maximal velocity of shortening falls relatively rapidly as load increases above zero (i.e., first portion of curve), and with relatively heavy loads, velocity of shortening increases relatively little as force is reduced from maximal isometric force. 26 U3L4P26 - Force-Velocity Relationship (cont.) 19 of 29 5/17/2001 3:44 PM

20 Let s look at the physiological explanations for this relationship. The point on the force-velocity curve that represents zero load is a theoretical one in terms of muscle activity in the body. Remember that the data the curve describes are from an isolated muscle, taken out of the body, and the muscle is externally stimulated with various weights hanging on it. The zero load means no external weight is hung on the muscle. In the intact body, there is never zero load on muscle. Even if there is no weight in the hand during an arm curl, for example, there is still the weight of the forearm and hand, as well as the resistance of the joint tissues and the muscles and connective tissue that oppose elbow flexion. Granted, these resistive loads are small, but they are definitely not zero. So, the zero-load condition is theoretical, which means we must use our imagination. What if a muscle had absolutely no resistance How much force would the muscle have to develop to shorten? I hope you answered, Not very much. In fact, in this theoretical situation, the force of only a single myosin molecule pulling on an actin would be enough to cause the muscle to shorten! (Remember, there is no force at all resisting.) So, what determines what the value is of this zero-load maximal velocity of shortening? On the graph shown earlier, this value is 9 (arbitrary units). Why 9, and not 10 or 8 or 4? Velocity of shortening is primarily determined by the type of myosin. (Remember fast-twitch and slow-twitch?) Following is a graph of the force-velocity relationship for two hypothetical muscles that are identical in every way except that one has only fast myosin and the other has only slow myosin. Note the difference between the two muscles in Vmax. At this theoretical point when there is no resistive load, a single fast myosin molecule is cycling rapidly (i.e., high Vmax) and the slow myosin is cycling slowly (i.e., slow Vmax). In actual human muscles, rarely if ever would a muscle have only slow or fast fibers. Nevertheless, Vmax can be two- to three-times greater in a predominantly fast muscle than in a predominantly slow muscle. This represents a huge advantage in terms of speed of movement. 20 of 29 5/17/2001 3:44 PM

21 27 U3L4P27 - Force-Velocity Relationship (cont.) Now notice the other end of the force-velocity curves of the two muscles. The PO values are the same. How can this be? What determines maximal isometric force? It s the maximal number of parallel myosin-actin attachments (active cross-sectional area). And in this example, I said these two muscles were alike in every way except in type of myosin. This includes having the same number of myosin-actin attachments in parallel during maximal stimulation. Fast and slow myosin differ greatly in velocity of crossbridge cycling but probably not in ability to generate force. Therefore, the two muscles can generate the same maximal isometric force. Now let s compare the muscles at the points in between the extremes. At every load below PO, the maximal velocity of shortening of the fast muscle is greater than the maximal shortening velocity of the slow muscle. If the attached myosins can overcome the resistive load, the fast myosin is able to cycle more rapidly and therefore move the load more rapidly than the slow myosin can. Is a stronger muscle also a faster muscle? To answer this question, let s consider two other hypothetical muscles that are alike in every way except in maximal active cross-sectional area (i.e., one is stronger than the other). Before going to the next page, picture in your mind what the force-velocity relationships of these two muscles look like graphically. Remember that the muscles are alike in every way (e.g., same proportion of fast and slow myosin) except that one is stronger than the other. 28 U3L4P28 21 of 29 5/17/2001 3:44 PM

22 - Force-Velocity Relationship (cont.) The graph illustrates the force-velocity relationship for each of the muscles. Once again we have two lines that share only one point. But this time the shared point is at Vmax. Vmax is determined by type of myosin, and these muscles are composed of the same myosin. The basic difference between the two muscles is in PO. The stronger muscle (i.e., the one with a greater active cross-sectional area) has a larger PO. Now, as we did with another example before, consider all of the points on the curves between the extremes. At every load (except zero), the stronger muscle has the ability to move the load more rapidly. Even though the stronger muscle has no inherent advantage in terms of velocity based on type of myosin, by having more parallel myosin-actin attachments pulling on a given resistive load, the load for a given crossbridge is relatively less (compared to the weaker muscle). Thus, the force opposing crossbridge cycling is relatively less and cycling can go more rapidly. 29 U3L4P29 22 of 29 5/17/2001 3:44 PM

23 - Force-Velocity Relationship (cont.) In summary, ability to move a load rapidly depends both on the Vmax and on the PO of the muscles involved. Vmax depends on the relative amounts of fast and slow myosin in the muscles. This is to a very large extent determined by genetics. It may be changeable through training, but is probably very difficult to do. PO depends on the maximal active cross-sectional area. This can certainly be changed with training. So, from the physiological perspective, even if Vmax does not change with training, velocity of movement can be improved with training that improves muscular strength. 30 U3L4P30 - Force-Velocity Relationship (cont.) The force-velocity curves we have examined have been based on data derived from isolated muscles, outside of the body. How do they relate to muscles in the intact body? The curves derived on isolated muscles apply to muscles in the body except for three considerations. One consideration, alluded to previously, is the theoretical zero-load condition. If we consider normal, healthy young adults, the lowest load is probably about 5-10% of PO for a strong person and perhaps 10-20% of PO for a weaker person, depending on the muscle group and movement involved. This lowest load could be much higher, even close to 100% of PO, in persons with (neuro-)muscular diseases, such as muscular dystrophy, or Lou Gehrig s Disease, or with severe atrophy due to aging. Whatever this percentage is, that equivalent load or muscular force would set the beginning operational point on the force-velocity curve for the given person. The points on the curve to the left of this minimum load would not apply. The second way the theoretical curve does not apply precisely to intact muscle has to do with the way velocity is measured. Velocity = distance / time. In experiments with isolated muscles, velocity is measured as the distance the isolated muscle shortens per unit of time. This is extremely difficult to measure in intact muscles. Furthermore, our interest in terms of velocity is usually how fast an object such as a weight or ball or implement is moved, rather than the muscle s velocity per se. We could, for example, measure fairly easily the speed at which a weight is lifted a given height in an arm curl or bench press. And we could do this with different weights and note an inverse relationship between external load and velocity of movement. But because of the lever systems in the body, such measurements of velocity are usually not rectilinearly related to velocity of shortening of the muscles involved as measured in isolated muscles. 23 of 29 5/17/2001 3:44 PM

24 the muscles involved as measured in isolated muscles. 31 U3L4P31 - Force-Velocity Relationship (cont.) The third consideration in comparing or contrasting the theoretical force-velocity relationship in isolated muscle vs. the relationship in the intact body is that the isolated muscle studies do not involve the nervous system. The isolated muscle is externally stimulated to develop maximal force. Intact muscle obviously does involve the nervous system, and contraction normally occurs only in response to impulses from the central nervous system. It is possible, of course, that the amount of force developed and/or the velocity of contraction could be limited by nervous stimulation. We could measure force (torque) and velocity data during a given movement in the intact body, such as during knee extensions on an isokinetic dynamometer. If we did, we could not be sure whether the maximal efforts were limited by intrinsic characteristics of the muscles (e.g., type of myosin, or active cross-sectional area) or by the nervous system activation of the muscles. There are experimental methods that can be used to sort these out, but they are not routine measurements. Let me pose one application related to this. Let s assume that maximal efforts during measurement of the force-velocity relationship in intact muscle groups were limited by nervous system stimulation of the muscles involved. In this case, the force-velocity curve would not represent the maximal velocities of shortening of the muscles at given loads. In fact, the muscles could actually shorten more rapidly, but the nervous system is not activating optimally. There are some training techniques that have been used in athletics, aimed at increasing speed of movement, that force movement to be faster than the athlete can do voluntarily. Examples have included sprinting down an incline or while attached to an elastic cord that pulls the runner. I am not aware of scientific research evidence supporting the benefit of such training techniques, but there is much testimonial evidence. From the physiological perspective, it is theoretically possible that such forcing of muscles to shorten at rates faster than they can shorten voluntarily may shift at least portions of the force-velocity curve upward. And this could be the result of changes in nervous system activation of muscles. 32 U3L4P32 24 of 29 5/17/2001 3:44 PM

25 - Force-Velocity Relationship (cont.) In spite of the differences between conditions of muscle in the intact body and conditions of isolated muscles, the basic force-velocity relationship still applies to intact muscle, and the underlying physiology is the same. In practice, if the goal is maximal force development (e.g., lifting the heaviest weight possible), the velocity of movement must necessarily be low. If the goal is speed of movement, the load must be low. Maximal velocity of moving a load can be improved by increasing the active cross-sectional area at a given load (which can result either from muscle enlargement or from better nervous system activation of motor units) or by increasing the ratio of fast myosin-to-slow myosin in the muscle. 33 U3L4P33 - Force-Velocity Relationship (cont.) Now let s shift to consideration of power. Very often in athletics, power is the critical factor in successful performance. Power is a combination of force developed and speed of movement. In fact, multiplying force (in pounds, for example) by speed (in feet per second, for example) gives power (in foot-pounds per second). Since every point on a force-velocity curve has a force value and a velocity value associated with it, maximal power can be calculated for each load. When this is done, a relationship between maximal power and load is derived as described by the following curve. The most important point that this curve shows is that maximal power can be generated when the external load or muscular force is about 30% of PO. We know that maximal velocity of shortening is greater with loads lighter than 30% of 25 of 29 5/17/2001 3:44 PM

26 velocity of shortening is greater with loads lighter than 30% of PO, and muscular forces obviously can be much greater, but with sacrifice of velocity. The optimal combination of load and velocity for generation of power, however, is with a load of about 30% of PO. In normal, healthy young adults, the resistive load of limbs, joint structures, forces exerted by antagonistic muscles, and the like without adding any external loads may be 5-20% of PO. Therefore, maximal power is achieved with most muscles in the intact body when only small external resistive loads are applied. Unfortunately, there is no simple way to predict exactly what such loads should be. But in an activity such as softball or baseball batting, or swinging a golf club, weight of the bat or club must be carefully chosen to optimize power (but with consideration of all other factors that could affect performance too). 34 U3L4P34 - Force-Velocity Relationship (cont.) Up till now, our consideration of the force-velocity relationship has dealt only with concentric and isometric muscle contractions, and has ignored eccentric contractions. In athletic performance, the application of force and development of speed and power during concentric contractions are usually the critical determinants of successful performance. So, the concentric portion of the force-velocity relationship has greatest application to athletic performance. Nevertheless, there is a very important point that must be recognized regarding eccentric contractions. I will deal with that now. If you look back at any of the force-velocity curves we studied above, do you see how the graph would have to be extended to include eccentric contractions? The Y-axis would have to be extended downward to include velocities less than zero, that is, negative velocities. If positive velocities involve shortening of muscles, lengthening of muscles involves negative velocities. Furthermore, in eccentric contractions, the resistive load is greater than the muscular force (which forces the muscle to lengthen). Therefore, when maximal contractions are considered, loads that cause eccentric contractions are greater than PO. So the typical force-velocity curve would have to be extended as follows to include eccentric contractions. 26 of 29 5/17/2001 3:44 PM

27 The key summary point is that forces within muscles can be very high during eccentric contractions, higher than during concentric or static contractions. (And resistive forces are always greater than active muscular force, which is what makes the muscle lengthen.) Some of the muscular force is, of course, active force developed by myosin molecules. But in eccentric contractions, much of the force is due to elasticity of various components of the muscle as they are forced to lengthen. In terms of practical application, the potential for injury to muscle is higher during eccentric contractions than during concentric or static contractions. 35 U3L4P35 - Force-Velocity Relationship (cont.) Let me present another simplistic example to hopefully clarify some of the points discussed in this section. Let s assume a person is doing arm curl exercises that stress the elbow flexor muscles. Assume also that the maximal weight this person can hold completely still (i.e., static contraction) with the elbow angle at 90 degrees is 25 kg. The person cannot lift 25 kg; he/she can only hold it in the same position (though probably not for long). After resting, this person could lift 24.5 kg. This would be the maximal concentric 1-RM load. The velocity of this lift would be very slow. After another rest period, another person places a 26-kg weight in the person s hand with the elbow at 90 degrees, and the person tries to exert maximal force with the elbow flexor muscles. What happens? The weight would overcome the muscle s active force, causing the elbow to extend and the flexor muscles to lengthen. The weight would lower slowly, because the active muscle force is close to the resistive force. Imagine repeating this with heavier and heavier weights, with the person exerting maximal effort each time trying to keep the weight from lowering. The velocity at which the weight lowers would increase with each increase in weight. Let me add one final absurd scenario to emphasize a point. Imagine that a 200-kg weight is put in the person s hand and he/she exerts maximal effort trying to resist its lowering. Of course, it would lower very rapidly. More importantly, the tension in the muscle during this lowering would be very high (close to 200 pounds!), much higher than the active muscular force. Damage to muscle and connective tissue would be very likely. 36 U3L4P36 27 of 29 5/17/2001 3:44 PM