BASIC PROPERTIES OF MUSCLE

Transcription

1 BASIC PROPERTIES OF MUSCLE 18-1 Lecture Overview Muscles and motion Muscles Muscle structure Relevant properties Force-length properties Muscle states Force-velocity relationship Muscle fiber types Isometric contraction Tendon (series elasticity) Tendon properties Activation Summary Review questions Why? Muscles are responsible for (most) of our motion.

2 MUSCLES AND MOTION 18-2 Newton s First Law If no external forces act on a body then the velocity of that body remains constant. Muscle exert forces on our skeletal system so produce movement. Resultant Joint Moment The net effect of the moments about a joint due to all structures crossing that joint, including muscles, ligaments, and bone forces. MJ = NM rmi i= 1.FMi + NL NC rli.fli + rci i= 1 i= 1. FCi Normally simplified to M J r = NM i= 1 Mi. F Mi

3 MUSCLES AND MOTION 18-3 Moment produced by a given muscle is a function of the muscle force and the moment arm of the muscle. F r T moment = moment arm of force x muscle force T = r. F The moment arms of muscles ( r ), vary with the joint angle, but what does the muscle force depend upon?

4 MUSCLES 18-4 The force produced by a muscle model ( F m) can be described using the following equation Where F = a. Fmax. F. m f L ( L ) F ( V ) f V f a f - normalized degree of activation of muscle fibers. F max - maximum isometric force muscle can produce. ( ) F L L f - normalized force length relationship of muscle, ( ) F V V f - normalized force-velocity relationship of muscle. Look at structure to understand the sources of these properties.

5 MUSCLE STRUCTURE 18-5 Myosin - protein forming thick part of myofibril Actin - protein forming thin part of myofibril Myofibril Muscle Muscle Fiber Bundle Single Muscle Fiber At the myofibril level it is the interaction of actin and myosin which generates force. (Cross-bridges.) The more of these proteins the higher the force which can be generated (bigger muscles produce more force). F = a. Fmax. F. m f L ( L ) F ( V ) f F αcsa max V f

6 MUSCLE STRUCTURE 18-6 At the simplest level muscle is assumed to have the following structure. Tendon Tendon Muscle Fibers Muscle-tendon complex the assumption is that there is no transition from tendon to muscle fibers.

7 RELEVANT PROPERTIES 18-7 Muscle Fibers Passive Active Activation Dynamics ( a f ) Different Length F L( L f ) Different Velocities F V ( V f ) Tendon Passive Force/Length Joint Muscle Moment Arms Passive Moment Profile

8 FORCE-LENGTH PROPERTIES 18-8 As the length of the muscle fibers change so does the force they can produce. [1] 3.7 [2] 2.2 [3] [4] Normalized Force Sarcomere Length (µm) Shortening - cross-bridges interfere with one another, force reduced. Lengthening - some cross bridges are too far apart to form, so force is reduced. [CF individual fibers and whole muscle.]

9 MUSCLE STATES 18-9 Isometric a muscle generating force without changing length. Isokinetic a muscle generating force whilst changing length at a constant velocity (a sub-class of isotonic). Isotonic a muscle generating force whilst changing length. Concentric Muscle Action - a muscle shortening to produce force (+tive velocity). Eccentric Muscle Action - a muscle lengthening to yield to a force (-tive velocity).

10 18-10 FORCE-VELOCITY RELATIONSHIP 25 Small Load Degree of Shortening (%) Large Load Time (s) In same time period Smaller load greater shortening Larger load less shortening Load Increases Force Increases Velocity decreases

11 18-11 FORCE-VELOCITY RELATIONSHIP 100% 75% F O R C E 50% Maximum tension 25% - Lengthen Shorten + VELOCITY Limit to maximum velocity of shortening is caused by limit to rate at which cross-bridges can cycle. As velocity increases the force decreases as time for formation of cross-bridges is reduced (concentric phase) As negative velocity increases in magnitude the force increases (eccentric phase).

12 MUSCLE FIBER TYPES Muscle fibers can be divided into two major groups: fast twitch and slow twitch. These fiber types have different histochemical and biochemical profiles. Type I fibers have a long contraction time (slow twitch), are well adapted for aerobic glycolysis. Type II fibers have short contraction times (fast twitch). Type IIa fibers have a high capacity for anaerobic metabolism but also have a capacity for aerobic metabolism. Type IIb fibers also have a high capacity for anaerobic glycolysis and some limited capacity for aerobic metabolism. Faulkner et al. (1986) examining bundles of human muscle fibers found that Type I fibers max. velocity of shortening of 2 fl.s -1. Type II fibers max. velocity of shortening of 6 fl.s -1.

13 ISOMETRIC CONTRACTION Isometric means staying them same length. Contraction means reduction. Question: How can a muscle produce force during an isometric contraction? The muscle must stay the same length BUT the muscle fibers must shorten to produce force.

14 ISOMETRIC CONTRACTION Series Elasticity (tendon) Contratile Element (fibers) No Force Low Force High Force

15 TENDON (SERIES ELASTICITY) When activated muscle fibers develop tension which is transferred to the skeleton via the elastic structures in series with the fibers the tendon. During an isometric contraction, the muscle fibers shorten producing tension, and the tendon stretches under this tension. The net length of the muscle tendon complex stays the same. Tendons are composed mostly of the protein collagen, it is this material which predominantly determines their properties.

16 TENDON PROPERTIES Often assumed to be rigid, but is not, the force exerted on it by the muscle fibers will cause it to stretch. Force Extension Hysteresis - difference between the curves during loading and unloading. This is small for a tendon because it is an efficient energy store.

17 TENDON PROPERTIES The properties of tendon vary from muscle to muscles but as a general Tendon tends to snap when stretched by 8% of its resting length. At maximum isometric force tendon stretched by 4%

18 ACTIVATION The force a muscle produces is modulated in two ways:- recruit more motor units (recruitment) increase the rate of discharge of the already active motor units (rate coding) What is the order of recruitment? Henneman size principle (Henneman et al., 1965) Slow Twitch Fast Twitch Fatigue Resistant Fast Twitch Fatigable

19 ACTIVATION Active state and degree of activation are often used synonymously. Muscle force depends on (using term activation) current level of activation which depends on previous level of activation level of stimulation.

20 ACTIVATION Active state Time (s) In figure maximum activation starts at 0 seconds, and ceases at 1 second. Question: What are nature of time delays? Question: When are these significant?

21 m SUMMARY F = a. Fmax. F. f L ( L ) F ( V ) f V f ) Maximum Isometric Force - proportional to amount of contractile proteins present in the muscle. 2) Force-Length Relationship dependent on overlap of muscle cross-bridges. 3) Force-Velocity Relationship force decreases as muscle shortens at a higher velocity, but as it lengthens (yields) can produce more force. 4) Activation there are significant time delays while muscles develop forces, or cease to develop forces.

22 SUMMARY 18-22

23 REVIEW QUESTIONS ) Explain the source of the maximum isometric force a muscle can produce. 2) What is the shape of the force-length relationship of isolated muscle fibers? What is this caused by? 3) What is the shape of the force-velocity relationship of isolated muscle fibers? What is this caused by? 4) What are the nature of the time delays caused by muscle activation? What is the significance of these delays? 5) What are key properties of tendon? 6) What are isometric, isokinetic, and isotonic contractions? How is it possible to produce an isometric contraction?