Chapter 6 The Biomechanics of Human Skeletal Muscle

Behavioral Properties of the Musculotendinous Unit 1) extensibility: ability to be stretched or to increase in length 2) elasticity: ability to return to normal resting length following a stretch Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -2

Behavioral Properties of the Musculotendinous Unit Components of elasticity: • parallel elastic component - passive elasticity derived from muscle membranes • series elastic component - passive elasticity derived from tendons when a tensed muscle is stretched Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -3

Behavioral Properties of the Musculotendinous Unit Parallel Elastic Component Contractile Component Series Elastic Component From a mechanical perspective, the musculotendinous unit behaves as a contractile component (muscle fibers) in parallel with one elastic component (muscle membranes) and in series with another elastic component (tendons). Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -4

Behavioral Properties of the Musculotendinous Unit What is the stretch-shortening cycle? • eccentric contraction (in which the muscle is actively stretched) followed immediately by concentric contraction • Can you think of examples? Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -5

Behavioral Properties of the Musculotendinous Unit 3) irritability: ability to respond to a stimulus 4) ability to develop tension: the contractile component of muscle function Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -6

Structural Organization of Skeletal Muscle What is a muscle fiber? (single muscle cell surrounded by a membrane called the sarcolemma and containing specialized cytoplasm called sarcoplasm) Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -7

Structural Organization of Skeletal Muscle Sarcolemma Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -8

Structural Organization of Skeletal Muscle What do we know about muscle fibers? • some fibers run the entire length of a muscle; others are shorter • skeletal muscle fibers grow in both length and diameter from birth through adulthood • fiber diameter can be increased through resistance training Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -9

Structural Organization of Skeletal Muscle Sarcomere The sarcomere is the basic structural unit of the muscle fiber. The alternating dark and light bands give muscle its striated appearance. The A bands contain thick, rough myosin filaments surrounded by six thin, smooth actin filaments. The I bands contain only thin actin filaments. Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -10

Structural Organization of Skeletal Muscle What is a motor unit? • single motor neuron and all fibers it innervates • considered the functional unit of the neuromuscular system Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -11

Structural Organization of Skeletal Muscle FT ST Twitch Tension Fast twitch (FT) fibers both reach peak tension and relax more quickly than slow twitch (ST) fibers. (Peak tension is typically greater for FT than for ST fibers. ) Time Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -12

Skeletal Muscle Fiber Characteristics TYPE IIA Fast-Twitch Oxidative Glycolytic (FOG) fast CHARACTERISTIC Contraction Speed Type I Slow-Twitch Oxidative (SO) slow Fatigue rate slow intermediate fast Diameter small intermediate large ATPase concentration low high Mitochondrial concentration Glycolytic enzyme concentration high low intermediate high Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. Type IIB Fast-Twitch Glycolytic (FG) fast 6 -13

Structural Organization of Skeletal Muscle How are muscle fibers organized? • parallel fiber arrangement: fibers are roughly parallel to the longitudinal axis of the muscle; examples are? • pennate fiber arrangement: short fibers attach to one or more tendons within the muscle; examples? Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -14

Structural Organization of Skeletal Muscle Parallel fiber arrangements Pennate fiber arrangements Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -15

Structural Organization of Skeletal Muscle The angle of pennation increases as tension progressively increases in the muscle fibers. Relaxed With tension development Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -16

Skeletal Muscle Function How are motor units (MUs) recruited? • slow twitch (ST) fibers are easier to activate than fast twitch (FT) fibers • ST fibers are always recruited first • increasing speed, force, or duration of movement involves progressive recruitment of MUs with higher and higher activation thresholds Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -17

Skeletal Muscle Function What terms are used to describe muscle contractions based on change in muscle length? • concentric: involving shortening • eccentric: involving lengthening • isometric: involving no change Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -18

Skeletal Muscle Function What roles are assumed by muscles? • agonist: acts to cause a movement • antagonist: acts to slow or stop a movement • stabilizer: acts to stabilize a body part against some other force • neutralizer: acts to eliminate an unwanted action produced by an agonist Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -19

Skeletal Muscle Function What are disadvantages associated with muscles that cross more than one joint? • active insufficiency: failure to produce force when slack • passive insufficiency: restriction of joint range of motion when fully stretched Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -20

Skeletal Muscle Function active insufficiency: failure to produce force when muscles are slack (decreased ability to form a fist with the wrist in flexion) Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -21

Skeletal Muscle Function passive insufficiency: restriction of joint range of motion when muscles are fully stretched (decreased ROM for wrist extension with the fingers extended) Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -22

The force-velocity relationship for muscle tissue: When resistance (force) is negligible, muscle contracts with maximal velocity. Force Factors Affecting Muscular Force Generation (Low resistance, high contraction velocity) Velocity Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -23

Factors Affecting Muscular Force Generation isometric maximum Force The force-velocity relationship for muscle tissue: As the load increases, concentric contraction velocity slows to zero at isometric maximum. Velocity Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -24

Factors Affecting Muscular Force Generation Total Tension The length-tension relationship: Tension present in a stretched muscle is the sum of the active tension provided by the muscle fibers and the passive tension provided by the tendons and membranes. Active Tension Passive Tension 50 100 150 Length (% of resting length) Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -25

Factors Affecting Muscular Force Generation What is electromechanical delay? Myoelectric activity Force Stimulus Electromechanical delay (time between arrival of a neural stimulus and tension development by the muscle) Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -26

Muscular Strength, Power and Endurance How do we measure muscular strength? (the amount of torque a muscle group can generate at a joint) Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -27

Muscular Strength, Power and Endurance How do we measure muscular strength? Ft Ft The component of muscle force that produces torque (Ft) at the joint is directed perpendicular to the attached bone. Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -28

Muscular Strength, Power and Endurance What factors affect muscular strength? • tension-generating capability of the muscle tissue, which is in turn affected by: • muscle cross-sectional area • training state of muscle Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -29

Muscular Strength, Power and Endurance What factors affect muscular strength? • moment arms of the muscles crossing the joint (mechanical advantage), in turn affected by: • distance between muscle attachment to bone and joint center • angle of the muscle’s attachment to bone Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -30

Skeletal Muscle Function Torque produced by a muscle (Tm) at the joint center of rotation is the product of muscle force (Fm) and muscle moment arm (d ). Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -31

Muscular Strength, Power and Endurance A B C The mechanical advantage of the biceps bracchi is maximum when the elbow is at approximately 90 degrees (A), because 100% of muscle force is acting to rotate the radius. As the joint angle increases (B) or decreases (C) from 90 degrees, the mechanical advantage of the muscle is lessened because more and more of the force is pulling the radius toward or away from the elbow rather than contributing to forearm rotation. th Basic Biomechanics, 6 edition By Susan J. Hall, Ph. D. 6 -32

Muscular Strength, Power and Endurance What is muscular power? • the product of muscular force and the velocity of muscle shortening • the rate of torque production at a joint • the product of net torque and angular velocity at a joint Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -33

Muscular Strength, Power and Endurance Power Force-Velocity Power-Velocity The general shapes of the force-velocity and power-velocity curves for skeletal muscle. Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -34

Muscular Strength, Power and Endurance What is muscular endurance? • the ability of muscle to exert tension over a period of time • the opposite of muscle fatigability Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -35

Muscular Strength, Power and Endurance What is the effect of muscle temperature on (warm up) ? (the speeds of nerve and muscle functions increase) Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -36

Muscular Strength, Power and Endurance Normal body temperature Elevated body temperature Force With warm-up, there is a shift to the right in the force-velocity curve, with higher maximum isometric tension and higher maximum velocity of shortening possible at a given load. Velocity Basic Biomechanics, 6 th edition By Susan J. Hall, Ph. D. 6 -37