© 2007 McGraw-Hill Higher Education. All rights reserved. Basic Biomechanics, (5th edition) by Susan J. Hall, Ph.D. Chapter 6 The Biomechanics of Human Skeletal Muscle

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. Behavioral Properties of the Musculotendinous Unit Parallel Elastic Component Series Elastic Component Contractile 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).

© 2007 McGraw-Hill Higher Education. All rights reserved. What is the stretch-shortening cycle? eccentric contraction (in which the muscle is actively stretched) followed immediately by concentric contraction examples? Behavioral Properties of the Musculotendinous Unit

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. 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)

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. Structural Organization of Skeletal Muscle 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.) Twitch Tension Time FT ST

© 2007 McGraw-Hill Higher Education. All rights reserved. Skeletal Muscle Fiber Characteristics TYPE IIA Type I Fast-Twitch Type IIB Slow-Twitch Oxidative Fast-Twitch Oxidative Glycolytic Glycolytic CHARACTERISTIC(SO)(FOG)(FG) Contraction Speedslowfast Fatigue rateslowintermediatefast Diametersmallintermediatelarge ATPase concentrationlowhigh Mitochondrialhigh low concentration Glycolytic enzymelow intermediatehigh concentration

© 2007 McGraw-Hill Higher Education. All rights reserved. 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?

© 2007 McGraw-Hill Higher Education. All rights reserved. Structural Organization of Skeletal Muscle Parallel fiber arrangements Pennate fiber arrangements

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. Skeletal Muscle Function active insufficiency: failure to produce force when muscles are slack (decreased ability to form a fist with the wrist in flexion)

© 2007 McGraw-Hill Higher Education. All rights reserved. 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)

© 2007 McGraw-Hill Higher Education. All rights reserved. Factors Affecting Muscular Force Generation The force-velocity relationship for muscle tissue: When resistance (force) is negligible, muscle contracts with maximal velocity. Velocity Force (Low resistance, high contraction velocity)

© 2007 McGraw-Hill Higher Education. All rights reserved. Factors Affecting Muscular Force Generation The force-velocity relationship for muscle tissue: As the load increases, concentric contraction velocity slows to zero at isometric maximum. Velocity Force isometric maximum

© 2007 McGraw-Hill Higher Education. All rights reserved. Factors Affecting Muscular Force Generation 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. Tension Length (% of resting length) Active Tension Passive Tension Total Tension

© 2007 McGraw-Hill Higher Education. All rights reserved. Factors Affecting Muscular Force Generation What is electromechanical delay? ( time between arrival of a neural stimulus and tension development by the muscle) Myoelectric activity Force Electromechanical delay Stimulus

© 2007 McGraw-Hill Higher Education. All rights reserved. Muscular Strength, Power and Endurance How do we measure muscular strength? (the amount of torque a muscle group can generate at a joint)

© 2007 McGraw-Hill Higher Education. All rights reserved. Muscular Strength, Power and Endurance How do we measure muscular strength? The component of muscle force that produces torque (F t ) at the joint is directed perpendicular to the attached bone. FtFt FtFt

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. Muscular Strength, Power and Endurance 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. C AB

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. Muscular Strength, Power and Endurance The general shapes of the force-velocity and power-velocity curves for skeletal muscle. Force Velocity Power Power-Velocity Force-Velocity

© 2007 McGraw-Hill Higher Education. All rights reserved. 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

© 2007 McGraw-Hill Higher Education. All rights reserved. Muscular Strength, Power and Endurance What is the effect of muscle temperature (warm up) ? (the speeds of nerve and muscle functions increase)

© 2007 McGraw-Hill Higher Education. All rights reserved. Muscular Strength, Power and Endurance 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 Force Normal body temperature Elevated body temperature

© 2007 McGraw-Hill Higher Education. All rights reserved. Chapter 6 The Biomechanics of Human Skeletal Muscle