Chapter 6 – The Biomechanics of Skeletal Muscle 1. Principal characteristics of skeletal muscle 2. Structural organization of skeletal muscle 3. Fast versus slow twitch motor units 4. Roles assumed by muscles 5. Types of muscular contraction 6. Factors affecting force production

Chapter 6 – The Biomechanics of Skeletal Muscle Four principal characteristics : Four principal characteristics : Excitability – ability to receive and respond to a stimulusExcitability – ability to receive and respond to a stimulus Contractilty (irritability) – ability of a muscle to contract and produce a forceContractilty (irritability) – ability of a muscle to contract and produce a force Extensibility – ability of a muscle to be stretched without tissue damageExtensibility – ability of a muscle to be stretched without tissue damage Elasticity – ability of a muscle to return to its original shape after shortening or extensionElasticity – ability of a muscle to return to its original shape after shortening or extension

Structural organization of skeletal muscle From Principles of Human Anatomy (7 th edition), 1995 by Gerard J. Tortora, Fig 9.5, p 213

6-6 From Basic Biomechanics by Susan Hall (3 rd edition), Fig 6.6, page 153

From Skeletal Muscle: Form and Function (2 nd ed) by MacIntosh, Gardiner, and McComas. Fig 1.4, p. 8.

6-5 From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.5, page 152

Structural organization of skeletal muscle From Principles of Human Anatomy (7 th edition), 1995 by Gerard J. Tortora, Fig 9.5, p 213

6-3 From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.3, page 150

From Exercise Physiology: Theory and Application to Fitness and Performance (6 th Edition) by Scott K. Powers and Edward T. Howley. Fig 8.6 P. 147

A motor unit: single motor neuron and all the muscle fibers it innervates From Basic Biomechanics Instructors manual by Susan Hall (2nd edition, 1995), Fig TM 31

6-7 From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.7, page 154

6-8 From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.8, page 154

Types of muscle fiber: Fast twitch vs Slow Twitch Types of muscle fiber: Fast twitch vs Slow Twitch Type I Type IIa Type IIb Type I Type IIa Type IIb ST Oxidative FT Oxidative - FT Glycolytic ST Oxidative FT Oxidative - FT Glycolytic (S0) Glycolytic (FOG) (FG) (S0) Glycolytic (FOG) (FG) Contraction speed slow fast (2xI) fast (4xI) Contraction speed slow fast (2xI) fast (4xI) Time to peak force slow fast fast Time to peak force slow fast fast

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

Types of muscle fiber: Fast twitch vs Slow Twitch Types of muscle fiber: Fast twitch vs Slow Twitch Type I Type IIa Type IIb Type I Type IIa Type IIb ST Oxidative FT Oxidative - FT Glycolytic ST Oxidative FT Oxidative - FT Glycolytic (S0) Glycolytic (FOG) (FG) (S0) Glycolytic (FOG) (FG) Contraction speed slow fast (2xI) fast (4xI) Contraction speed slow fast (2xI) fast (4xI) Time to peak force slow fast fast Time to peak force slow fast fast Fatigue rate slow inter. fast Fatigue rate slow inter. fast Fiber diam. small inter. large Fiber diam. small inter. large Aerobic capacity high inter. low Aerobic capacity high inter. low Mitochondrial conc. high inter. low Mitochondrial conc. high inter. low Anaerobic capacity low inter. High Anaerobic capacity low inter. High Sedentary people – 50% slow/50% fast, whereas elite athletes may differ e.g., cross country skiers – 75% slow 25% fast sprinters - 40% slow 60% fast sprinters - 40% slow 60% fast

Roles 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 Synergist: acts to perform the same action as another muscle

Types of muscular contraction Concentric: fibers shorten Eccentric: fibers lengthen Isometric: no length change

Factors affecting force Production 1. Cross-sectional area 2. Frequency of stimulation 3. Spatial recruitment 4. Velocity of shortening 5. Muscle length 6. Action of the series elastic component 7. Muscle architecture 8. Electromechanical delay 9. Muscle temperature

Factors affecting force Production 1. Cross-sectional area Hypertrophy: increase in the # of myofibrils and myofilaments Hypertrophy: increase in the # of myofibrils and myofilaments Hyperplasia: increase in the number of fibers??? Hyperplasia: increase in the number of fibers??? 1. Cross sectional area

Parallel vs serially arranged sarcomeres Optimal for velocity of shortening and range of motion Optimal for force production In seriesIn parallel From Exercise Physiology: Human Bioenergetics and its applications (2 nd edition) by Brooks, Fahey, and White (1996) Fig 17-20, P. 318

2. Rate Coding – frequency of stimulation From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.9, page 155

3. Spatial recruitment Increase # of active motor units (MUs) Increase # of active motor units (MUs) Order of recruitment Order of recruitment I ---> IIa -----> IIb Henneman's size principle: MUs are recruited in order of their size, from small to large Henneman's size principle: MUs are recruited in order of their size, from small to large Relative contributions of rate coding and spatial recruitment. Relative contributions of rate coding and spatial recruitment. Small muscles - all MUs recruited at approximately 50% max. force; thereafter, rate coding is responsible for force increase up to maxSmall muscles - all MUs recruited at approximately 50% max. force; thereafter, rate coding is responsible for force increase up to max Large muscles - all MUs recruited at approximately 80% max. force.Large muscles - all MUs recruited at approximately 80% max. force.

4. Velocity of shortening: Force inversely related to shortening velocity The force-velocity relationship for muscle tissue: When resistance (force) is negligible, muscle contracts with maximal velocity. Velocity Force (Low resistance, high contraction velocity)

The force-velocity relationship for muscle tissue: As the load increases, concentric contraction velocity slows to zero at isometric maximum. Velocity Force isometric maximum

Force-Velocity Relationship in different muscle fiber types Type II fiber Type I fiber

Force -Velocity Relationship (Effect of strength-Training)

Force/Velocity/Power Relationship Force Velocity Power 30% Force/velocity curve Power/velocity curve From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.25, page 175

Effect of Muscle Fiber Types on Power-Velocity Relationship

Force-velocity Relationship During Eccentric Muscular Contractions

From Skeletal muscle structure, function, and plasticity (2 nd Edition) by R.L. Leiber, P 312

5. Muscle length From Skeletal muscle structure, function, and plasticity by R.L. Leiber, P. 55

From Exercise Physiology: Human Bioenergetics and its applications (2nd edition) by Brooks, Fahey, and White (1996) P. 306

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 fascia, and titin

6.Action of the series elastic component The stretch-shortening phenomenon The stretch-shortening phenomenon The effectiveness and efficiency of human movement may be enhanced if the muscles primarily responsible for the movement are actively stretched prior to contracting concentrically. The effectiveness and efficiency of human movement may be enhanced if the muscles primarily responsible for the movement are actively stretched prior to contracting concentrically. Mechanism: storage and release of elastic strain energy. Mechanism: storage and release of elastic strain energy.

7. Muscle Architecture Parallel fiber arrangements Pennate fiber arrangements Fibers are roughly parallel to the longitudinal axis of the muscle Short fibers attach at an angle to one or more tendons within the muscle From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.11, page 159

From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.13, page 161

8. Electromechanical delay Time between arrival of a neural stimulus and tension development by the muscle ms From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.20, page 171

9. Temperature: Effect on the Force-Velocity Relationship (22 o C, 25 o C, 31C o, and 37 o C)

Two- joint Muscles Advantages Advantages Actions at two joints for the price of one muscle. Possible metabolic saving if coordinated optimallyActions at two joints for the price of one muscle. Possible metabolic saving if coordinated optimally Shortening velocity of a two-joint muscle is less than that of its single-joint synergistsShortening velocity of a two-joint muscle is less than that of its single-joint synergists Results in a more favorable position on the force velocity curve. Act to redistribute muscle torque and joint power throughout a limb.Act to redistribute muscle torque and joint power throughout a limb.

Two- joint Muscles Disadvantages: Disadvantages: Active insufficiency: unable to actively shorten sufficiently to produce a full range of motion at each joint crossed simultaneouslyActive insufficiency: unable to actively shorten sufficiently to produce a full range of motion at each joint crossed simultaneously Passive insufficiency: unable to passively lengthen sufficiently to produce a full range of motion at each joint crossed simultaneouslyPassive insufficiency: unable to passively lengthen sufficiently to produce a full range of motion at each joint crossed simultaneously