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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.

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Chapter 6 – The Biomechanics of Skeletal Muscle

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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:

    • Excitability – ability to receive and respond to a stimulus

    • Contractilty (irritability) – ability of a muscle to contract and produce a force

    • Extensibility – ability of a muscle to be stretched without tissue damage

    • Elasticity – ability of a muscle to return to its original shape after shortening or extension


Structural organization of skeletal muscle

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


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

6-6


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


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

6-5


Structural organization of skeletal muscle

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


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

6-3


From Exercise Physiology: Theory and Application to Fitness and Performance (6th 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


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

6-7


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

6-8


  • Types of muscle fiber: Fast twitch vs Slow Twitch

    Type I Type IIa Type IIb

    ST OxidativeFT Oxidative - FT Glycolytic

    (S0)Glycolytic (FOG) (FG)

  • Contraction speed slow fast (2xI) fast (4xI)

  • Time to peak force slow fast fast


FT ST

Twitch Tension

Time

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.)


  • Types of muscle fiber: Fast twitch vs Slow Twitch

    Type I Type IIa Type IIb

    ST OxidativeFT Oxidative - FT Glycolytic

    (S0)Glycolytic (FOG) (FG)

  • Contraction speed slow fast (2xI) fast (4xI)

  • Time to peak force slow fast fast

  • Fatigue rate slow inter. fast

  • Fiber diam. small inter. large

  • Aerobic capacity high inter. low

  • Mitochondrial conc. high inter. low

  • 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


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


1. Cross-sectional area

Hypertrophy: increase in the # of myofibrils and myofilaments

Hyperplasia: increase in the number of fibers???

Factors affecting force Production

1. Cross sectional area


Parallel vs serially arranged sarcomeres

Optimal for force production

Optimal for velocity of shortening and range of motion

In series

In parallel

From Exercise Physiology: Human Bioenergetics and its applications (2nd 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)

  • Order of recruitment

    I ---> IIa -----> IIb

  • Henneman's size principle: MUs are recruited in order of their size, from small to large

  • 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 max

    • Large muscles - all MUs recruited at approximately 80% max. force.


(Low resistance, high contraction velocity)

Force

Velocity

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.


isometric maximum

Force

Velocity

The force-velocity relationship for muscle tissue: As the load increases, concentric contraction velocity slows to zero at 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 curve

Power/velocity curve

Force

Power

30%

From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.25, page 175

30%

Velocity


Effect of Muscle Fiber Types on Power-Velocity Relationship


Force-velocity Relationship During Eccentric Muscular Contractions


From Skeletal muscle structure, function, and plasticity (2nd 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


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.

Mechanism: storage and release of elastic strain energy.

6.Action of the series elastic component


Pennate fiber arrangements

Parallel fiber arrangements

7. Muscle Architecture

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

20-100 ms

Time between arrival of a neural stimulus and tension development by the muscle

From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.20, page 171


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


Two- joint Muscles

  • Advantages

    • Actions 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 synergists

      Results in a more favorable position on the force velocity curve.

    • Act to redistribute muscle torque and joint power throughout a limb.


Two- joint Muscles

  • Disadvantages:

    • Active 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 simultaneously


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