Biomechanics of the skeletal muscles

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Biomechanics of the skeletal muscles
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Objectives

• Identify the basic behavioral properties of the
  musculotendinous unit.
• Explain the relationships of fiber types and
  fiber architecture to muscle function.
• Explain how skeletal muscles function to produce
  coordinated movement of the human body.
• Discuss the effects of the force-velocity and
  length-tension relationships and
  electromechanical delay on muscle function.
• Discuss the concepts of strength, power, and
  endurance from a biomechanical perspective.

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Structural Organization of Skeletal Muscle

• Human body has approx. 434 muscles
• 40-45 of total body weight in adults
• 75 muscle pairs responsible for bodily movements
  and posture
• Muscle Fibers
• Motor Units
• Fiber Types
• Fiber Architecture

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Muscle Fibers

• During contraction, cross-bridges form
• Sarcoplasmic Reticulum
• Transverse Tubules
• Endomysium
• Perimysium
• Fascicles
• Epimysium
• Variation of length and diameter within muscles
  seen in adults.

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Muscle Fibers

• Contain
• sarcolemma
• sarcoplasm
• nuclei
• mitochondria
• myofibrils
• myofilaments

• Sarcomere
• Z lines
• M line
• A band
• myosin filaments
• I band
• actin filaments
• H zone

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Motor Units

• Motor unit
• Axon
• Motor end plate
• Twitch Type
• Tonic Type
• Summation
• Tetanus

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Behavioral Properties of the Musculotendinous Unit

• Behavioral properties of muscle tissue
• Extensibility
• Elasticity
• Irritability
• Ability to develop tension
• Behavioral properties common to all muscle
• Cardiac, smooth, skeletal

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Extensibility and Elasticity

• Extensibility
• Elasticity
• Two components
• Parallel elastic component (PEC)
• Series elastic component (SEC)
• Contractile component
• Visoelastic

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Irritability and the Ability to Develop Tension

• Irritability
• The ability to respond to electrical or
  mechanical stimulus.
• Response is the development of tension.
• Not necessarily a contraction

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Fiber Types

• Fast Twitch (FT)
• Type IIa
• Type IIb
• Slow Twitch (ST)
• Type I
• Peak tension reached in FT in 1/7 time of ST
• ST and FT compose skeletal muscles
• Percentages of each range from muscle to muscle
  and individual to individual.

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Fiber Types

• Effects of training
• Endurance training can increase ST contraction
  velocity by 20
• Resistance training can convert FT fibers from
  Type IIb to Type IIa
• Elite athlete fiber type distribution does not
  significantly differ from untrained individuals
• Affected by
• Age and Obesity

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Fiber Architecture

• Parallel fiber arrangement
• Resultant tension from shortening of muscle
  fibers
• Shortens the muscle
• Pennate fiber arrangement
• Resultant tension from shortening of muscle
  fibers
• Increases the angle of pennation (attachment) to
  a tendon.

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Skeletal Muscle Function

• Recruitment of motor units
• Change in length with tension development
• Roles assumed by muscles
• Two-joint and multijoint muscles

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Recruitment of Motor Units

• CNS enables matching of speed and magnitude of
  muscle contraction to requirement of movement.
• Threshold activation
• ST activated first (low threshold)
• With an increase in speed, force, and/or duration
  requirement, higher threshold motor units are
  activated (FT fibers)

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Change in Muscle Length with Tension Development

• Concentric
• Bicep shortening with the bicep curl (flexion)
• Isometric
• Body builders develop isometric contraction in
  competition
• Eccentric
• Acts as a breaking mechanism to control movement

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Roles Assumed by Muscles

• Agonist
• Primary Secondary
• Antagonist
• Stabilizer
• Neutralizer
• Agonists and Antagonists are typically positioned
  on opposite sides of a joint.

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Two-joint and Multijoint Muscles

• Movement effectiveness depends on
• Location and orientation of muscles attachment
  relative to the joint
• Tightness or laxity of musculotendinous unit
• Actions of other muscles crossing the joint
• Disadvantages
• Active insufficiency
• Passive insufficiency

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Factors Affecting Muscular Force Generation

• Force-Velocity Relationship
• Length-Tension Relationship
• Electromechanical Delay
• Stretch-Shortening Cycle

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Force-Velocity Relationship

• Maximal force developed by muscle governed by
  velocity of muscles shortening or lengthening.
• Holds true for all muscle types
• Does not imply
• Its impossible to move heavy resistance at a
  fast speed.
• Its impossible to move light loads at low speeds

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Force-Velocity Relationship

• Maximum isometric tension
• Eccentric conditions
• Volitionally
• Represents contribution of the elastic components
  of muscle
• Eccentric Strength Training
• More effective than concentric training in
  increasing muscle size and strength.

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Length-Tension Relationship

• In human body, force generation increases when
  muscle is slightly stretched.
• Parallel fibers at max just over resting length
• Pennate fibers at max with 120-130 resting
  length.
• Due to contribution of elastic components of
  muscle (primarily the SEC)

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Electromechanical Delay

• Electromechanical Delay (EMD)
• Varies among human muscles (20-100 msec)
• Short EMDs produced by muscles with high
  percentage of FT fibers
• Associated with development of higher contraction
  forces
• Not effected by muscle length, contraction type,
  contraction velocity, or fatigue

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Stretch-Shortening Cycle

• Stretch-Shortening Cycle (SSC)
• Elastic Recoil
• Stretch Reflex Activation
• Muscle can perform more work with active stretch
  prior to shortening contraction
• Less metabolic costs when SSC utilized.
• Eccentric training increases ability of
  musculotendinous unit to store and produce more
  elastic energy.

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Muscular Strength, Power, and Endurance

• Muscular Strength
• Muscular Power
• Muscular Endurance
• Muscular Fatigue
• Effect of Muscle Temperature

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Muscular Strength

• The ability of a given muscle group to generate
  torque at a particular joint.
• Two orthogonal components
• 1) Rotary Component
• 2) Parallel to bone
• Derived from
• amount of tension the muscles can generate
• moment arms of contributing muscles with respect
  to joint center.

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Muscular Strength

• Tension-generating capability of a muscle
  affected by
• Cross-sectional area
• Training state
• Moment arm of a muscle affected by
• Distance between the muscles anatomical
  attachment to bone and the axis of rotation at
  the joint center
• Angle of muscles attachment to bone.

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Muscular Power

• The product of muscular force and the velocity of
  muscular shortening.
• The rate of torque production at a joint
• Max. power occurs at
• approx. 1/3 max. velocity, and
• approx. 1/3 max concentric force
• Affected by muscular strength and movement speed

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Muscular Endurance

• The ability to exert tension over a period of
  time.
• Constant gymnast in iron cross
• Vary rowing, running, cycling
• Length of time dramatically effected by force and
  speed requirements of activity.
• Training involves many repetitions with light
  resistance.

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Muscular Fatigue

• Opposite of endurance
• Characteristics
• Reduction in force production
• Reduction in shortening velocity
• Prolonged relaxation of motor units between
  recruitment
• Absolute Fatigue
• Resistance
• SO gt FOG gt FG
• Causes

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Effect of Muscle Temperature

• Increased body temperature, increases speed of
  nerve and muscle function
• Fewer motor units needed to sustain given load
• Metabolic processes quicken
• Benefits of increased muscular strength, power
  and endurance
• Key point Be sure to warm-up!

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Common Muscle Injuries

• Strains
• Mild, moderate or severe
• Contusions
• Myositis ossificans
• Cramps
• Delayed-Onset Muscle Soreness (DOMS)
• Compartment Syndrome

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Summary

• Muscle is the only biological tissue capable of
  developing tension.
• Resulting actions can be concentric, eccentric,
  isometric for muscle shortening, lengthening or
  remaining unchanged in length
• Force production is the combination of many
  relationships (ex force-velocity)
• Specific activity performance is related power,
  endurance, and strength

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• The End