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Chapter 6:
The Biomechanics of
Human Skeletal Muscle
Basic Biomechanics, 4th edition
Susan J. Hall
Presentation Created by
TK Koesterer, Ph.D., ATC
Humboldt State University
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 lengthtension relationships and electromechanical delay on
muscle function.
• Discuss the concepts of strength, power, and
endurance from a biomechanical perspective.
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
Extensibility and Elasticity
• Extensibility
• Elasticity
• Two components:
– Parallel elastic component (PEC)
– Series elastic component (SEC)
• Contractile component
• Visoelastic
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
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
Muscle Fibers
Contain:
sarcolemma
sarcoplasm
nuclei
mitochondria
myofibrils
myofilaments
Sarcomere
Z lines
M line
A band
myosin filaments
I band
actin filaments
H zone
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.
Motor Units
• Motor unit:
– Axon
– Motor end plate
• Twitch Type
• Tonic Type
• Summation
• Tetanus
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.
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
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.
Skeletal Muscle Function
• Recruitment of motor units
• Change in length with tension development
• Roles assumed by muscles
• Two-joint and multijoint muscles
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)
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
Roles Assumed by Muscles
• Agonist
– Primary & Secondary
• Antagonist
• Stabilizer
• Neutralizer
• Agonists and Antagonists are typically
positioned on opposite sides of a joint.
Two-joint and Multijoint
Muscles
• Movement effectiveness depends on:
– Location and orientation of muscle’s
attachment relative to the joint
– Tightness or laxity of musculotendinous
unit
– Actions of other muscles crossing the joint
• Disadvantages:
– Active insufficiency
– Passive insufficiency
Factors Affecting Muscular
Force Generation
• Force-Velocity Relationship
• Length-Tension Relationship
• Electromechanical Delay
• Stretch-Shortening Cycle
Force-Velocity Relationship
• Maximal force developed by muscle
governed by velocity of muscle’s shortening
or lengthening.
• Holds true for all muscle types
• Does not imply:
– It’s impossible to move heavy resistance at
a fast speed.
– It’s impossible to move light loads at low
speeds
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.
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)
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
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.
Muscular Strength, Power,
and Endurance
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•
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Muscular Strength
Muscular Power
Muscular Endurance
Muscular Fatigue
Effect of Muscle Temperature
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.
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 muscle’s anatomical
attachment to bone and the axis of rotation
at the joint center
– Angle of muscle’s attachment to bone.
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
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.
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 > FOG > FG
• Causes
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!
Common Muscle Injuries
• Strains
– Mild, moderate or severe
• Contusions
– Myositis ossificans
• Cramps
• Delayed-Onset Muscle Soreness (DOMS)
• Compartment Syndrome
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 the the combination of
many relationships (ex: force-velocity)
• Specific activity performance is related power,
endurance, and strength