Advanced Biomechanics of Physical Activity (KIN 831)

1
Advanced Biomechanics of Physical Activity (KIN
831)

• Lecture 2
• Biomechanics of Tendons and Ligaments
• Material included in this presentation is
  derived primarily from two sources
• Enoka, R. M. (1994). Neuromechanical
  basis of kinesiology. (2nd ed.). Champaign, Il
  Human Kinetics.
• Nordin, M. Frankel, V. H. (2001).
  Basic Biomechanics of the Musculoskeletal System.
  (3rd ed.). Philadelphia
• Lippincott Williams Wilkins.

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What do you know about the macroscopic structure
and function of tendons and ligaments?
3
What do you know about the microscopic structure
and function of tendons and ligaments?
4
Functions of Ligaments and Joint Capsules

• connect bone to bone
• act as static restraint to
• help with joint stability
• guide joint motion
• prevent excessive motion

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Functions of Tendons

• connect muscle to bone
• transmit tensile loads from muscle to bone to
• produce joint torque
• stabilize joint during isometric contractions and
  in opposition to other torques
• cause joint motion during isotonic contractions
• act as a dynamic joint restraint
• interact with ligaments and joint capsule to
  mitigate loads that they receive
• --------------------------------------------------
  ---
• Interesting points
• tendon extends the reach of muscle
• tendon may conserve muscle tissue mass (i.e.,
  muscle tissue not required to extend from origin
  to insertion)

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Tendons and Ligaments

• Dense connective tissues (parallel-fibered
  collagenous tissues)
• Sparsely vascularized
• Composed primarily of collagen (fibrous protein
  which gives tendons and ligaments strength and
  flexibility)
• Consist of relatively few cells or fibroblasts (
  20 of total tissue volume)
• Contain abundant extracellular matrix
• 80 of total tissue volume
• 70 of extracellular matrix is water and 30
  solids (collagen (75 of extracellular matrix),
  ground substance, and small amount of elastin)
• Structure and chemical composition identical to
  other animal species (extrapolate behavior from
  animals)

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Tendons and Ligaments

• Tendons
• Join muscle to bone
• Organization of collagen fibers to accommodate
  specialized function
• Fibers longitudinal and parallel
• Transmit tensile muscle forces

• Ligaments
• Join bone to bone
• Organization of collagen fibers to accommodate
  specialized function
• Fibers generally longitudinal and parallel, some
  oblique and spiral
• Primarily transmit forces in functional
  direction, but also multidirectional

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How can you make string able to support a large
load?

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How do manufacturers of string make it able to
support a large load?

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Collagen Molecule

• Synthesized by within fibroblast as procollagen
  (precursor to collagen)
• Consists of 3 polypeptide chains (? chains) each
  coiled in left hand helix
• 3 ? chains combined in a right handed triple
  helix
• Bonding (cross-linking) between ? chains enhances
  strength of collagen molecules
• Develops extracellularly into collagen molecules

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Collagen

• Groups of 5 collagen molecules form microfibrils
• Cross links formed between collagen molecules
  that aggregate at the fibril level
• Cross links between collagen molecules give
  strength to tissues (e.g., tendons and ligaments)
  they compose
• Fibrils aggregate further to form collagen fibers
• Fibers aggregate to form bundles

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Collagen Fiber Arrangement in Tendons and
Ligaments
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Macroscopic and Microscopic Structure of Tendon
and Ligaments
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Macroscopic and Microscopic Structure of Tendon
and Ligaments
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Macroscopic and Microscopic Structure of Tendon
and Ligaments

• Epitendidium -outer covering
• Fascicle - bundle of fibrils
• Fibril - basic load bearing unit of tendon and
  ligaments
• Microfibril - 5 rows of triple helixes in
  parallel (see figure)

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Schematic illustration depicting the hierarchical
structure of collagen in ligament midsubstance
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Macroscopic and Microscopic Structure of Tendon
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Schematic representation of the microarchitecture
of a tendon
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Structural hierarchy of a tendon. Connective
tissue layers or sheaths envelop the collagen
fascicles (endotenon), bundles of fascicles
(epitenon), and the entire tendon (paratenon)
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Macroscopic and Microscopic Structure of Tendon
and Ligaments

• Collagen molecule - triple helix in series 5
  rows stacked side-by side (parallel)
• Triple helix - cross links occur both between and
  within rows of triple helixes ? strength ( and
  state of cross links influence strength) ?
  determined by age, gender, and activity level

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Elastin

• tendons and ligaments contain protein elastin
• influences elastic properties of tendons and
  ligaments (? elastin ? ? elasticity)
• proportion varies by function
• little in tendons and extremity ligaments
• much present in ligamentum flavum between laminae
  of vertabrae
• protect spinal nerve roots
• pre-stress the motion segment
• provide intrinsic stability to spine

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Ground Substance

• amorphous material in which structural elements
  occur
• in connective tissues, composed of proteoglycans,
  plasma constituents, metabolites, water, and ions
  between cells and fibers

Ground Substance in Tendons and Ligaments

• Proteoglycans act as cement-like substance
  between collagen microfibrils contributing to
  overall strength of tendons and ligaments

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Water and Proteoglycans

• Forms a gel
• Viscosity decreases with activity
• Thixotrophy (property seen in catsup)
• Increased ability to accommodate higher velocity
  stretches
• Advantage of a warm-up

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Vascularization of Tendons and Ligaments

• Dual Pathway for Tendons
• Vascular (tendon surrounded by paratenon)
• receives blood supply from vessels in perimysium,
  periosteal insertion, and surrounding tissues
• Avascular (tendon surrounded by tendon sheath)
• Synovial diffusion
• Healing and repair in the absence of blood supply

• Ligaments
• Vascularity
• Originates from ligament insertion sites
• Small size and limited blood flow

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• Take home message
• Amount of tissue vascularization is directly
  related to rate of tissue metabolism and healing
• Tendons and ligaments have limited vascularization

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Macroscopic and Microscopic Structure of Tendon
and Ligaments

• Tendons surrounded by loose connective tissue
  (paratenon)
• Paratenon forms sheath
• Protects tendon
• Enhances gliding
• Epitenon
• Synovial-like membrane beneath paratenon in
  locations of high friction
• Absent in low friction locations
• Surrounds several fiber bundles
• Endotendon
• Surrounds each fiber bundle
• Joins musculotendinous junction into perimysium

• Ligaments surrounded by very loosely structured
  connective tissue (not named)
• Vascularity
• Originates from ligament insertion sites
• Small size and limited blood flow

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Tendon Insertion in Bone
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What comes to mind when you hear the word toe?
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Load Deformation Relationships in Collagenous
Tissues

• Toe - collagen fibrils stretched to line up, from
  zigzag to straighten
• linear region - elastic capability of tissue
  elastic modulus
• failure region - fibers disrupted
• Hysteresis failure to return to resting length

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Stress-Strain Relationship in Collagenous Tissues
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Collagen Fibers Unloaded (Toe) and Loaded
(Elastic Region)
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Typical Load-Elongation Curve
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Load-Elongation Curve of Ligaments with High
Levels of Elastin

• Elastin (protein) scarcely present in tendons and
  extremity ligaments
• Ligamentum flavum
• Substantial proportion of elastin
• Connect laminae of adjacent vertebrae
• Function to protect spinal nerve roots
• Provide intrinsic stability to spine

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Load-Deformation Relationships for Connective
Tissues
1kN 224.8 pounds
Note that text gives value of failure of ACL
between 76.4 and 87.67 lbs (340-390 N)
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Is there any movement in isometric contractions?
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Physiological Loading of Tendons and Ligaments

• P (max) of ligaments and tendons not achieved
  during normal activities
• normally 30 of P (max) achieved
• upper limit during running and jumping ? 2 - 5
  P (max)

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Ligament and Tendon Injury Mechanisms

• Injury mechanisms similar in tendons and
  ligaments
• Microfailures take place before yield point
• After yield point, gross failure results and
  joint begins to displace abnormally
• Joint displacement can also damage surrounding
  structures (e.g., joint capsule, other ligaments,
  blood vessels)

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Anterior Drawer Loading the ACL to Failure
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Anterior Drawer Loading the ACL to Failure

• Microfailure begins before physiological loading
  range is exceded

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What is the numerical categorization system used
by athletic trainers to differentiate between
levels of ligamentous injury?
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Categorization of Ligamentous Injury

• Negligible clinical symptoms, some pain,
  microfailure of some collagen fibers
• Severe pain, clinical detection of some joint
  instability, progressive collagen fiber failure
  resulting in partial ligament rupture, strength
  and stiffness may decrease 50 or more, muscle
  guarding, perform clinical testing under
  anesthesia

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Categorization of Ligamentous Injury

• 3. Severe pain, joint completely unstable, most
  collagen fibers ruptured, loading joint produces
  abnormally high stress on the articular cartilage
  ? correlated with osteoarthritis

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Additional Factors in Injuries to Tendons

• Amount of force of contraction produced by muscle
  attached to tendon
• Tensile stress on tendon directly related to
  force of muscle contraction
• High levels of tensile stress can be produced by
  eccentric contraction, possibly reaching failure

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Additional Factors in Injuries to Tendons

• Cross sectional area of tendon in relation to
  cross sectional area of its muscle
• Cross sectional area of muscle directly related
  to force of contraction
• Cross sectional area of tendon directly related
  to tensile strength
• Tensile strength of healthy tendon may be more
  than twice that of force of muscle contraction
  (clinically, muscle ruptures more common than
  tendon ruptures)
• Large muscles usually have large tendons

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Viscoelastic Behavior (Rate Dependency) in
Tendons and Ligaments

• Increased strain ? increased slope of
  stress-strain curve (i.e., greater stiffness at
  higher strain)
• Higher strain rate ? more energy stored, require
  more force to rupture, undergo greater elongation

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Typical loading (top and unloading curves
(bottom) from tensile testing of knee ligaments.
The two nonlinear curves, called the area of
historesis, represents the energy losses within
the tissue.
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Two Standard Tests of Viscoelastic Behavior

• Stress-relaxation test
• Loading halted in safe region of stress-strain
  curve
• Strain kept constant over extended period of time
• Stress decreases rapidly at first, then gradually
• Decrease in stress less pronounced with repeat
  tests

Viscoelastic variation in mechanical
properties of tissue with different rates of
loading
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If you were asked to develop a creep test, what
would you use to make measurements?
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Two Standard Tests of Viscoelastic Behavior

• 2. Creep test
• Loading halted in safe region of stress-strain
  curve
• Stress kept constant over extended period of time
• Strain increases rapidly at first, then gradually
• Clinically used in casting club foot and bracing
  in scoliosis

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Schematic creep curve for ligament
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Influence of Loading Rates on Bone-Ligament-Bone
Complex

• At slow loading rates (60 sec. much slower than
  in vivo injury mechanism), avulsion produced
• At fast loading rates (0.6 sec. simulates in
  vivo injury mechanism), ligamentous injury
  typical

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Factors Affecting Biomechanical Properties of
Tendons and Ligaments

• Maturation and aging
• Up to 20 years of age,
• number and quality of cross-links in collagen
  molecules increases ? increased tensile strength
• Collagen fibril diameter increased ? increased
  tensile strength
• After maturation,
• Collagen content of tendon and ligaments
  decreases ? decreased tensile strength

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Factors Affecting Biomechanical Properties of
Tendons and Ligaments

• Pregnancy and postpartum period
• Clinical observation increased laxity of
  tendons and ligaments in pubic area during latter
  stages of pregnancy and during early postpartum
  period ? hormonal influence
• Research studies of rats increased laxity of
  tendons and pubic symphasis during latter stages
  of pregnancy and during postpartum period
  stiffness of these structures later returned

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Factors Affecting Biomechanical Properties of
Tendons and Ligaments

• Pregnancy and postpartum period (continued)
• Hormones may have influence on ligament laxity in
  women at various stages of menstrual cycle ?
  influence ligamentous injury rates in females
  (e.g., higher incidence of injury in women in
  basketball and soccer in comparison to men)

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Factors Affecting Biomechanical Properties of
Tendons and Ligaments

• Mobilization and immobilization
• Tendons and ligaments remodel in response to
  mechanical demands
• Become stronger and stiffer when subjected to
  increased stress
• Become weaker and less stiff when stress removed
• Physical training found to increase tensile
  strength of tendons and ligament-bone interface

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Factors Affecting Biomechanical Properties of
Tendons and Ligaments

• Mobilization and immobilization
• Immobilization found to decrease tensile strength
  of ligaments
• Immobilization decreased mechanical properties of
  bone-ligament-bone complex in knee of primates (8
  weeks of casting)
• Considerable reconditioning required in primate
  knees to regain former complex strength (approx.
  12 months) (see figure)

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Influence of Immobilization on Primate ACL
Ligament
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Influence of Immobilization on Primate ACL
Ligament
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Factors Affecting Biomechanical Properties of
Tendons and Ligaments

• Nonsteroidal Anti-Inflammatory Drugs (NSAID)
  (e.g., aspirin, acetaminophen, indomethacin)
• In animal studies, short term administration of
  NSAIDs (indomethacin) found to increase the rate
  of biomechanical restoration of tissues (tendons)