Kines Week 2

Load

non-specific term for forces applied to the human body

Deformation

a change in shape

Mechanical (material) properties

describe how a tissue responds to applied loads/forces

Stress

force applied to a tissue per unit area, distribution of forces within a tissue, force/cross-sectional area

Stress has a ______ relationship to cross sectional area

inverse

Stress has a ____ relationship to force

direct

Strain

the amount a tissue deforms in response to a load, relative to its original length, %=(change in length/original length)*100%

Tissues undergo _______ in response to stress

strain/deformation

Young’s modulus (PA) represents _______ of a tissue by taking __________

stiffness, stress/strain (slope (y/x))

Stiff materials undergo _______ deformation in response to loading, representing a ________ slope

little, steep

Compliant materials undergo _______ deformation in response to loading, representing a _________ slope

a lot, shallow slope

A steeper slope in the linear region of a stress-strain curve means the material is ____

stiffer

An object that resists strain well (like a metal bar) will have a ___ slope on the stress strain curve

steeper

Bone handles ____ well

compression

Bone handles _____ poorly

shear

Tendon handles ______ well

tension

Tendon handles ______ poorly

compression

Articular cartilage handles ______ well in the deep zone, and _____ well in the superficial zone

compression, shear

Ligament/capsule handles __________ well

tension (multidirectional for capsule)

All ____________ in the human body are viscoelastic

soft tissue

Extent of deformation of a viscoelastic tissue is dependent on

magnitude, history of loading, rate, time, and temperature

Viscoelastic

tissue displaying both viscous and elastic properties

Viscoelastic magnitude

higher loads → more deformation

Viscoelastic rate

speed of load application, rapid application → stiffer tissue (think obleck)

Viscoelastic time

duration of load application, longer time of application → decreased deformation over time

Viscoelastic temperature

higher temperature → more compliant

Viscosity

a fluid’s resistance to flow/deformation, caused by internal friction between molecules, rate, time, and temperature dependent

Rate-dependence (higher rate leads to…)

When loaded rapidly, tissue is stiffer and more stress is required to achieve a given deformation, and more energy is stored

The Achilles tendon is ______ while running than walking

stiffer

Creep

Under a constant load, deformation increases over time, non-linear deformation recovery once load lifted

Stress relaxation

under constant deformation, the load required to maintain that deformation decreases over time

Temperature dependence

viscosity and compliance increase with temperature, takes less stress to achieve a given deformation at higher temps

Hysteresis

loading and unloading curves look different because energy is lost as heat viscoelastic materials are less efficient at storing energy than purely elastic materials

Clinical importance of viscoelastic properties

warming up tissue before stretching (temperature dependence), and why slow vs fast stretching feels/behaves differently (rate dependence)

Physical Stress Theory definition

Changes in the relative level of physical stress cause a predictable adaptive response in all biological tissues

3 determinants of physical stress level

Magnitude, direction, time

Direction determinant of physical stress includes

Tension, compression, shear, torsion

Time determinant of physical stress includes

Duration, repetition, rate

With stress well below the adapted range, tissue will

Atrophy (decreased stress tolerance)

Continued understress beyond atrophy leads to

Tissue death

With appropriately progressive increased stress, tissue will

Hypertrophy (increased stress tolerance)

Continued overstress beyond tolerance leads to

Injury, and eventually tissue death

Movement and alignment factors affecting physical stress (examples)

Muscle performance, motor control, posture/alignment, physical activity

Extrinsic factors affecting physical stress (examples)

Orthotic devices, taping, assistive devices, footwear, ergonomic environment, modalities, gravity

Physiological factors affecting physical stress (examples)

Medication, age, systemic pathology, obesity, genetics

Clinical application of Physical Stress Theory

Basis for rehab dosing — too little stress stalls recovery, too much exceeds tissue tolerance

Stress-strain curve definition

Describes how much stress must be applied to strain a tissue a given amount; unique to each tissue

Number of regions/points on the stress-strain curve

6

Nonlinear (toe) region

Uncrimping of collagen fibers ("taking up the slack"); little stress develops as strain increases

Elastic (linear) region

Stress and strain display a linear relationship; tissue returns to original length when load is removed

Slope of the elastic region =

Young's Modulus (stiffness)

Yield point

Point where tissue is strained beyond physiologic range; marks the beginning of the plastic region

Plastic region

Microscopic damage has occurred; permanent change in tissue length (does not return to original length)

Ultimate strength

Highest point on the stress-strain curve; max stress the tissue can sustain

Ultimate failure point

Tissue partially or completely tears, loses ability to hold tension

Ductile material

Undergoes relatively large plastic deformation before breaking

Brittle material

Undergoes very little plastic deformation before breaking

Linear elastic material behavior

Stress/strain proportional, no plastic deformation, loading/unloading lines are the same

Linear elastoplastic material behavior

Stress/strain proportional up to yield point; plastic deformation after yield; unloading line parallel to loading line but offset

3 fibrous protein types in connective tissue

Collagen (Type I, II, III), elastin

Ground substance is composed of

GAGs, water, solutes

Type I collagen characteristics and location

Thick, strong fibers, resist tension well; found in ligament, tendon, fascia, capsule, bone

Type II collagen characteristics and location

Thinner fibers, scaffold function, resist compression; found in hyaline/articular cartilage

Type III collagen characteristics and location

Thinner/weaker than Type I; associated with scar tissue and injury

Tendon composition

70-80% Type I collagen, parallel organized fibers, tenocytes, surrounded by epitenon/paratenon

Tendon function (3)

Transmit muscle force to bone, energy conservation (spring), protect muscle from injury

Joint capsule layers (2)

Fibrous (outer) layer, synovial (inner) layer

Ligament composition compared to tendon

Less collagen, more elastin/proteoglycans than tendon

Articular (hyaline) cartilage composition

Chondrocytes + Type II collagen, 70-85% water, avascular/aneural, no perichondrium

Fibrocartilage composition

Dense multidirectional Type I collagen + moderate proteoglycans

Bone composition

Primarily Type I collagen + calcium phosphate ground substance

Irregular dense CT orientation and function

Haphazard orientation, resists multidirectional forces (ex: joint capsule)

Regular dense CT orientation and function

Orderly orientation, resists forces in a single/few directions (ex: tendon)

Weakest of the 3 cartilage types

Articular (hyaline) cartilage

Strongest of the 3 cartilage types

Fibrocartilage

Elastic cartilage locations

External ear, arteries, lung tissue

Fibrocartilage locations

Intervertebral discs, pubic symphysis, menisci, labrum (hip/shoulder), TFCC (wrist)

Tendon structure relates to function how

Parallel collagen bundles → efficient one-direction tensile force transmission

Joint capsule structure relates to function how

Irregular collagen orientation → resists multidirectional forces

Capsular ligament example

Iliofemoral (Y) ligament

Intra/extra-capsular ligament examples

ACL, PCL (intracapsular); LCL (extracapsular)

Articular cartilage deep zone structure relates to function how

Collagen perpendicular to surface, high proteoglycans → resists compression

Articular cartilage superficial zone structure relates to function how

Collagen parallel to surface, flat chondrocytes → resists shear

Compact (cortical) bone structure relates to function how

Osteons oriented in direction of stress → protection/support, resists weight-bearing stress

Cancellous (trabecular) bone structure relates to function how

Spongy, metabolically active, flexible → transmits/distributes force along long axis

Big-picture structure-function rule for all tissues in this unit

Fiber orientation predicts load tolerance

Tendon response to underloading/short-term immobilization

Reduced tendon stiffness and collagen production

Tendon mechanobiology

Tenocytes experience shear/compression during loading → biological cascade → remodeling

Dense CT tensile strength loss after 8 weeks of immobilization

~50%

Time to restore dense CT strength after immobilization

12 to >18 months

Why adolescents are more likely to get avulsion fractures than ligament ruptures

Dense CT is stronger than bone at that age

Dense CT degenerative changes in older adults appear more related to

Decreased activity than aging itself

Articular cartilage response to immobilization/unloading

Decreased thickness, decreased chondrocyte density, decreased collagen content → softer/weaker

Fibrillation

Cartilage becomes less smooth (damage to the collagen network) from excessive loading

Articular cartilage response to impact (fast-rate) loading

Becomes stiffer, unable to deform/redistribute loads quickly

Articular cartilage aging changes

Breakdown > repair, decreased thickness, calcification, increased cross-linking → increased stiffness, increased fatigue failure

Bone response to compressive loading

Grows stronger in direction of load (increased mineral deposition, increased collagen production)

Bone response to unloading

Less bone deposition (same resorption rate) → reduced bone mass → weaker

Bone aging changes

Resorption > deposition → reduced bone mass, stiffer/brittle, weaker

Osteopenia

Reduced bone mineral density