PT7701- Intro to Tissue Mechanics

Force

"Push" or "pull".

Vector

Forces are directional — __________________________.

Magnitude

Forces have _____________________________.

Torque

Forces that are applied at a distance away from an axis will produce ______________________________.

Internal forces

Produces from muscle (tendon), ligaments, skin, fascia.

External forces

Gravity, external resistance (weights, bands, etc.), frictional, water (drag).

Parallel forces

- Forces will be summed.
- Direction matters.
- Also called tangential forces.

Perpendicular forces

- Summed to move into a different direction.
- Also called normal forces.

Oblique forces

Have component parts (X-component and Y-component).

Y-component

Rotational force, normal force.

X-component

Compressive or distractive force, tangential force.

Internal torque

If weight is not moving, external torque is equal to _____________________________.

Class 3 levers

Insertion of muscle is closer to joint than external force.

ROM

Majority of levers in human body are class 3 levers, giving us greater _________________________ and speed of contraction.

Deformable

Biological tissues are __________________________ bodies, not rigid bodies.

Unbalanced force

If an object is in static equilibrium, then it will most likely change shape or deform when acted on by an ________________________________.

Duration; temperature

Extent of deformation depends on:
- Applied external force (magnitude, direction, _________________________)
- Characteristics of tissue being deformed (material properties, size and shape of object)
- Environmental factors (_________________________ and humidity)

Tensile force

Pulling

Compressive force

Pushing

Shear force

Force that would go parallel to surface of segment.

Bending force

Tensioning convex side and compressing concave side.

Torsion force

Tensioning external and compressing internal.

Normal forces

Tensile and compressive forces are colinear and known as _____________________________.

Tangential forces

Shear forces are parallel to the object and known as ______________________________.

Injury

Human body doesn't respond to shear force very well, often causes ____________________________.

Shearing

Tension and bending cause __________________________ across all parts of segment.

Normal stress (σ)

Resultant force is perpendicular to the surface of an object.

Force/area

Assuming that the intensity of the external and internal forces are equivalent then the normal stress is equal to the ________________________ of the object.

Newtons/meter²

Normal stress is measured in ____________________________ or Pascal (Pa).

Normal strain (ε)

Measure of degree of deformation.

ε=Δx/x

Normal strain is the ratio of the change in length (increase or decrease) from the resting length.

Percentage

Normal strain may be reported as __________________________ of change.

Shear stress (τ)

Force is applied parallel to the surface of an object, then the internal forces are acting in tangent.

τ=F/A

If the intensity of the force is uniform across the surface of the object, then ___________________________.

- Normal stress for normal force.
- Shear stress for tangential force.

What is difference between shear stress and normal stress?

Shear strain (γ)

Occur due to distortions caused by shear stresses.

γ= Δx/h

Relative displacement of the object (Φ) caused from shear stress is Δx. Average shear strain is ______________________________.

Stress

Internal resistance to change.

Strain

Percentage (%) of stretch past resting length.

Elastic region

Where the tissue can resist change and will return to resting position once the external force has been removed.

Plastic region

Where there is long-term changes to the tissue.

Failure point

All tissues have a ________________________, where the tissue will break/tear.

Origin (O)

Area with no load and no deformation.

Proportionality limit (P)

Between O and P, the stress and strain are linear (known as Young's modulus).

Elastic limit (E)

End of elastic region; often same as yield point.

Yield point (Y)

Elongation can occur without an increase in load (this is the start of the plastic region).

σᵧ

Yield strength of the material; amount of stress tissue can withstand without permanent change in length of tissue.

U

Highest stress point on the diagram.

σᵤ

Ultimate strength of the material.

R

Rupture or failure point.

Strength

Load at ultimate stress point; often at or just before failure point.

Stiffness

Load required to deform the structure a given amount = load/elongation.

Slope

Stiffness is a ratio specific to each sample specimen; it is ____________________________ of stress-strain curve.

Compliance

Measure of ease of deforming of a specific structure; reciprocal of stiffness.

Toughness

Area under the stress-strain curve; defined as work or energy.

Young's (elastic) modulus

E=σ/ε

Small

E is usually a large number because strain is generally ____________________________.

Flexible

When strain is relatively large, E is small, implying a gentler slope and tissue/material is more __________________________.

Solids

Everything but fluids; various stress/strain relationships.

Steel alloy

Ductile material where stress and strain can be linear.

Uniform

Nothing __________________________ with solids group.

Glass

Brittle → little strain occurs for stress application

Elasticity

A property of solids; perfectly linear relationship (i.e., spring).

Elasticity

Ability of material to return to starting size and shape when load is removed.

Independent

Elasticity is time _________________________________ and has a predictable relationship (F=kd).

Viscoelastic materials

High liquid content, usually water resulting in fluid flow when material is loaded; ALSO have some properties that are more "solid-like".

Viscosity

Resistance to flow.

Increased

Increased viscosity leads to __________________________ slope in elastic region of stress-strain curve.

Viscoelastic materials

All biomaterials are _______________________________.

Predictive

Viscoelastic materials are not "ideal" in the sense of material properties being _________________________; BUT these materials serve our bodies well (EXCEPT when they don't).

Combination

Viscoelastic materials have a _______________________ of viscous and solid (elastic) response. Depending on the material, the tissue will respond more with elastic or viscous properties.

Time; rate

Viscoelastic materials are ____________________ and ______________________ dependent.

Solid

A __________________________ material will deform to a certain extent when an external force is applied.

Fluid

A continuously applied force on a __________________________ body will cause a continuous deformation (known as flow).

Viscosity

Property that is the quantitative measure of resistance to flow.

Hysteresis

Change in the stress/strain relationship with elongation and relaxation.

Greater

__________________________ energy is absorbed during loading than is released during unloading.

Heat

The curve will move to the right during hysteresis. The area within the loop represents the energy dissipated as ____________________________ during the process.

Lag

This __________________________ in rebounding is hysteresis.

Elastic; viscous

Warm-ups prior to exercise are a good idea because they warm the tissues, making them more ______________________ and less __________________________.

Rebound resilience

Ratio of area under unloading curve to area under loading curve expressed as a percentage.

Elastic

When rebound resilience ratio is 1, the material is purely linearly _________________________.

Repetitions

Rebound resilience is effected by ______________________ and temperature.

Reduced

If hysteresis is ________________________, then the response of the tissue is quicker and would become more "ideal".

Stiffness

Decrease in hysteresis implies decrease in ____________________________.

Creep

Maintain load over time and observe reaction.

Elongates

Creep: Change in elongation or strain over time → tissue _____________________________.

10 minutes

Creep: Change occurs mostly within the first __________________________ (2 minutes in ligament).

Cyclic creep tests

Seem to mimic athletic activities better (hold is short but up to 100 cycles would be performed) → examines fatigue characteristics of viscoelastic materials.

Increased

Increased time of load application leads to __________________________ deformation.

Arthritis

Resting on hip ligaments → Degradation of fibrocartilage labrum and articular cartilage → _________________________

ROM

Stretching elbow contracture to second resistance and beyond → Greater ________________________________

End

In regard to creep, there is no increase in force with time, but ROM increases so elongation/deformation is greatest at _________________________ of range.

Stress relaxation

Maintain elongation over time and observe the load changes. Load gradually decreases with time.

6-8 hours

Stress relaxation changes take _______________________ to occur in biological tissue.

Cartilage

Stress relaxation is not well tested; not as well understood how it relates to biological tissue; only time this may occur in the body is with ____________________________.

Plastic region; elastic region

Stretching occurs in _____________________________ for muscles, whereas strengthening occurs in ____________________________.

Uncrimping

When collagen fibers go from being "wavy" to straight with an applied stress.

Viscoelasticity

Primary mechanism to increase tissue length.

Elastic deformation

Ability of the material to return to its resting length after a load has been applied.