Movement Science Biomechanics Unit

What is deformation?

Change in body configuration in response to imposition of stress

What is strain?

Strain is a “normalized” measure of deformation

Strain = Deformation/Original length

What is the toe region in the stress-strain relationship?

Region where not much stress/force can cause deformation

Elastic region

• Linear region; doesn’t matter which point to quantify stiffness

• As soon as stress is relieved, shape returns back to normal (minimal damage)

Nonelastic (Plastic) region

• Will not return to original shape

Yield point

Point of change from linear → plastic

Optimal Strength

The highest stress/force that can be handled

Failure/Fracture Point

End of test; tissue fails/fractures

Elastic (Young’s) Modulus

• A measure of the stiffness of an elastic material

• Elastic Modulus = ΔStress/ΔStrain = (Force/Area)/Ratio of original length

• Measured in Pascals (Pa = N/m²)

Stiffness

Slope of the curve

Ductile material

• Plastic range energy absorption

• Long plastic region (takes more to break)

Brittle material

• Short plastic region (takes less to break)

Viscosity

• The property of fluids regarding the resistance to flow

Elasticity

• The property of a solid object to return to its original shape after being unloaded

Viscoelasticity in human tissues

• Collagen + Elastin

• Ground substance (largely water)

Time Dependent Material Properties

1) Loading rate

2) Hysteresis

3) Creep

4) Stress relaxation

1) Property depends on loading rate

• Change how fast you apply a stress

• Faster = more stiff and more brittle

• Ex: Haliburton achilles rupture

2) Hysteresis

• Energy loss during a loading cycle

• Some of the loaded energy gets lost to friction and heat

• Ex: a squishy returning back to original shape after being stretched (very slow)

3) Creep

• Force stays the same, time changes (deformation increases with time)

• Ex: stretching a hamstring

  • Slowly stretch to max length (creep) → hold → tissue will continue to stress or deform

4) Stress relaxation

• Constant deformation → stress will DECREASE over time

• Ex: Serial casting for kids w/ cerebral palsy

Anisotropic Behavior

• Material properties depend on loading direction

• Bone is good for compression and tension, but not shear

Effects of Stress

Types of Bone Fractures

Collagen Fibril Matrix

• Anisotropic: depends on loading direction

• Strong in tension

• Little resistance to compression

Tendons

• Attach muscle to bone

• Transmit muscle tensile forces to bone

• Contribute to or restrain joint motion

Typical Tendon Load Elongation Curve

1 = Toe

2 = Linear

3 = End of linear region, failure occurring (yield point)

4 = Maximum load followed by complete failure (failure point)