Connective tissues exhibit viscoelastic properties that make their mechanical behavior dependent on time, rate, and loading history. When a constant strain is applied, stress will decrease over time as the tissue relaxes. Conversely, under constant stress, strain will increase over time as the tissue creeps. This viscoelastic behavior, along with the tissue's adaptive responses, allow connective tissues to absorb loads while maintaining structural integrity.

1. General properties of
connective tissues.

2. • Connective tissues change their structure in response to applied
forces; that is, they adapt.
• This adaptive behavior illustrates the dynamic nature of connective
tissue and the strong relationships among structure, composition,
and function.
• The remarkable ability of connective tissues to respond to load
alterations is often referred to as the SAID principle (specific
adaptation to imposed demand).

3. Mechanical Behavior
• Load, Force, and Elongation:
• Load refers to a force or forces applied to a structure.
• The magnitude, direction, and rate of force application, as well as the
size and composition of the tissue, will all affect the tissue’s response
to load.
• When a force acts on an object, it produces a deformation.
• A tensile load produces elongation;
• a compressive force produces compression.

4. • The load-deformation curve is the result of plotting the applied load (force) against the
deformation, providing information about the strength properties of a particular material or
structure.
• The load-deformation curve shows the elasticity, plasticity, ultimate strength, and stiffness of the
material, as well as the amount of energy that the material can absorb before it fails.

5. • The portion of the curve between point A and point B is the elastic
region.
• If the response to loading is confined to the elastic region, the
deformation of the material will not be permanent; the structure will
return to its original dimensions immediately after the load is
removed
• After point B, the yield point at the end of the elastic region, the
material will no longer immediately return to its original state when
the load is removed, although it may recover in time.

6. • The portion of the curve between B and C is the plastic region.
Although the structure will appear to be intact, after the load is
removed the material will not recover its original length—the
deformation is permanent
• If loading continues through the plastic region, the material will
continue to deform until it reaches the ultimate failure point, C. The
load being applied when this point is reached is the failure load.

7. • Force values on the load-deformation curve depend on both the size
of the structure and its composition.
• A structure with a greater cross-sectional area can withstand more
force with less deformation than a structure of the same original
length with less cross-sectional area.
• A longer structure deforms more when a force is applied than does a
shorter structure of similar cross-section

8. Stress and Strain
• When loads (forces) are applied to a structure or material, forces
within the material are produced to oppose the applied forces.
• These forces within the material depend on the composition of the
material.
• When the applied force is tensile, we calculate the stress on the
tissue.
• Stress, the force per cross-sectional unit of material, can be expressed
mathematically with the following formula, where S = stress, F =
applied force, and A = area:
• S = F/A

9. • The percentage change in the length or cross-section of a structure or
material is called strain.
• Strain = (L2 – L1) ÷ L1
• The types of stress and strain that develop in human tissues depend
on the material, the type of load applied, the point at which the load
is applied, the direction and magnitude of the load, and the rate and
duration of loading.

10. • If two applied forces act along the same line but in opposite
directions, they create a distractive or tensile load and cause tensile
stress and tensile strain in the structure or material

11. • If two applied forces act in a line toward each other, they constitute
compressive loading and compressive stress and, as a result,
compressive strain will develop in the structure.

12. • If two applied forces are parallel and are applied in opposite
directions but are not in-line with one another, they constitute shear
loading
• Because stress and strain are independent of size of the material, the
stress-strain curve is said to reflect the material properties of the
tissue.
• With size accounted for, only changes in the material constituting the
tissue will alter the stress-strain curve.

13. • The stress-strain curve can be used to compare the strength
properties of one material with that of another material or to
compare the same tissue under different conditions (e.g., ligaments
before and after immobilization).
• The stress-strain curve contains the same defining points (A, B, and C)
as the load deformation curve, but the shape of the curve and the
amount of stress and strain will vary with the composition of the
material
• The curve will be flatter in more elastic materials and steeper in stiffer
materials.

14. Young’s Modulus
• modulus of elasticity, of a material under compressive or tensile
loading is represented by the slope of the linear portion of the curve
between point A and point B
• The modulus of elasticity is a measure of the material’s stiffness (its
resistance to external loads).
• A value for stiffness can be found by dividing the change in (Δ) stress
by the change in (Δ) strain for any two consecutive sets of points in
the elastic range of the curve.

15. • The inverse of stiffness is compliance.
• If the slope of the curve is steep and the modulus of elasticity is high,
the material exhibits high stiffness and low compliance.
• If the slope of the curve is gradual and the modulus of elasticity is
low, the material exhibits low stiffness and a high compliance

16. Load Deformation and Stress-Strain Curves
• Each material has its own unique stress-strain curve.
• The first region of the curve (0 to A) is called the toe region.
• Very little force is required to deform the tissue
• In this region, a minimal amount of force produces a relatively large
amount of deformation (elongation); stress is low, and the strain is
typically in the 1% to 2% range.

18. • The second portion of the curve A to B is the elastic region, in which
elongation (strain) has a linear relationship with stress.
• Each additional unit of applied force creates an equal stress and strain
in the tissue.
• In this region of the curve, collagen fibrils are being stretched and are
resisting the applied force.
• When the load is removed, the ligament or tendon will return to its
prestressed dimensions, although this return will take some time.
• This level of loading includes the stresses and strains that occur with
normal activities and typically extends to about 4% strain

19. • In the third region (B to C, the plastic region), the failure of collagen
fibers (microfailure) begins, and the ligament or tendon is no longer
capable of returning to its original length after the force is removed.
• Clinical examples include grade I and II ligament sprains and tendon
strains.
• Recovery after this level of loading requires considerable time
because it involves aspects of healing such as synthesis of new tissue
and cross-linking of collagen molecules.

20. • If force continues to be applied beyond the plastic region, the
remaining collagen fibrils experience increased stress and rapidly
rupture sequentially, creating overt failure (macrofailure) of the
tissue.
• In the case of a ligament or tendon, if the failure occurs in the middle
of the structure through a disruption of the connective tissue fibers, it
is called a rupture.
• If the failure occurs at the bony attachment of the ligament or
tendon, it is called an avulsion.
• When failure occurs within bony tissue, it is called a fracture

21. • Each type of connective tissue is able to withstand a different
percentage of strain before failure.
• In general, ligaments and tendons are able to deform more than
cartilage, and cartilage is able to deform more than bone.
• However, the total deformation also depends on the size (length,
width, or depth) of the structure.

22. Viscoelasticity
• All connective tissues are viscoelastic materials: they combine the
properties of elasticity and viscosity, making their behavior time-,
rate-, and history-dependent.
• Elasticity refers to the material’s ability to return to its original length
or shape after the removal of a deforming load

23. • Viscosity refers to a material’s resistance to flow.
• It is a fluid property, and depends on the PG and water composition
of the tissue.
• A tissue with high viscosity will exhibit high resistance to deformation,
whereas a less viscous fluid will deform more readily
• When forces are applied to viscous materials, the tissues exhibit time-
dependent and rate-dependent properties.
• Viscosity diminishes as temperature rises or loads are slowly applied
and increases as pressure increases or loads are rapidly applied.

24. Time-Dependent and Rate-Dependent
Properties
• Viscoelastic materials are capable of undergoing deformation under
either tensile or compressive forces and returning to their original
state after removal of the force.
• However, their viscous qualities make the deformation and return
time-dependent.
• A viscoelastic material possesses characteristics of creep, stress-
relaxation, strain-rate sensitivity, and hysteresis.

25. Creep
• If a force is applied to a tissue and maintained at the same level while
the deformation produced by this force is measured, the deformation
will gradually increase.
• Force remains constant while length changes.
• For example, if you hang a weight on the end of an elastic band, you
will get an immediate elastic deformation.
• However, it will also gradually elongate further over time.

26. • Connective tissues also will gradually elongate (creep) after an initial
elastic response to a constant tensile load and then gradually return
to their original length (recovery) after the load is removed.
• In a clinical setting, this might apply to stretching shortened tissue:
The clinician applies a constant force and the tissue gradually
elongates.
• For cartilage and bone, compressive loading is used to test creep, and
so the depth of indentation represents creep and recovery

28. Stress-Relaxation
• If a tissue is stretched to a fixed length while the force required to
maintain this length is measured, the force needed will decrease over
time.
• Length remains constant while force decreases.
• In a clinical setting, a therapist may perceive this as a reduced
resistance to stretch (less force is required to maintain tissue length).

30. Hysteresis
• When the force and length of the tissues are measured as force is
applied (loaded) and removed (unloaded), the resulting load-
deformation curves do not follow the same path.
• Not all of the energy gained as a result of the lengthening work is
recovered during the exchange from energy to shortening work.
• Some energy is lost, usually as heat

32. Strain-Rate Sensitivity
• Most tissues behave differently if loaded rapidly or slowly.
• When a load is applied rapidly, the tissue is stiffer, and a larger peak
force can be applied to the tissue than if the load was applied slowly.
• The subsequent stressrelaxation also will be larger than if the load
was applied slowly.
• Creep will take longer to occur under conditions of rapid loading