This document discusses the mechanical properties of noncontractile soft tissue and different stretching techniques. It begins by describing the composition of connective tissue, including collagen, elastin, and ground substance. It then explains how the mechanical behavior of tissue is determined by the proportion of fibers and their orientation. The stress-strain curve is examined, outlining regions like the toe region, elastic range, and plastic range. Different types of stretching techniques are defined, such as static, cyclic, ballistic, and proprioceptive neuromuscular facilitation stretching. PNF techniques like hold-relax, agonist contraction, and hold-relax with agonist contraction are explained in detail.
2. Mechanical Properties of
Noncontractile Soft Tissue
› Composition of Connective Tissue
› Connective tissue is composed of three types of fiber: collagen,
elastin and reticulin, and nonfibrous ground substance (proteoglycans
and glycoproteins).
› Collagen fibers. Collagen fibers are responsible for the strength and
stiffness of tissue and resist tensile deformation.
› Elastin fibers. Elastin fibers provide extensibility. They show a great
deal of elongation with small loads and fail abruptly without
deformation at higher loads. Tissues with greater amounts of elastin
have greater flexibility.
› Reticulin fibers: Reticulin fibers provide tissue with bulk.
› Ground substance. Ground substance is made up of proteoglycans
(PGs) and glycoproteins. The PGs function to hydrate the matrix,
stabilize the collagen networks, and resist compressive forces.
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3. Mechanical Behavior of
Noncontractile Tissue
› The mechanical behavior of the various noncontractile tissues is determined
by the proportion of collagen and elastin fibers and by the structural
orientation of the fibers.
› The proportion of proteoglycans (PGs) also influences the mechanical
properties of connective tissue.
› Those tissues that withstand high tensile loads are high in collagen fibers;
those that withstand greater compressive loads have greater concentrations
of PGs. The composition of the tissue changes when the load changes.
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4. › Collagen is the structural element that absorbs most of
the tensile stress. Its mechanical behavior is explained
with reference to the stress-strain curve.
› When tensile forces are applied, the maximum
elongation of collagen is less than 10%, whereas elastin
may lengthen 150% and return to its original
configuration. Collagen is five times as strong as elastin.
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5. The Stress-Strain Curve
› Stress
› Strain
› Types of Stress:
› There are three kinds of
stress that cause strain
to structures:
› ■ Tension—a force applied
perpendicular to the cross-sectional
area of the tissue in a direction away
from the tissue. A stretching force is a
tension stress.
› ■ Compression—a force applied
perpendicular to the cross sectional
area of the tissue in a direction toward
the tissue.
› Muscle contraction and loading of a
joint during weight bearing cause
compression stresses in joints.
› ■ Shear—a force applied parallel to
the cross-sectional area of the tissue.
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6. Regions of the Stress-Strain
Curve
› Toe region. That area of the stress-
strain curve in which there is
considerable deformation without
the use of much force is called the
toe region. This is the range in
which most functional activity
normally occurs.
› Elastic range/linear phase. Strain is
directly proportional to the ability of
tissue to resist the force. This occurs
when tissue is taken to the end of its
ROM, and gentle stretch is applied.
› With stress in this phase, the collagen
fibers line up with the applied force; the
bonds between fibers and between the
surrounding matrix are strained; some
microfailure between the collagen
bonds begins; and some water may be
displaced from the ground substance.
› There is complete recovery from this
deformation, and the tissue returns to its
original size and shape when the load is
released if the stress is not maintained
for any length of time.
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7. › Elastic limit. The point
beyond which the
tissue does not return
to its original shape
and size is the elastic
limit.
› Plastic range. The
range beyond the
elastic limit extending
to the point of rupture
is the plastic range.
› Tissue strained in this
range has permanent
deformation when the
stress is released.
› The region of necking is
reached in which there
is considerable
weakening of the
tissue, and it rapidly
fails.
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8. › Failure. Rupture of the
integrity of the tissue is
called failure.
› Structural stiffness.
The slope of the linear
portion of the curve
(elastic range) is known
as Young’s modulus or
modulus of elasticity
and represents the
stiffness of the tissue.
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9. › The grades of ligament
injuries (strains) can be
related to the
› stress-strain curve.
› Grade I—Microfailure:
rupture of a few fibers
in the lower part of the
plastic range.
› Grade II—Macrofailure:
rupture of a greater
number of fibers
resulting in partial tear
further into the plastic
range.
› Grade III—Complete
rupture or tissue failure.
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10. Time and Rate Influences on
Tissue Deformation
› Tissue response to
prolonged stretch
forces as a result of
viscoelastic properties.
Effects of creep. A
constant load, applied
over time, results in
increased tissue length
until equilibrium is
reached.
› Effects of stress-
relaxation. A load
applied with the tissue
kept at a constant
length results in
decreased internal
tension in the tissue
until equilibrium is
reached.
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11. Changes in Collagen Affecting
Stress-Strain
Response
› Effects of
immobilization
› Effects of age
› Effects of injury
› Effects of
corticosteroids
› Effects of Inactivity
(Decrease of Normal
Activity)
› There is a decrease in the size
and amount of collagen fibers,
resulting in weakening of the
tissue. There is a proportional
increase in the predominance
of elastin fibers, resulting in
increased compliance
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14. Static Stretching
› Static stretching is a commonly
used method of stretching in
which soft tissues are elongated
just past the point of tissue
resistance and then held in the
lengthened position with a
sustained stretch force over a
period of time.
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15. Cyclic Stretching
› A relatively short-duration stretch force that is
repeatedly but gradually applied, released, and then
reapplied is described as a cyclic (intermittent) stretch.
› Cyclic stretching, by its very nature, is applied for
multiple repetitions (stretch cycles) during a single
treatment session.
› With cyclic stretching, the end-range stretch force is
applied at a slow velocity, in a controlled manner, and
at relatively low-intensity
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16. Ballistic Stretching
› A rapid, forceful intermittent stretch—that is, a high-
speed and high-intensity stretch—is commonly called
ballistic stretching.
› It is characterized by the use of quick, bouncing
movements that create momentum to carry the body
segment through the ROM to stretch shortened
structures.
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17. Manual Stretching
› During manual stretching,
a therapist applies an
external force to move the
involved body segment
slightly beyond the point
of tissue resistance and
available ROM.
› The therapist manually
controls the site of
stabilization as well as the
direction, speed, intensity,
and duration of stretch.
› Stretching can be
performed passively, with
assistance from the
patient, or even
independently by the
patient.
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18. Self-Stretching
› Self-stretching (also
referred to as flexibility
exercises or active
stretching) is a type of
stretching procedure a
patient carries out
independently after
careful instruction and
supervised practice.
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20. Mechanical Stretching
› Mechanical stretching devices
apply a very low-intensity
stretch force (low-load) over a
prolonged period of time to
create relatively permanent
lengthening of soft tissues,
presumably due to plastic
deformation.
› The equipment can be as
simple as a cuff weight or
weight-pulley system or as
sophisticated as some
adjustable orthotic devices or
automated stretching
machines.
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22. Hold-Relax and Contract-Relax
› With the HR and CR
procedures, the range
limiting target muscle is
first lengthened to the
point of tissue
resistance or to the
extent that is
comfortable for the
patient.
› The patient then performs
a prestretch, end-range,
isometric contraction (for
about 5 seconds) followed
by voluntary relaxation of
the range-limiting target
muscle.
› The limb is then
› passively moved into the
new range as the range-
limiting muscle is
elongated.
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24. Agonist Contraction
› To perform the AC procedure, the
patient concentrically contracts
(shortens) the muscle opposite the
range-limiting muscle and then
holds the end-range position for at
least several seconds.
› The movement of the limb is
controlled independently by the
patient and is deliberate and slow,
not ballistic. In most instances, the
shortening contraction is
performed without the addition of
resistance.
› For example,
› if the hip flexors are the range-
limiting target muscle group, the
patient performs end-range, prone
leg lifts by contracting the hip
extensors concentrically; the end-
range contraction of the hip
extensors is held for several
seconds. After a brief rest period,
the patient repeats the procedure.
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25. Hold-Relax with Agonist
Contraction
› The HR-AC stretching
technique combines
the HR and AC
procedures.
› The HR-AC technique is
also referred to as the
CR-AC procedure or
slow reversal hold-relax
technique.
› To perform the HR-AC
procedure, move the limb to
the point that tissue
resistance is felt in the range-
limiting target muscle; then
have the patient perform a
resisted, prestretch isometric
contraction of the range-
limiting muscle followed by
voluntary relaxation of that
muscle and an immediate
concentric contraction of the
muscle opposite the range-
limiting muscle.
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