-
Soft Tissue Mechanics
Soft tissues
A primary group of tissue which binds, supports
and protects our human body and structures
such as organs is soft connective tissue.
Examples for soft tissues are muscles, tendons,
ligaments, blood vessels, skins or articular
cartilages among many others.
-
Tendons are muscle-to-bone linkages to stabilize the
bonyskeleton (or to produce motion), while ligaments are
bone-to-bone linkages to restrict relative motion.
Blood vessels are prominent organs composed of soft tissueswhich
have to distend in response to pulse waves.
The skin is the largest single organ (16% of the human
adultweight).
Soft connective tissues are complex fiber-reinforced
compositestructures.
Their mechanical behavior is strongly influenced by
thestructural arrangement of constituents such as collagen
andelastin, and respective function in the organism.
-
Collagen: Collagen is a protein which is a major constituent
ofthe extracellular matrix of connective tissue. It is the main
loadcarrying element in soft tissues.
Elastin: Elastin, like collagen, is a protein which is a
majorconstituent of the extracellular matrix of connective
tissue.
It is present as thin strands in soft tissues such as skin,
lung,ligaments.
-
Types of Muscles
Skeletal muscle
Skeletal Muscles are those which attach to bones and have the
main function of contracting to facilitate movement of our
skeletons.
Smooth muscle
- Smooth muscle is also sometimes known as involuntary muscle
due to our inability to control its movements.
- Smooth muscle is found in the walls of hollow organs such as
the Stomach, Oesophagus, Bronchi and in the walls of blood
vessels.
Cardiac muscle (heart muscle)
This type of muscle is found solely in the walls of the heart.
It has similarities with skeletal muscles in that it is
striated.
-
Muscles are described as running from a proximal origin to a
distal insertion.
A muscle generally receives its blood supply from one main
artery, which enters the muscle in a single/branches.
A single nerve, which carries both motor efferents and sensory
afferents.
Efferent nerves, otherwise known as motor neurons, carry
nerveimpulses away from the CNS to effectors such as muscles or
glands.
The opposite activity of direction or flow is afferent.
MuscleGross Morphology
-
Skeletal muscle
- Skeletal muscle is a fascinating biological tissue able to
transform
chemical energy to mechanical energy.
- Skeletal muscle has three basic performance parameters
that
describe its function:
Movement production
Force production
Endurance
- The production of movement and force is the mechanicaloutcome
of skeletal muscle contraction.
-
Skeletal muscle..
The skeletal muscles like the joints, are designed to contribute
to the bodys needs for mobility and stability.
Muscles serve a mobility function by producing or controlling
the movements of a bony lever around a joints axis.
A human skeleton without muscles will collapse when placed in
the erect standing position.
Skeletal muscles are length- and velocity-dependent force
generators.
The muscles transmit force to bones via tendons.
-
Biological soft tissues are nonlinear, anisotropic, fibrous
composites.
One can separate these tissues based on their mode of loading:
cartilage is generally loaded in compression;
tendons and ligaments are loaded in tension; and muscles
generate active tension.
The structure and material properties differ to accommodate the
tissue function.
-
Structure of Skeletal Muscle
- The functional unit that produces motion at a joint
consists
of two discrete units, the muscle belly and the tendon.
- The muscle belly consists of the muscle cells, or fibers,
thatproduce the contraction.
Structure of an Individual Muscle Fiber
- A skeletal muscle fiber is a long cylindrical,
multinucleated
cell that is filled with smaller units of filaments (Fig).
- These filamentous structures are roughly aligned parallel
tothe muscle fiber itself.
-
The largest of the filaments is the myofibril, composed
ofsubunits called sarcomeres that are arranged end to end thelength
of the myofibril.
Each sarcomere also contains filaments, known asmyofilaments.
There are two types of myofilaments within eachsarcomere.
The thicker myofilaments are composed of myosin
proteinmolecules, and the thinner myofilaments are composed
ofmolecules of the protein actin.
Structure of muscle..
-
The myofilaments in each sarcomere are 1 to 2 m long; themyosin
myofilaments are longer than the actin myofilaments.
Thus sarcomeres in humans are a few micrometers in
length:varying from approximately 1.25 to 4.5 m with
musclecontraction and stretch.
Sliding of the actin myofilament on the myosin chain is thebasic
mechanism of muscle contraction.
-
The Sliding Filament Theory of Muscle Contraction
The sarcomere, containing the contractile proteins actin and
myosin, is the basic functional unit of muscle.
Contraction of a whole muscle is actually the sum of
singularcontraction events occurring within the individual
sarcomeres.
The organization of the sarcomere.
The thinner actin chains are more abundant than the myosin
myofilaments in a sarcomere.
The actin myofilaments are anchored at both ends of thesarcomere
at the Z-line and project into the interior of thesarcomere where
they surround a thicker myosin myofilament(Fig. 4.2).
-
The amount of these contractile proteins within the cells is
strongly related to a muscles contractile force.
Contraction results from the formation of cross-bridges
between the myosin and actin myofilaments, causing the actin
chains to slide on the myosin chain (Fig 4.3).
The tension of the contraction depends upon the number of
cross-bridges formed b/w the actin and myosin myofilaments.
The number of cross-bridges formed also depends on the
frequency of the stimulus to form cross-bridges.
-
Contraction is initiated by an electrical stimulus from the
associated motor neuron causing depolarization of the muscle
fiber.
When the fiber is depolarized, calcium is released into
the cell and binds with the regulating protein troponin.
The combination of calcium with troponin acts as a
trigger,causing actin to bind with myosin, beginning the
contraction.
Cessation (stop) of the nerves stimulus causes a reduction
incalcium levels within the muscle fiber, inhibiting the
crossbridgesb/w actin and myosin.
-
The muscle relaxes stimulation of the muscle fiber occurs at
asufficiently high frequency, new cross-bridges are formed
beforeprior interactions are completely severed, causing a fusion
ofsucceeding contractions.
Ultimately a sustained, or tetanic, contraction is produced.
Modulation of the frequency and magnitude of the initialstimulus
has an effect on the force of contraction of a wholemuscle.
-
The Connective Tissue System within the Muscle Belly
The muscle belly consists of the muscle cells, or fibers, andthe
connective tissue that binds the cells together (Fig. 4.4).
The outermost layer of connective tissue that surrounds
theentire muscle belly is known as the epimysium.
The muscle belly is divided into smaller bundles or fasciclesby
additional connective tissue known as perimysium.
Finally individual fibers within these larger sheaths
aresurrounded by more connective tissue, the endomysium.
Thus the entire muscle belly is invested in a large network
ofconnective tissue.
-
The amount of connective tissue within a muscle and the size
of the connecting tendons vary widely from muscle to muscle.
The amount of connective tissue found within an individual
muscle influences the mechanical properties of that muscle.
-
Effect of Fiber Length on Joint Excursion
Fiber length has a significant influence on the magnitude of
thejoint motion that results from a muscle contraction.
The fundamental behavior of muscle is shortening, and it isthis
shortening that produces joint motion.
Each sarcomere can shorten to approximately the length of
itsmyosin molecules.
Because the sarcomeres are arranged in series in a myofibril,
amuscle fiber can produce is the sum of the shortening in all ofthe
sarcomeres.
-
Thus the total shortening of a muscle fiber depends upon
thenumber of sarcomeres arranged in series within
eachmyofibril.
The more sarcomeres in a fiber, the longer the fiber is andthe
more it is able to shorten (Fig. 4.5).
The amount a muscle fiber can shorten is proportional to
itslength. A fiber can shorten roughly 50 to 60% of its length.
-
Sliding filament model of muscle
-
Basic Behaviors of the Skeletal Muscle
Extensibility the ability to be stretched or to increase in
length
Elasticity the ability to return to the original length after
a
stretch
Irritability the ability to respond to a a stimulus
Ability to develop tension: the ability to decrease in
length
-
Contractions
A concentric contraction is a type of muscle contraction in
which the muscles shorten while generating force.
During a concentric contraction, a muscle is stimulated to
contract according to the sliding filament mechanism.
An eccentric contraction occurs when a muscle is contracting,
and an external force is trying to lengthen the muscle (
strain).
- An eccentric contraction is also a type of strengthening
exercise for a muscle, when performed in a controlled manner.
-
An isometric contraction of a muscle generates force without
changing length. An example can be found when the muscles of the
hand and forearm grip an object.
-
Effect of Muscle
The moment arm depends on the location of the musclesattachment
on the bone and on the angle between the line of pullof the muscle
and the limb to which the muscle attaches.
This angle is known as the angle of application (Fig).
The sine of the angle of application, , can be measured
directly.
Fig.: The relationship between a muscles moment arm and
excursion.
-
Structural Organizaiton of Skeletal Muscle
Depends on muscle fiber, motor unit,
fiber types, and fiber architecture
- parallel fiber arrangement parallel to the
longitudinal axis of the muscle,
e.g. sartorius, biceps brachii, etc.
- pennate fiber arrangement at an angle to the longitudinal axis
of the muscle, e.g. rectus femoris, deltoid, etc.
The greater the angle of pennation, the smaller the amount of
effective force transmitted to the tendon
The angle of the pennation increases as tension progressively
increases in the muscle fibers
-
Muscle Size and Its Effect on Force Production
The angle at which the fibers insert into the tendon also
influences the total force.
This angle is known as the angle of pennation.
The tensile force generated by the whole muscle is the vector
sum of the force components that are applied parallel to the
muscles tendon (Fig. 4.11).
Therefore, as the angle of pennationincreases, the tensile
component of the contraction force decreases.
Figure 4.11: The overall tensile force (FM) of a muscle is
the vector sum of the force of contraction of the pennate
fibers (FF).
-
Muscle strength
The muscles tensile force of contraction and its resulting
moment are related by the following:
M = r X F
where M is the moment generated
F = the muscles tensile force applied at a distance,
r is the muscles moment arm
The primary factors influencing the muscles strength are
Muscle size, Muscle moment arm, Stretch of the muscle
Contraction velocity, Level of muscle fiber recruitment
Fiber types composing the muscle
-
Mechanical Properties of the Skeletal Muscle
Force-Length/Length-Tension Relationship
The tension that a muscle generates
varies with its length
Found when a muscle under isometric
contraction and for maximum activation
of the muscle
In a single muscle fiber,
peak force is noted at normal resting length
a bell-shaped length-tension curve
In a muscle, force generation capacity increases when the muscle
is slightly stretched because of the effect of both active and
passive components.
-
Force-Velocity Relationship
Muscle force decreases as the velocity of contraction increases
(Hill, 1938)
- only true for concentric contraction
Muscle force decreases with increased velocity of contraction
during concentric contraction whereas it increases with increased
velocity of contraction during eccentric contraction.
-
Eccentric strength of a muscle can exceed isometric strength by
a factor of 1.5 to 2.0, but this is true only under electric
stimulation of the motor neuron.
Maximum strength can be generated either by recruitment of more
motor unit or by increase in muscle length
-
Forcevelocity relationship
The speed at which a muscle changes length also affects the
force it can generate.
Force declines in a hyperbolic fashion relative to the isometric
force as the shortening velocity increases, eventually reaching
zero at some maximum velocity.
The reverse holds true for when the muscle is stretched force
increases above isometric maximum, until finally reaching an
absolute maximum.
This has strong implications for the rate at which muscles can
perform mechanical work (power).
-
Effects of the magnitude of the contraction velocity on force
production in muscle
Contractile velocity of a muscle is
determined by the change in length per
unit time.
Thus an isometric contraction has zero
contraction velocity.
In contrast, a concentric contraction (shortening contraction),
is shortening of the muscle, has a positive contraction
velocity.
The contractile force is maximum when contraction velocity is
zero (isometric contraction) and decreases as contraction velocity
increases (Fig. 4.19).
-
Since power is equal to force times velocity, the muscle
generates no power
at either isometric force (due to zero
velocity) or maximal velocity
(due to zero force).
Instead, the optimal shortening velocity for power generation is
approximately one-third of maximum shortening velocity.
These two fundamental properties of muscle have numerous
biomechanical consequences, including limiting running speed,
strength, and jumping distance and height.
-
Muscle Architecture
Where M = muscle mass, = muscle density (1.056 g/cm3
in fresh tissue), = surface pennation angle, and L f = myofiber
length.
This formulation provides a good estimate of experimentally
measured
isometric muscle force output
It is typically described in terms of muscle length, mass,
myofiber length, and physiological cross-sectional area (PCSA).
PCSA: The standard measure used to approximate the number
offibers of a whole muscle, projected along the muscles line of
action, it is calculated as:
-
Muscle Force-Velocity Relationship Under conditions of constant
load the relationship between force and velocity is nearly
hyperbolic.
The shortening force - velocity relation can be described
by:
Where a and b are constants derived experimentally, P is muscle
force,
Po is maximum tetanic tension, and v is muscle velocity.
-
Mechanical Model
The combined effects of muscle contraction and stretch of
the elastic components are represented mechanically by a
contractile element in series and in parallel with the
elastic
components (Fig).
Fig: A mechanical model of the
contractile and elastic components of a
muscle.
A muscles contractile (actin andmyosin) and elastic (connective
tissue)
components are modeled mechanically
as a combination of a contractile
element (CE) with springs that
represent the elastic elements that are
both in series (SE) and in parallel (PE)
with the contractile component.
-
Mechanical Model of a Muscle
contractile component muscle fiber series elastic component
(SEC) tendon
parallel elastic component (PEC) muscle membrane
-
Neuromuscular disease
It is either directly, via intrinsic muscle pathology, or
indirectly, via nerve pathology, impair the functioning of the
muscles.
Neuromuscular diseases are those that affect the muscles and/or
their nervous control.
In general, problems with nervous control can cause either
spasticity or some degree of paralysis, depending on the location
and the nature of the problem.
-
A large proportion of neurological disorders leads to problems
with movement.
Some examples of these disorders include
- Amyotrophic Lateral Sclerosis (ALS or Lou Gehrig's
Disease),
- Cerebrovascular accident (stroke),
- Parkinson's disease,
- Multiple sclerosis,
- Muscular dystrophy,
- Myasthenia gravis, etc
-
Cartilage
Cartilage is a flexible connective tissue found
in the joints between bones, the ear, the
nose , in joints, and the intervertebral discs.
It is not as hard and rigid as bone but is stiffer
and less flexible than muscle.
Cartilage is composed of specialized cells called
Chondroblasts , composed of Type II collagen fibers,
and elastin fibers.
It does not contain blood vessels. thus it heals very
slowly.
These mechanical properties include the response of cartilage in
frictional, compressive, shear and tensile loading.
-
Such experiments reveal that when the muscle is very short,
stimulation produces a small contractile force.
As the stretch increases and stimulations continue, the tension
in the muscle increases.
The overall tension of the muscle is greatest when the muscle is
stretched maximally.
-
Structure of Soft Tissue
Cartilage
Articular cartilage is found at the
ends of bones, where it serves as
a shock absorber and lubricant between bones of 1-2mm
thickness.
The composition of articular cartilage consists of approximately
20% collagen, 5% proteoglycan, & remaining 75% water.
-
Tendons
A tendon is a tough band of fibrous connective tissue that
usually connects muscle to bone and is capable of withstanding
tension.
Tendons are similar to ligaments and fasciae as they are all
made of collagen except that ligaments join one bone to another
bone, and fasciae connect muscles to other muscles.
Tendons and muscles work together.
Normal healthy tendons are composed mostly of parallel arrays of
collagen fibers closely packed together.
The mechanical properties of the tendon are dependent on the
collagen fiber diameter and orientation.
-
- Finally, multiple fascicles are bundled into a complete tendon
or ligament encased in a reticular membrane.
- Individual collagen fibrils also display some inherent
elasticity, and these two features are believed to determine the
bulk properties of passive tensile tissues.
FIGURE 2.1 Tendons are organized in progressively larger
filaments, beginning with molecular tropocollagen, and
building
to a complete tendon encased in a reticular sheath.
-
Ligaments
It is most commonly refers to a band of
tough, fibrous dense regular connective tissue
comprising attenuated collagenous fibers.
Ligaments connect bones to other bones to form a joint.
Ligaments are viscoelastic. They gradually lengthen when
under tension, and return to their original shape when the
tension is removed.
They act as mechanical reinforcement.
Instability of a joint can over time lead to wear of the
cartilage and eventually to osteoarthritis.
-
Tendon and Ligament
Because the biomechanical behavior of a tissue is deter-mined by
its composition and structure, the mechanical properties of
ligaments, tendons, and cartilage are also considerably
different.
The structural properties are inuenced not onlyby the properties
and geometry of the tissue but also by the mechanical properties of
the bonetissue and muscletissue junctions.
The passive tensile tissues, tendon and ligament, are composed
largely of water and collagen.
The collagen fibrils, with the 2040-nm fibrils being bundled
into 0.212-m fibers.
The fibers are bundled into fascicles, supported by fibroblasts
or tenocytes, and surrounded by a fascicular membrane.
-
Tendon & Ligament
Ultimate tensile stress of tendon considerably high (50-
100 MPa)
Viscoelastic behaviors
creep, stress-relaxation
strain rate sensitivity, different from bone
fast strain rate ligament injuries, slow rate
(avulsion fracture)
Partial failure
Geometry
-
Tendon & Ligament
Age
before maturity: more viscous & compliant
maturity: stiffness & modulus of elasticity
After middle age: viscosity, weak insertions