What are Tendons?
Tue, 3 Nov 2009
A tendon (or sinew) 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 fascia
as they are all made of collagen except that ligaments join one bone to
another bone, and fascia connect muscles to other muscles. Tendons and
muscles work together and can only exert a pulling force.
Structure of Tendons
Normal healthy tendons are mostly composed of parallel arrays of collagen
fibers closely packed together. The dry mass of normal tendons, which
makes up about 30% of the total mass in water, is composed of about 86%
collagen, 2% elastin, 1–5% proteoglycans, and 0.2% inorganic components such as copper, manganese, and calcium.
The collagen portion is made up of 97-98% type I collagen, with small
amounts of other types of collagen. These include type II collagen in
the cartilaginous zones, type III collagen in the reticulin fibers of the vascular walls, type IX collagen, type IV collagen in the basement membranes of the capillaries, type V collagen in the vascular walls, and type X collagen in the mineralized fibrocartilage near the interface with the bone. Collagen fibers coalesce into macroaggregates. After secretion from the cell, the terminal peptides are cleaved by procollagen
N- and C-proteinases, and the tropocollagen molecules spontaneously
assemble into insoluble fibrils. A collagen molecule is about 300 nm
long and 1-2 nm wide, and the diameter of the fibrils that are formed
can range from 50-500 nm. In tendons, the fibrils then assemble further
to form fascicles, which are about 10 μm in length with a diameter of
50-300 μm, and finally into a tendon fiber with a diameter of 100-500
μm. Groups of fascicles are bounded by the epitendon and peritendon to form the tendon organ.
The collagen in tendons are held together with proteoglycan components, including decorin and, in compressed regions of tendon, aggrecan, which are capable of binding to the collagen fibrils at specific locations. The proteoglycans are interwoven with the collagen fibrils and that their glycosaminoglycan
(GAG) side chains have multiple interactions with the surface of the
fibrils, showing that the proteoglycans are important structurally in
the interconnection of the fibrils. The major glycosaminoglycan (GAG) components of the tendon are dermatan sulfate and chondroitin sulfate,
which associate with collagen and are involved in the fibril assembly
process during tendon development. Dermatan sulfate is thought to be
responsible for forming associations between fibrils, while chondroitin
sulfate is thought to be more involved with occupying volume between
the fibrils to keep them separated and help withstand deformation.
The dermatan sulfate side chains of decorin aggregate in solution, and
this behavior can assist with the assembly of the collagen fibrils.
When decorin molecules are bound to a collagen fibril, their dermatan
sulfate chains may extend and associate with other dermatan sulfate
chains on decorin that is bound to separate fibrils, therefore creating
interfibrillar bridges and eventually causing parallel alignment of the
produce the collagen molecules which aggregate end-to-end and
side-to-side to produce collagen fibrils. Fibril bundles are organized
to form fibers with the elongated tenocytes closely packed between
them. There is a three-dimensional network of cell processes associated
with collagen in the tendon. The cells communicate with other through gap junctions, and this signaling gives them the ability to detect and respond to mechanical loading.
Blood vessels may be visualized within the endotendon running parallel to collagen fibers, with occasional branching transverse anastomoses.
The internal tendon bulk is thought to contain no nerve fibers, but the epi- and peritendon contain nerve endings, while Golgi tendon organs are present at the junction between tendon and muscle.
Tendon length varies in all major groups and from person to person.
Tendon length is practically the discerning factor where muscle size
and potential muscle size is concerned. For example, should all other
relevant biological factors be equal, a man with a shorter tendons and
a longer biceps muscle will have greater potential for muscle mass than
a man with a longer tendon and a shorter muscle. Successful bodybuilders
will generally have shorter tendons. Conversely, in sports requiring
athletes to excel in actions such as running or jumping, it is
beneficial to have longer than average Achilles tendon and a shorter calf muscle.
Tendon length is determined by genetic predisposition, and has not
been shown to either increase or decrease in response to environment,
unlike muscles which can be shortened by trauma, use imbalances and a
lack of recovery and stretching.
Function of Tendons
Tendons have been traditionally considered to simply be a mechanism
by which muscles connect to bone, functioning simply to transmit
forces. However, over the past two decades, much research focused on
the elastic properties of tendons and their ability to function as
springs. This allows tendons to passively modulate forces during
locomotion, providing additional stability with no active work. It also
allows tendons to store and recover energy at high efficiency. For
example, during a human stride, the Achilles tendon stretches as the
ankle joint dorsiflexes. During the last portion of the stride, as the
foot plantar-flexes (pointing the toes down), the stored elastic energy
is released. Furthermore, because the tendon stretches, the muscle is
able to function with less or even no change in length, allowing the muscle to generate greater force.
The mechanical properties of the tendon are dependent on the
collagen fiber diameter and orientation. The collagen fibrils are
parallel to each other and closely packed, but show a wave-like
appearance due to planar undulations, or crimps, on a scale of several
In tendons, the collagen I fibers have some flexibility due to the
absence of hydroxyproline and proline residues at specific locations in
the amino acid sequence, which allows the formation of other
conformations such as bends or internal loops in the triple helix and
results in the development of crimps.
The crimps in the collagen fibrils allow the tendons to have some
flexibility as well as a low compressive stiffness. In addition,
because the tendon is a multi-stranded structure made up of many
partially independent fibrils and fascicles, it does not behave as a
single rod, and this property also contributes to its flexibility.
The proteoglycan components of tendons also are important to the
mechanical properties. While the collagen fibrils allow tendons to
resist tensile stress, the proteoglycans allow them to resist
compressive stress. The elongation and the strain of the collagen
fibrils alone have been shown to be much lower than the total
elongation and strain of the entire tendon under the same amount of
stress, demonstrating that the proteoglycan-rich matrix must also
undergo deformation, and stiffening of the matrix occurs at high strain
These molecules are very hydrophilic, meaning that they can absorb a
large amount of water and therefore have a high swelling ratio. Since
they are noncovalently bound to the fibrils, they may reversibly
associate and disassociate so that the bridges between fibrils can be
broken and reformed. This process may be involved in allowing the
fibril to elongate and decrease in diameter under tension.
Pathology of Tendons
Tendons are subject to many types of injuries. There are various forms of tendinopathies
or tendon injuries due to overuse. These types of injuries generally
result in inflammation and degeneration or weakening of the tendons,
which may eventually lead to tendon rupture.
Tendinopathies can be caused by a number of factors relating to the
tendon extracellular matrix, and their classification has been
difficult because their symptoms and histopathology often are similar.
The first category of tendinopathy is paratenonitis, which refers to
inflammation of the paratenon, or paratendinous sheet located between
the tendon and its sheath. Tendinosis
refers to non-inflammatory injury to the tendon at the cellular level.
The degradation is caused by damage to collagen, cells, and the
vascular components of the tendon, and is known to lead to rupture.
Observations of tendons that have undergone spontaneous rupture have
shown the presence of collagen fibrils that are not in the correct
parallel orientation or are not uniform in length or diameter, along
with rounded tenocytes, other cell abnormalities, and the ingrowth of
Other forms of tendinosis that have not led to rupture have also shown
the degeneration, disorientation, and thinning of the collagen fibrils,
along with an increase in the amount of glycosaminoglycans between the
The third is paratenonitis with tendinosis, in which combinations of
paratenon inflammation and tendon degeneration are both present. The
last is tendinitis which refers to degeneration with inflammation of the tendon as well as vascular disruption.
Tendinopathies may be caused by several intrinsic factors including
age, body weight, and nutrition. The extrinsic factors are often
related to sports and include excessive forces or loading, poor
training techniques, and environmental conditions.
Healing of Tendons
The tendons in the foot are highly complex and intricate. If any
tendons break it is a long, painful healing process, not to mention the
intricacy of the repairing (if fully severed) process. Most people that
do not receive medical attention within the first 48 hours of the
injury will suffer from severe swelling, pain, and an on-fire feeling
where the injury occurred. They are very painful when they are inflamed
or not in use.
It was believed previously that tendons could not undergo matrix
turnover and that tenocytes were not capable of repair. However, it has
been shown more recently that throughout the lifetime of a person,
tenocytes in the tendon actively synthesize ECM components as well as
enzymes such as matrix metalloproteinases (MMPs) can degrade the matrix.
Tendons are capable of healing and recovering from injuries in a
process that is controlled by the tenocytes and their surrounding
extracellular matrix. However, the healed tendons never regain the same
mechanical properties as before the injury.
The three main stages of tendon healing are inflammation, repair or
proliferation, and remodeling, which can be further divided into
consolidation and maturation. These stages can overlap with each other.
In the first stage, inflammatory cells such as neutrophils are recruited to the injury site, along with erythrocytes. Monocytes and macrophages are recruited within the first 24 hours, and phagocytosis of necrotic materials at the injury site occurs. After the release of vasoactive and chemotactic factors, angiogenesis and the proliferation of tenocytes are initiated. Tenocytes then move into the site and start to synthesize collagen III.
The inflammation stage usually lasts for a few days, and the repair or
proliferation stage then begins. In this stage, which lasts for about
six weeks, the tenocytes are involved in the synthesis of large amounts
of collagen and proteoglycans at the site of injury, and the levels of
GAG and water are high.
After about six weeks, the remodeling stage begins. The first part of
the remodeling stage is consolidation, which lasts from about six to
ten weeks after the injury. During this time, the synthesis of collagen
and GAGs is decreased, and the cellularity is also decreased as the
tissue becomes more fibrous as a result of increased production of
collagen I and the fibrils become aligned in the direction of
The final maturation stage occurs after ten weeks, and during this time
there is an increase in crosslinking of the collagen fibrils, which
causes the tissue to become stiffer. Gradually, over a time period of
about one year, the tissue will turn from fibrous to scar-like.
or MMPs have a very important role in the degradation and remodeling of
the ECM during the healing process after a tendon injury. Certain MMPs
including MMP-1, MMP-2, MMP-8, MMP-13, and MMP-14 have collagenase
activity, meaning that unlike many other enzymes, they are capable of
degrading collagen I fibrils. The degradation of the collagen fibrils
by MMP-1 along with the presence of denatured collagen are factors that
are believed to cause weakening of the tendon ECM and an increase in
the potential for another rupture to occur. In response to repeated mechanical loading or injury, cytokines
may be released by tenocytes and can induce the release of MMPs,
causing degradation of the ECM and leading to recurring injury and
A variety of other molecules are involved in tendon repair and
regeneration. There are five growth factors that have been shown to be
significantly upregulated and active during tendon healing: insulin-like growth factor 1 (IGF-I), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and transforming growth factor beta (TGF-β).
These growth factors all have different roles during the healing
process. IGF-1 increases collagen and proteoglycan production during
the first stage of inflammation, and PDGF is also present during the
early stages after injury and promotes the synthesis of other growth
factors along with the synthesis of DNA and the proliferation of tendon
cells. The three isoforms of TGF-β (TGF-β1, TGF-β2, TGF-β3) are known to play a role in wound healing and scar formation.
VEGF is well known to promote angiogenesis and to induce endothelial
cell proliferation and migration, and VEGF mRNA has been shown to be
expressed at the site of tendon injuries along with collagen I mRNA.
Bone morphogenetic proteins (BMPs) are a subgroup of TGF-β superfamily
that can induce bone and cartilage formation as well as tissue
differentiation, and BMP-12 specifically has been shown to influence
formation and differentiation of tendon tissue and to promote
Effects of activity on healing
In animal models, extensive studies have been conducted to
investigate the effects of mechanical strain in the form of activity
level on tendon injury and healing. While stretching can disrupt
healing during the initial inflammatory phase, it has been shown that
controlled movement of the tendons after about one week following an
acute injury can help to promote the synthesis of collagen by the
tenocytes, leading to increased tensile strength and diameter of the
healed tendons and fewer adhesions than tendons that are immobilized.
In chronic tendon injuries, mechanical loading has also been shown to
stimulate fibroblast proliferation and collagen synthesis along with
collagen realignment, all of which promote repair and remodeling.
To further support the theory that movement and activity assist in
tendon healing, it has been shown that immobilization of the tendons
after injury often has a negative effect on healing. In rabbits,
collagen fascicles that are immobilized have shown decreased tensile
strength, and immobilization also results in lower amounts of water,
proteoglycans, and collagen crosslinks in the tendons.
mechanisms have been proposed as reasons for the response of tenocytes
to mechanical force that enable them to alter their gene expression,
protein synthesis, and cell phenotype and eventually cause changes in
tendon structure. A major factor is mechanical deformation of the extracellular matrix,
which can affect the actin cytoskeleton and therefore affect cell
shape, motility, and function. Mechanical forces can be transmitted by
focal adhesion sites, integrins,
and cell-cell junctions. Changes in the actin cytoskeleton can activate
integrins, which mediate “outside-in” and “inside-out” signaling
between the cell and the matrix. G-proteins,
which induce intracellular signaling cascades, may also be important,
and ion channels are activated by stretching to allow ions such as
calcium, sodium, or potassium to enter the cell.
Uses of sinew
Sinew was widely used throughout pre-industrial eras as a tough, durable fiber. Some specific uses include using sinew as thread for sewing, attaching feathers to arrows (see fletch),
lashing tool blades to shafts, etc. It is also recommended in survival
guides as a material from which strong cordage can be made for items
like traps or living structures. Tendon must be treated in specific
ways to function usefully for these purposes. Inuit and other circumpolar
people utilized sinew as the only cordage for all domestic purposes due
to the lack of other suitable fiber sources in their ecological
The elastic properties of particular sinews were also used in composite recurved bows favoured by the steppe nomads of Eurasia. The first stone throwing artillery also used the elastic properties of sinew.
Sinew makes for an excellent cordage material for three reasons: It
is incredibly strong, it contains natural glues, and it shrinks as it
dries, doing away with the need for knots.
All mammals have two strips of sinew running on either side of their backbones, commonly called backstrap sinew.
The Achilles tendon is a particularly large tendon connecting the heel to the muscles of the calf. It is so named because the mythic hero Achilles was said to have been killed due to an injury to this area.
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