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Tendon
The Achilles tendon, one of the tendons in the human body (from Gray's Anatomy, 1st ed., 1858)
Micrograph of a piece of tendon; H&E stain
Details
Identifiers
Latintendo
MeSHD013710
TA22010
THH3.03.00.0.00020
FMA9721
Anatomical terminology

A tendon or sinew is a tough band of dense fibrous connective tissue that connects muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tension.

Tendons, like ligaments, are made of collagen. The difference is that ligaments connect bone to bone, while tendons connect muscle to bone. There are about 4,000 tendons in the adult human body.[1][2]

Structure

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A tendon is made of dense regular connective tissue, whose main cellular components are special fibroblasts called tendon cells (tenocytes).[3] Tendon cells synthesize the tendon's extracellular matrix, which abounds with densely-packed collagen fibers. The collagen fibers run parallel to each other and are grouped into fascicles. Each fascicle is bound by an endotendineum, which is a delicate loose connective tissue containing thin collagen fibrils[4][5] and elastic fibers.[6] A set of fascicles is bound by an epitenon, which is a sheath of dense irregular connective tissue. The whole tendon is enclosed by a fascia. The space between the fascia and the tendon tissue is filled with the paratenon, a fatty loose connective tissue.[7] Normal healthy tendons are anchored to bone by Sharpey's fibres.

Extracellular matrix

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The dry mass of normal tendons, which is 30–45% of their total mass, is made of:

Although most of a tendon's collagen is type I collagen, many minor collagens are present that play vital roles in tendon development and function. These include type II collagen in the cartilaginous zones, type III collagen in the reticulin fibres 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.[8][12]

Ultrastructure and collagen synthesis

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Collagen fibres coalesce into macroaggregates. After secretion from the cell, cleaved by procollagen N- and C-proteases, 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 mm in length with a diameter of 50–300 μm, and finally into a tendon fibre with a diameter of 100–500 μm.[13]

The collagen in tendons are held together with proteoglycan (a compound consisting of a protein bonded to glycosaminoglycan groups, present especially in connective tissue) components including decorin and, in compressed regions of tendon, aggrecan, which are capable of binding to the collagen fibrils at specific locations.[14] The proteoglycans are interwoven with the collagen fibrils – 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.[15] The major 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.[16] 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 fibrils.[17]

Tenocytes

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The tenocytes produce the collagen molecules, which aggregate end-to-end and side-to-side to produce collagen fibrils. Fibril bundles are organized to form fibres 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 each other through gap junctions, and this signalling gives them the ability to detect and respond to mechanical loading.[18] These communications happen by two proteins essentially: connexin 43, present where the cells processes meet and in cell bodies connexin 32, present only where the processes meet.[19]

Blood vessels may be visualized within the endotendon running parallel to collagen fibres, with occasional branching transverse anastomoses.

The internal tendon bulk is thought to contain no nerve fibres, but the epitenon and paratenon contain nerve endings, while Golgi tendon organs are present at the myotendinous junction between tendon and muscle.

Tendon length varies in all major groups and from person to person. Tendon length is, in practice, the deciding factor regarding actual and potential muscle size. For example, all other relevant biological factors being 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.[20]

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.[21] In addition tendons allow muscles to be at an optimal distance from the site where they actively engage in movement, passing through regions where space is premium, like the carpal tunnel.[19]

List of tendons

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There are about 4,000 tendons in the human body, of which 55 are listed in the following table:

Functions

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Magnified view of a tendon

Traditionally, tendons have been considered to be a mechanism by which muscles connect to bone as well as muscles itself, functioning to transmit forces. This connection allows tendons to passively modulate forces during locomotion, providing additional stability with no active work. However, over the past two decades, much research has focused on the elastic properties of some tendons and their ability to function as springs. Not all tendons are required to perform the same functional role, with some predominantly positioning limbs, such as the fingers when writing (positional tendons) and others acting as springs to make locomotion more efficient (energy storing tendons).[22] Energy storing tendons can 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 more 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 micrometers.[23] In tendons, the collagen fibres 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.[24] 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.[25]

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. 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.[26] However, the proteoglycans may also have a role in the tensile properties of tendon. The structure of tendon is effectively a fibre composite material, built as a series of hierarchical levels. At each level of the hierarchy, the collagen units are bound together by either collagen crosslinks, or the proteoglycans, to create a structure highly resistant to tensile load.[27] 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 rates.[28] This deformation of the non-collagenous matrix occurs at all levels of the tendon hierarchy, and by modulating the organisation and structure of this matrix, the different mechanical properties required by different tendons can be achieved.[29] Energy storing tendons have been shown to utilise significant amounts of sliding between fascicles to enable the high strain characteristics they require, whilst positional tendons rely more heavily on sliding between collagen fibres and fibrils.[30] However, recent data suggests that energy storing tendons may also contain fascicles which are twisted, or helical, in nature - an arrangement that would be highly beneficial for providing the spring-like behaviour required in these tendons.[31]

Mechanics

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Main article: Soft tissue

Tendons are viscoelastic structures, which means they exhibit both elastic and viscous behaviour. When stretched, tendons exhibit typical "soft tissue" behavior. The force-extension, or stress-strain curve starts with a very low stiffness region, as the crimp structure straightens and the collagen fibres align suggesting negative Poisson's ratio in the fibres of the tendon. More recently, tests carried out in vivo (through MRI) and ex vivo (through mechanical testing of various cadaveric tendon tissue) have shown that healthy tendons are highly anisotropic and exhibit a negative Poisson's ratio (auxetic) in some planes when stretched up to 2% along their length, i.e. within their normal range of motion.[32] After this 'toe' region, the structure becomes significantly stiffer, and has a linear stress-strain curve until it begins to fail. The mechanical properties of tendons vary widely, as they are matched to the functional requirements of the tendon. The energy storing tendons tend to be more elastic, or less stiff, so they can more easily store energy, whilst the stiffer positional tendons tend to be a little more viscoelastic, and less elastic, so they can provide finer control of movement. A typical energy storing tendon will fail at around 12–15% strain, and a stress in the region of 100–150 MPa, although some tendons are notably more extensible than this, for example the superficial digital flexor in the horse, which stretches in excess of 20% when galloping.[33] Positional tendons can fail at strains as low as 6–8%, but can have moduli in the region of 700–1000 MPa.[34]

Several studies have demonstrated that tendons respond to changes in mechanical loading with growth and remodeling processes, much like bones. In particular, a study showed that disuse of the Achilles tendon in rats resulted in a decrease in the average thickness of the collagen fiber bundles comprising the tendon.[35] In humans, an experiment in which people were subjected to a simulated micro-gravity environment found that tendon stiffness decreased significantly, even when subjects were required to perform restiveness exercises.[36] These effects have implications in areas ranging from treatment of bedridden patients to the design of more effective exercises for astronauts.

Clinical significance

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Injury

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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.[37] Tendinopathies can be caused by a number of factors relating to the tendon extracellular matrix (ECM), and their classification has been difficult because their symptoms and histopathology often are similar.

Types of tendinopathy include:[38]

  • Tendinosis: 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.[39] 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 blood vessels.[37] 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 fibrils.[40]
  • Tendinitis: degeneration with inflammation of the tendon as well as vascular disruption.[8]
  • Paratenonitis: inflammation of the paratenon, or paratendinous sheet located between the tendon and its sheath.[38]

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.[41]

Healing

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It was believed that tendons could not undergo matrix turnover and that tenocytes were not capable of repair. However, it has since been shown that, throughout the lifetime of a person, tenocytes in the tendon actively synthesize matrix components as well as enzymes such as matrix metalloproteinases (MMPs) can degrade the matrix.[41] Tendons are capable of healing and recovering from injuries in a process that is controlled by the tenocytes and their surrounding extracellular matrix.

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.[37][40] After a few days, the repair or proliferation stage begins. In this stage, 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.[42] After about six weeks, the remodeling stage begins. The first part of this 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 mechanical stress.[40] 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 about one year, the tissue will turn from fibrous to scar-like.[42]

Matrix metalloproteinases (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.[43] 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 chronic tendinopathies.[40]

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).[42] 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.[42] The three isoforms of TGF-β (TGF-β1, TGF-β2, TGF-β3) are known to play a role in wound healing and scar formation.[44] 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.[45] 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 fibrogenesis.[citation needed]

Effects of activity on healing

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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.[42] 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.[37]

Several mechanotransduction 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.[42]

Society and culture

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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 habitats. The elastic properties of particular sinews were also used in composite recurved bows favoured by the steppe nomads of Eurasia, and Native Americans. The first stone throwing artillery also used the elastic properties of sinew.

Sinew makes for an excellent cordage material for three reasons: It is extremely strong, it contains natural glues, and it shrinks as it dries, doing away with the need for knots[clarification needed].

Culinary uses

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Main article: Tendon (meal)

Tendon (in particular, beef tendon) is used as a food in some Asian cuisines (often served at yum cha or dim sum restaurants). One popular dish is suan bao niu jin, in which the tendon is marinated in garlic. It is also sometimes found in the Vietnamese noodle dish phở.

Other animals

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Ossified tendon from an Edmontosaurus bone bed in Wyoming (Lance Formation)

In some organisms, notably birds,[46] and ornithischian dinosaurs,[47] portions of the tendon can become ossified. In this process, osteocytes infiltrate the tendon and lay down bone as they would in sesamoid bone such as the patella. In birds, tendon ossification primarily occurs in the hindlimb, while in ornithischian dinosaurs, ossified axial muscle tendons form a latticework along the neural and haemal spines on the tail, presumably for support.

See also

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This article uses anatomical terminology.
Wikimedia Commons has media related to Tendons.

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Tendon

View on Grokipedia
from Grokipedia
A tendon is a tough, flexible band of dense fibrous that connects muscle to or other structures, such as the eyeball, serving as a mechanical bridge to transmit the force generated by to enable movement and skeletal stability. Unlike ligaments, which connect to to maintain structural integrity, tendons specifically facilitate motion by linking contractile muscle tissue to the skeletal . Tendons exhibit a hierarchical structure optimized for tensile strength and force transmission, consisting of collagen molecules organized into , fibers, fascicles, and finally the tendon proper, with all components aligned parallel to the tendon's long axis. Their composition is dominated by , accounting for 60–85% of dry weight—primarily (about 95%)—along with smaller amounts of types III, V, XI, XII, and XIV, as well as non-collagenous elements like proteoglycans (e.g., , comprising ~80% of this category), glycoproteins such as for , and elastic fibers including and fibrillins. This provides the tendon with remarkable mechanical properties, allowing it to withstand high tensile forces while exhibiting varying strain capacities: energy-storing tendons, like the Achilles, can stretch over 10%, whereas positional tendons typically strain 2–3%. Tendons play a critical role in locomotion and load-bearing across the body, with over 4,000 named tendons in humans varying in size, shape, and location to suit specific biomechanical demands, such as the robust at the ankle or the slender flexor tendons in the hand. They receive blood supply primarily from surrounding tissues and intrinsic vascular networks, which influences healing potential, and are susceptible to injuries like or rupture due to overuse, aging, or trauma, highlighting their importance in musculoskeletal health.

Overview

Definition and Basic Characteristics

A tendon is a tough, flexible band of dense fibrous that connects muscle to or other structures, serving as a mechanical bridge to transmit the forces generated by to the skeletal system. This structure enables efficient movement by converting muscular effort into precise actions while minimizing energy loss. Tendons are primarily composed of , which constitutes 60–85% of their dry weight, predominantly that provides structural integrity. They exhibit high tensile strength, typically ranging from 50 to 100 MPa, allowing them to withstand substantial loads without rupture, alongside low elasticity that ensures force transmission with minimal deformation. In adults, tendons are poorly vascularized, with nutrition supplied by both from surrounding fluids (particularly in sheathed tendons via ) and limited vascular networks, contributing to their durability but limiting rapid repair. Unlike ligaments, which connect to to stabilize joints, tendons specifically link muscles to bones to facilitate motion. The contains approximately 4,000 tendons, a number that varies across depending on locomotor demands and anatomical complexity.

Etymology and Historical Context

The term "tendon" derives from the tendōn-, tendō, borrowed from tendron and ultimately tracing back to the ténōn (τένων), meaning "sinew" or "tendon," which evokes the structure's stretched, fibrous quality. This Greek root aligns with the Proto-Indo-European ten-, signifying "to stretch," reflecting the tendon's role in extending muscular force, while the Latin influence through tendere ("to stretch") further emphasized its elastic properties in early anatomical . In ancient contexts, similar concepts appeared in Egyptian medical texts, where sinews and tendons were part of the metu system—encompassing vessels, ducts, muscles, and cords vital to life—often viewed mystically as conduits for vital energies sustaining the body. Early historical understanding of tendons emerged in Greek medicine, with (c. 460–370 BCE) describing them as "sinews" (neura in Greek), robust cords connecting muscles to bones and essential for movement, though sometimes conflated with due to shared . Building on this, (129–216 CE), the Roman physician and anatomist, advanced the distinction between tendons and ligaments in works like Anatomical Procedures, portraying tendons as white, pliant, tear-resistant fibers transmitting muscle contractions to bones, while ligaments served primarily to bind joints, marking a shift toward functional differentiation based on dissection of animal models. These ancient contributions laid foundational observations, transitioning from empirical wound descriptions to systematic anatomical inquiry. The revitalized tendon study through detailed illustrations, as seen in Andreas Vesalius's De Humani Corporis Fabrica (1543), which featured precise woodcuts of human tendons integrated with musculature, correcting Galenic errors via direct cadaver dissection and emphasizing their gross structural attachments. By the , early advanced the understanding of structures, pioneering histological analysis beyond macroscopic views. In the , further refined tendon , describing their cellular and extracellular components—such as nucleated fibroblasts amid dense fibrous matrices—in his Manual of Human Histology (1853–1854), establishing tendons as specialized connective tissues and bridging microscopic detail with physiological function. This progression from ancient mystical and descriptive accounts evolved into modern , where tendons are analyzed as viscoelastic structures optimizing force transmission, informed by quantitative imaging and material science.

Anatomy and Structure

Gross Anatomy and Location

Tendons are tough, fibrous connective tissues that typically appear as elongated, cord-like structures or broader flat sheets known as aponeuroses, serving to connect muscle to or to other muscles. These structures vary in shape and size depending on their location and function, with many enclosed within synovial sheaths that provide to facilitate smooth gliding over bony surfaces during movement. In the human body, tendons are distributed across major regions, including the upper and lower limbs and the trunk. For example, in the upper limb, the biceps brachii tendon originates from the supraglenoid tubercle of the scapula and the coracoid process, extending through the shoulder joint to insert on the radial tuberosity, forming a key component of elbow flexion. In the lower limb, the Achilles tendon represents one of the largest examples, connecting the gastrocnemius and soleus muscles to the calcaneal tuberosity of the heel bone; it measures approximately 15 cm in length with an average thickness of 4–7 mm. Similarly, the patellar tendon links the inferior patella to the tibial tuberosity, spanning roughly 5 cm in length and 20-30 mm in width, while the quadriceps tendon unites the four heads of the quadriceps femoris muscle to the superior patella. In the trunk, tendons of the erector spinae muscles attach along the vertebral column and ribs, providing support for spinal extension and posture. Notable variations in tendon organization include sesamoid tendons, which incorporate small sesamoid bones embedded within them to reduce friction and alter force direction at joints; prominent examples occur in the hand, such as within the flexor pollicis brevis tendon at the of . Additionally, some muscles feature tendinous intersections, transverse fibrous bands that segment the muscle belly, as seen in the rectus abdominis where three such intersections divide the muscle into distinct segments along its length. The tendons in the exemplify a specialized grouping, comprising the tendons of the supraspinatus, infraspinatus, teres minor, and subscapularis muscles, which converge to form a continuous musculotendinous cuff encircling the humeral head for stability.

Microscopic Composition

Tendons exhibit a hierarchical organization at the microscopic level, consisting of fibers bundled into primary fiber units, which are further grouped into larger fascicles that form the core of the tendon unit. These fascicles are surrounded by delicate sheaths: the endotenon, which forms internal septa separating individual fascicles and providing structural support, and the epitenon, an outer sheath that encases the entire bundle of fascicles. This layered architecture allows for efficient force distribution while maintaining flexibility and resilience. The collagen fibers within fascicles are arranged in a parallel, unidirectional alignment, optimizing the tendon's ability to withstand high tensile loads during . A characteristic feature of these fibers is the crimp pattern, a wavy configuration with a crimp typically ranging from about 20 μm in energy-storing tendons to 100–400 μm in positional tendons, which contributes to the tendon's elasticity by allowing limited extension under low loads before straightening for maximum strength. This microstructural alignment ensures that tendons can transmit forces effectively without fracturing under physiological stress. Tendons possess sparse compared to other connective tissues, reflecting their for mechanical durability over metabolic activity. In flexor tendons, which are often intra-synovial, blood supply is provided through vincula—thin mesothelial folds containing arteries and veins that connect the tendon to surrounding structures. In contrast, extensor tendons and other extra-synovial types rely on the paratenon, a loose fibroelastic layer that facilitates diffusion and vascular ingress. This limited supports tendon longevity but can pose challenges during . Nerve innervation in tendons is of low density, primarily serving proprioceptive functions rather than sensory or . Specialized mechanoreceptors, such as Golgi tendon organs, are embedded among the fibers near the musculotendinous junction, detecting tension changes to provide feedback on muscle force and joint position. This sparse helps regulate movement and prevent overload without compromising the tendon's compact structure.

Extracellular Matrix

The (ECM) of tendons is predominantly composed of , which constitutes 60-85% of the dry weight and forms the primary structural scaffold responsible for the tissue's tensile strength. This is organized into with diameters ranging from 50 to 500 nm, assembled through a quarter-stagger where individual collagen molecules are offset by approximately 67 nm along their length, enabling the formation of banded structures visible under electron microscopy. The structure of these collagen molecules follows a repeating sequence of ([Gly](/page/Glycine)-X-Y)n(\text{[Gly](/page/Glycine)-X-Y})_n, where () occupies every to facilitate tight packing, X is frequently (Pro), and Y is often (), stabilizing the through hydrogen bonding. Intermolecular cross-links, such as pyridinoline, further enhance the stability of these by forming covalent bonds between and hydroxylysine residues, contributing to the ECM's resistance to mechanical stress. Other key matrix elements include , which account for 1-5% of the dry weight and include as the predominant small leucine-rich that regulates assembly and spacing. comprises 1-2% of the dry mass, providing limited recoil properties to the otherwise stiff matrix, while reaches 60-70% of the total weight, ensuring hydration and facilitating diffusion within the avascular tissue. With aging, the tendon ECM undergoes remodeling characterized by increased cross-linking density, particularly of and enzymatic cross-links like pyridinoline, which stiffens the matrix and reduces its elasticity, potentially predisposing older tendons to . These changes alter the , leading to larger diameters and decreased compliance without significant shifts in overall content.

Cellular Components

The primary cellular components of tendons are tenocytes, which comprise approximately 90% of the resident cell population and function as highly specialized fibroblasts responsible for synthesizing and maintaining the . Recent studies have revealed heterogeneity among tenocyte subpopulations with distinct profiles. In younger tendon tissue, tenoblasts predominate as the immature precursors to tenocytes, exhibiting greater proliferative capacity to support tissue growth and repair. These cells are characteristically elongated and aligned longitudinally parallel to the fibers, enabling efficient force transmission and interaction with the surrounding matrix. Tenocytes display a distinctive morphology with elongated nuclei and a thin, stretched that contains abundant rough and Golgi apparatus, facilitating the production of and other extracellular proteins. Their stellate shape in cross-section allows sparse distribution in rows between collagen bundles. Due to the relative avascularity of tendon mid-substance, tenocytes maintain a low , characterized by quiescence and limited proliferation, which contributes to the tissue's slow remodeling and healing processes. In addition to tenocytes, tendons contain minor populations of other cell types, including chondrocytes within fibrocartilaginous regions such as entheses, where they form a graded interface between tendon and to distribute mechanical stress. Resident immune cells, notably macrophages, are present in small numbers and contribute to ongoing tissue remodeling by modulating and matrix turnover. Tendon cellularity is notably low, with cell densities decreasing progressively with age, which further constrains regenerative potential.

Physiology and Function

Mechanical Properties

Tendons exhibit remarkable mechanical properties that enable them to withstand substantial loads while transmitting forces efficiently. The of human tendons typically ranges from 50 to 150 MPa, allowing them to endure high stresses without rupture under normal physiological conditions. The modulus of elasticity, which quantifies the tendon's , generally falls between 1.0 and 2.0 GPa, reflecting their ability to deform elastically under tension. These properties vary slightly across tendon types and individuals, influenced by factors such as age and loading history, but they collectively ensure tendons function as durable, compliant connectors between muscle and . Tendons display viscoelastic behavior, meaning their mechanical response depends on the strain rate and includes time-dependent phenomena like creep and . This is evident in the characteristic stress-strain curve, which consists of three regions: the initial toe region (up to ~2% strain), where collagen fibers uncrimp; the linear region, where the tendon behaves elastically; and the region, marked by progressive damage leading to rupture. in this curve indicates energy dissipation as during loading-unloading cycles, with the area between the curves representing lost energy, which increases at higher strain rates due to the viscoelastic nature. The Young's modulus EE is defined as the ratio of stress σ\sigma (force per unit area) to strain ε\varepsilon (relative deformation) in the linear region: E=σεE = \frac{\sigma}{\varepsilon} This parameter typically yields values of 1-2 GPa for human tendons, with failure occurring at strains of 10-15%, beyond which irreversible damage predominates. A key aspect of tendon is their capacity for storage and , particularly in tendons like the Achilles, which can return approximately 90% of stored energy during gait cycles to enhance efficiency. This contributes to overall locomotion by recycling , though the exact efficiency varies with activity intensity and individual tendon properties.

Role in Locomotion and Force Transmission

Tendons serve as critical intermediaries in the musculoskeletal system, transmitting contractile forces generated by muscles to s, thereby enabling joint motion and overall locomotion. In the classic Hill-type muscle model, the tendon functions as the series elastic component (SEC), positioned in series with the contractile muscle fibers to absorb and relay forces without significant energy loss. This arrangement allows muscle contractions to produce precise and efficient movements, such as during walking or jumping, where the tendon's elasticity decouples muscle shortening from bone movement, optimizing force application across joints. A key physiological role of tendons lies in enhancing energy efficiency through the stretch-shortening cycle (SSC), particularly in dynamic activities like running. During the eccentric phase of the SSC, tendons such as the Achilles store elastic energy as they stretch under load, which is then released during the subsequent concentric phase to augment muscle power output. This mechanism reduces the metabolic cost of locomotion by minimizing the work required from muscles; for instance, elastic energy recovery from the Achilles tendon during running can decrease overall muscle work by up to 35%, contributing to greater endurance and speed. Tendons also provide essential passive stability to joints and posture by generating tension that resists excessive motion. In upright standing, the maintains ankle through its non-linear elastic properties, preventing unintended dorsiflexion and supporting balance without constant muscular effort. This passive tension helps stabilize the body against gravitational forces, ensuring efficient energy use during prolonged static postures. Adaptations in tendon length and properties further refine their role in locomotion, particularly in high-power activities. The length-tension relationship of associated muscles is influenced by tendon length, allowing longer tendons—as observed in elite sprinters—to position muscle fibers near their optimal operating lengths on the force-velocity curve, thereby enhancing power output and stride efficiency. These adaptations enable greater storage and rapid recoil, supporting explosive movements like sprinting.

Sensory and Regulatory Functions

Tendons contribute to sensory functions primarily through specialized mechanoreceptors that provide feedback on muscle tension and position. , located at the musculotendinous junction, serve as key proprioceptors by detecting active and passive tension in the tendon. These encapsulated sensory endings are innervated by group Ib afferent fibers, which transmit signals to the to modulate muscle activity. When tension exceeds the GTO threshold—typically in the range of low to moderate forces—these organs activate, triggering autogenic inhibition of the agonist muscle via inhibitory interneurons, thereby preventing excessive force and protecting the tendon-muscle unit from overload. This reflex mechanism enhances proprioceptive awareness during movement by integrating tension feedback with . In addition to proprioception, tendons play a role in pain signaling through nociceptive pathways. Free nerve endings, primarily from small-diameter Aδ and C fibers, are distributed within the tendon tissue and serve as polymodal nociceptors, responding to mechanical, thermal, and chemical stimuli. These endings detect damaging levels of tension or inflammation, initiating pain perception that alerts the body to potential injury. During inflammatory responses, such as in tendinopathy, sensory nerves release neuropeptides including substance P and calcitonin gene-related peptide (CGRP), which amplify nociception and promote vasodilation, edema, and further neuropeptide release in a positive feedback loop. Substance P, in particular, correlates with increased pain and neural sprouting in pathological tendons, while CGRP contributes to neurogenic inflammation. Tendons also exhibit regulatory functions through cellular mechanotransduction and metabolic adaptations. Tenocytes, the primary resident cells, sense mechanical loading via and focal adhesions, transducing physical cues into biochemical signals that regulate . A prominent pathway involves the Hippo effectors (Yes-associated protein) and TAZ (transcriptional coactivator with PDZ-binding motif), which translocate to the nucleus under appropriate strain to upregulate genes for production, such as type I, thereby maintaining tendon and adapting to load. This mechanoregulation ensures long-term tissue integrity without excessive remodeling. Metabolically, tendons have a low basal rate, with oxygen consumption approximately 7.5 times lower than , reflecting their avascular nature and reliance on . Under stress or hypoxia, tenocytes shift to increased lactate production via elevated glycolytic flux, supporting needs while tolerating low oxygen environments.

Development and Maintenance

Embryonic and Postnatal Development

Tendons originate during embryonic development from distinct mesodermal compartments. Axial tendons derive from the sclerotome of somites, while limb tendons arise from the lateral plate mesoderm. Tendon progenitor cells are marked by the expression of the scleraxis (Scx) gene, a basic helix-loop-helix transcription factor that initiates and maintains tendon lineage specification. The initial stages of tendon formation involve mesenchymal condensation, occurring around weeks 6-7 of , where progenitor cells aggregate near developing muscles and bones. By week 12, further maturation includes differentiation of tenocytes and alignment of fibers along the longitudinal axis to form the tendon proper, facilitating force transmission. At the tendon-bone interfaces, known as entheses, transitional tissues begin to develop during the fetal period, with fibrocartilaginous zones forming postnatally to enable graded mechanical transitions. Genetic regulation plays a pivotal role in tendon patterning and differentiation. , particularly Hox11 paralogs, coordinate regional specification and integration of tendon, muscle, and tissues during embryogenesis. Additionally, transforming growth factor-β (TGF-β) signaling induces Scx expression in progenitors and promotes tenocyte differentiation, ensuring proper tendon morphogenesis. Postnatally, tendons undergo significant growth and maturation, particularly during childhood, with increasing approximately 2-3 times in proportion to overall body growth from birth to adulthood. Mechanical properties, including stiffness and strength, continue to develop through , reaching peak levels by ages 20-30 due to progressive organization and cross-linking.

Collagen Synthesis and Tissue Remodeling

Collagen synthesis in tendons primarily occurs within tenocytes, the resident fibroblast-like cells, where type I procollagen is assembled intracellularly through a series of post-translational modifications. The process begins with the transcription and translation of procollagen chains, followed by of and residues for stability and to facilitate folding and . Once properly modified, the procollagen trimer is packaged into vesicles and secreted into the via . In the , procollagen undergoes proteolytic cleavage to mature : the N-terminal propeptide is removed by ADAMTS-2, -3, or -14 enzymes, while the C-terminal propeptide is cleaved by bone morphogenetic protein 1 (). This cleavage enables the spontaneous self-assembly of molecules into quarter-staggered , which are further stabilized by cross-linking enzymes such as lysyl oxidase. These aggregate into larger fibers, forming the hierarchical structure essential for tendon tensile strength. Tendon tissue remodeling maintains matrix integrity through a balance of synthesis and degradation, with exhibiting a of approximately 300-1000 days in humans, reflecting relatively slow turnover compared to other connective tissues. Degradation is mediated by matrix metalloproteinases (MMPs), particularly MMP-1, -2, and -13, which cleave , while tissue inhibitors of metalloproteinases (TIMPs), such as TIMP-1 and -2, regulate MMP activity to prevent excessive breakdown. This dynamic equilibrium allows tendons to adapt to mechanical demands without compromising structural integrity. Mechanical loading, such as from exercise, upregulates synthesis in tenocytes via mechanotransduction pathways, including the PI3K/Akt signaling cascade, which activates transcription factors and enhances procollagen production. Studies in humans show that acute resistance exercise can increase tendon synthesis rates by 50-100% within hours to days post-loading, promoting thickening and improved tensile properties. With aging, tendon collagen turnover decreases due to reduced tenocyte proliferative capacity and increased advanced end-product (AGE) cross-links, leading to greater matrix rigidity and brittleness. solubility, a measure of extractability, declines with age, correlating with diminished remodeling efficiency and heightened susceptibility.

Factors Influencing Tendon

Several lifestyle, environmental, and physiological factors influence tendon health by modulating collagen synthesis, tissue remodeling, and mechanical integrity. Moderate mechanical loading through exercise promotes tendon adaptations, including and increased stiffness, which can enhance tendon strength by approximately 5-10% over training periods of several weeks to months. In contrast, excessive or overuse loading contributes to the accumulation of microdamage within tendon fibers, impairing self-repair mechanisms and leading to degeneration if not adequately managed. Nutritional factors play a critical role in maintaining tendon extracellular matrix stability, with vitamin C serving as an essential cofactor for prolyl hydroxylase in the hydroxylation of proline residues during collagen synthesis. Deficiency in vitamin C, as seen in scurvy, disrupts this process, resulting in weakened collagen structures and tendon fragility due to impaired cross-linking and increased susceptibility to rupture. Supplementation with collagen peptides has shown mixed efficacy in supporting tendon health, with some studies reporting improvements in tendon cross-sectional area and biomechanical properties when combined with resistance training, while others indicate inconsistent benefits for preventing degeneration. Hormonal influences significantly affect tendon resilience, particularly in sex-specific ways. Estrogen exerts protective effects on female tendons by enhancing synthesis and maintaining biomechanical properties, potentially reducing degeneration risk during reproductive years. Conversely, systemic or local administration of corticosteroids accelerates tendon degeneration by suppressing , inducing in tenocytes, and inhibiting production, thereby weakening tendon structure over time. Age and genetic predispositions further shape tendon health trajectories. Tendons typically achieve peak structural integrity and functional capacity between 20 and 40 years of age, after which age-related declines in cellular activity and turnover lead to reduced and increased vulnerability to overload. Genetic variations, such as in the COL1A1 gene, underlie conditions like certain forms of Ehlers-Danlos syndrome, where defective production results in tendon hyperextensibility, fragility, and heightened injury risk.

Clinical Significance

Common Injuries and Pathologies

Tendon injuries can be broadly classified into acute and chronic categories, each presenting distinct pathological features. Acute injuries typically result from sudden, high-force events and include tendon strains and ruptures. Tendon strains are graded from 1 to 3 based on severity: grade 1 involves minor stretching or micro with minimal fiber disruption and no significant loss of function; grade 2 represents partial affecting a portion of the tendon fibers, leading to moderate functional impairment; and grade 3 denotes complete or ruptures where the tendon is fully severed, resulting in substantial loss of strength and mobility. Ruptures are particularly common in tendons like the Achilles, often occurring in athletes or active individuals aged 40 to 60 years during explosive activities such as sprinting or jumping. Chronic conditions encompass degenerative and inflammatory disorders that develop over time due to repetitive stress. , a prevalent overuse injury, involves tendon degeneration without prominent and affects sites like the , where it is common among athletes in overhead sports such as or , with shoulder injury rates up to 30% in collegiate overhead athletes including rotator cuff tendinopathy. Another common chronic pathology is , characterized by of the , often seen in the or hand tendons and leading to restricted gliding motion. The of these injuries includes mechanical overload, degenerative changes, and systemic factors. Overload, particularly from eccentric loading where the tendon lengthens under tension, is a primary trigger for both acute strains and chronic , as it exceeds the tendon's adaptive capacity. Degeneration in hypovascular zones—regions with poor blood supply, such as the mid-portion of the —predisposes to weakening and failure over time. Systemic conditions like contribute by promoting widespread inflammation that affects tendon integrity. Symptoms of tendon injuries generally include localized that worsens with activity, swelling, and due to impaired force transmission. In chronic cases like , patients may experience stiffness and reduced , while often presents with —a grating sensation during movement. Diagnostic signs aid in identification; for instance, the for Achilles rupture involves squeezing the calf while observing for absent plantarflexion of the foot, indicating discontinuity.

Healing Mechanisms

Tendon healing following is a complex, overlapping process involving three distinct phases: , proliferation, and remodeling. The inflammatory phase, lasting approximately 1 to 7 days, is characterized by the influx of inflammatory cells such as neutrophils and macrophages, which release cytokines like interleukin-1 and tumor necrosis factor-alpha to clear debris and initiate repair signals. This phase sets the stage for subsequent tissue regeneration but can contribute to excessive if prolonged. The proliferative phase follows, spanning roughly weeks 1 to 6, during which fibroblasts migrate to the injury site and begin synthesizing components, predominantly type III , to form a preliminary bridge. Tenocyte proliferation is driven by growth factors such as (PDGF), which stimulates cell migration and division, while (VEGF) promotes to supply nutrients and oxygen to the healing area. This results in the formation of a disorganized matrix, often leading to that lacks the hierarchical structure of native tendon. The remodeling phase begins around 3 months post-injury and can extend for 6 to 12 months or longer, involving the gradual replacement of type III collagen with aligned fibers to restore tensile strength and organization. During this period, matrix metalloproteinases and other enzymes facilitate tissue maturation, though the healed tendon typically achieves only 80-90% of its original biomechanical properties. Biomechanically, the repaired tendon exhibits significant initial weakness, emphasizing the need for protected loading to prevent failure. Full functional recovery, including near-normal strength and stiffness, generally requires 6 to 12 months, as collagen cross-linking and fiber alignment continue progressively. Complications in tendon healing include scar adhesions, which can limit motion by tethering the tendon to surrounding tissues, and re-rupture, with rates of 1-4% following surgical repair and 5-12% in conservative management, influenced by rehabilitation protocols. These issues arise from inadequate matrix organization and excessive mechanical stress during early phases.

Diagnosis and Treatment Approaches

Diagnosis of tendon disorders typically begins with a thorough clinical evaluation, including patient history and to assess , swelling, and functional limitations. Specific clinical tests, such as the empty can test for supraspinatus tendon involvement, involve positioning the arm in 90 degrees of abduction and internal with the thumb down, resisting downward pressure to detect indicative of a tear; this test demonstrates high sensitivity for supraspinatus pathology when combined with imaging. Imaging modalities play a crucial role in confirming tendon abnormalities. Ultrasound is particularly valuable for dynamic assessment, allowing real-time visualization of tendon structure, motion, and blood flow during movement, with sensitivity often exceeding that of MRI for detecting certain tears like those in ankle tendons. Magnetic resonance imaging (MRI) serves as the gold standard for detailed evaluation of tendon tears, especially partial or full-thickness ones greater than 3 mm in depth, providing high-resolution images of tendon integrity, surrounding tissues, and associated pathologies. Conservative treatments form the first-line approach for most tendon disorders, emphasizing non-invasive methods to reduce and promote recovery. The RICE protocol—rest, ice, compression, and elevation—is widely recommended initially to manage acute symptoms and minimize swelling. Eccentric exercises, such as the Alfredson protocol for Achilles tendinopathy, involve three sets of 15 repetitions twice daily for 12 weeks, focusing on controlled lengthening of the tendon under load to stimulate remodeling and improve tensile strength. Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used to alleviate and , though evidence suggests they do not alter long-term outcomes and should be limited to short-term use. Surgical interventions are reserved for cases unresponsive to conservative management or severe structural damage. Debridement involves removing degenerative or calcified tissue to preserve healthy tendon portions, often yielding good outcomes with return to in 2 weeks using supportive devices. Tendon repair techniques, such as methods for Achilles ruptures, utilize small incisions to suture the tendon ends, minimizing tissue disruption and facilitating faster rehabilitation compared to open surgery. Augmentation with grafts, including autografts or allografts, reinforces repairs in chronic or large defects, enhancing biomechanical stability during the phases of , proliferation, and remodeling. Emerging therapies aim to accelerate tendon repair through biologic augmentation, though evidence remains mixed and further research is needed. (PRP) injections, derived from autologous blood, deliver growth factors to the injury site; clinical trials show variable results, with some demonstrating improvements in pain and function while meta-analyses indicate no consistent superiority over . therapies, particularly mesenchymal stem cells, promote tissue regeneration by modulating and enhancing production; as of 2024, a phase IIa trial has demonstrated safety and proof-of-concept efficacy in treating non-insertional Achilles , with ongoing studies evaluating long-term outcomes.

Comparative and Evolutionary Aspects

Tendons in Non-Human Animals

Tendons in non-human animals exhibit diverse structural and functional adaptations tailored to species-specific locomotor demands, differing notably from those in humans by emphasizing , recoil efficiency, or robustness in extreme environments. In mammals, these adaptations are particularly evident in specialized locomotion. For instance, kangaroos possess highly elastic hindlimb tendons analogous to the , which stretch during hopping to store and return elastic strain energy, enhancing jumping efficiency and reducing metabolic costs by up to 70% at speeds of 6 m/s. Similarly, in , the superficial digital flexor tendon (SDFT) functions as a key energy-storing structure during galloping, where it stretches and recoils to absorb and release , supporting high-speed propulsion while bearing substantial strain up to 16% per stride. These mammalian tendons often feature optimized collagen-elastin compositions to balance and elasticity for repetitive, high-impact activities. In birds and reptiles, tendon adaptations prioritize rapid force transmission over extensive , reflecting aerial or terrestrial constraints. muscles, such as the pectoralis, are typically parallel-fibered with short tendons that minimize compliance, enabling direct power delivery for flapping wings without significant delay. In contrast, the supracoracoideus muscle has a longer tendon for upstroke control, but overall, avian tendons are reduced in relative to body size to optimize speed and reduce mass. Reptilian tendons show similar trends, but crocodiles display robust tendons integrated with their adductor musculature, supporting powerful bite forces exceeding 16,000 Newtons through reinforced tendinous insertions that enhance mechanical stability during prey capture. Elastin content varies markedly across species, influencing tendon compliance for environmental adaptations. Cetaceans, such as whales and dolphins, have tendons with notably high elastin levels to facilitate elastic recoil during prolonged diving and tail-powered swimming, allowing energy storage for efficient propulsion in aquatic media. In amphibians, tendons contain elastic fibers that support recoil, suited to short bursts of jumping or crawling. These variations underscore how tendon composition evolves to match ecological niches, such as buoyancy challenges in cetaceans or terrestrial hopping in amphibians. Veterinary medicine highlights the clinical impact of these adaptations, particularly in performance animals. In racehorses, SDFT tendinitis is a prevalent , with incidence rates ranging from 6% to 20% across age groups, often triggered by repetitive galloping strains that exceed the tendon's energy-storage limits and lead to microdamage accumulation. Such conditions underscore the trade-off between enhanced locomotor performance and injury vulnerability in domesticated species.

Evolutionary Origins and Adaptations

Tendons trace their evolutionary origins to early chordates during the Cambrian period, approximately 500 million years ago, when the phylum first emerged. In the basal chordate Branchiostoma (commonly known as amphioxus), the earliest tendinous structures appear as myosepta—thin, sheet-like connective tissues composed of a two-dimensional array of collagen fibers arranged to transmit tension across multiple directions between muscle segments. These myosepta represent a primitive adaptation for force transmission in an aquatic environment, predating the more specialized linear tendons seen in jawless vertebrates like hagfish. Homologs of scleraxis, a key transcription factor involved in tendon progenitor cell specification in vertebrates, have been identified in amphioxus, suggesting conserved genetic mechanisms for connective tissue development across chordates. Over phylogenetic time, tendons exhibited increased structural complexity, particularly with the transition to tetrapods and around 360 million years ago. In early tetrapods, tendons co-evolved with and to support and on land, evolving from simple myoseptal sheets into hierarchical bundles capable of handling unidirectional tensile loads. This co-evolution is evident in the development of specialized tendon insertions at muscle-bone interfaces, enhancing force transmission efficiency and enabling more dynamic movement compared to the axial undulations of aquatic chordates. In jawed vertebrates, further adaptations included fibrocartilaginous pads and sesamoid bones within tendons to manage compressive forces, first appearing in cartilaginous fishes and becoming widespread in bony fishes and tetrapods. Tendons display diverse adaptations shaped by locomotor demands and ecological pressures. In cursorial mammals, such as antelopes and horses, evolutionary selection has favored longer distal tendons, particularly the , which store and release to improve running efficiency and reduce metabolic cost during high-speed locomotion. in tendon properties also occurs, with males often exhibiting greater tensile strength and cross-sectional area to support larger body sizes and agonistic behaviors; for instance, male possess robust tendons adapted for powerful upper limb exertion in territorial displays. These variations highlight tendons' role in optimizing force transmission for species-specific lifestyles. Fossil evidence provides direct insights into ancient tendon function, including impressions preserved in dinosaur tracks that reveal patterns of force transmission through soft tissues. Such impressions, recording the contours of , , and tendons alongside bony structures, indicate that non-avian utilized tendons for efficient load distribution during locomotion, similar to modern vertebrates. Ossified tendons, common in ornithischian and theropod , further demonstrate evolutionary continuity in tendon mineralization as an adaptation for stiffening the tail and vertebral column to enhance stability.

Cultural and Practical Uses

Culinary and Nutritional Roles

Tendons, primarily composed of , are utilized in various culinary traditions where their tough, fibrous texture requires slow cooking methods to break down into tender, gelatinous forms. In , tendons are a staple in , a , where they are simmered for hours in aromatic to contribute a chewy texture and rich flavor. Similarly, in Chinese dim sum, tendons appear in braised preparations such as suan bao niu jin, marinated with and slow-cooked until soft and succulent. These preparations often involve blanching to remove impurities before prolonged or pressure cooking, enhancing the extraction of into the for a silky consistency. In , tendons may be incorporated into , a spicy where connective tissues from or shank cuts gelatinize during extended with chiles and spices. The high content in tendons makes them an excellent source for production, achieved through where raw tendons are treated with or and heated to yield a gel-forming protein. derived from tendons typically exhibits a high bloom strength of 200-300 grams, indicating strong gelling properties suitable for culinary applications like stocks and desserts. This process not only tenderizes the tissue but also enriches soups and stews with natural thickening agents. Nutritionally, tendons are a dense source of protein, with 100 grams of cooked beef tendon providing approximately 146 calories and 35 grams of protein, predominantly from . This protein is rich in essential amino acids such as (comprising about one-third of collagen's structure) and , which support health but are not complete proteins due to low levels of and other essentials. Their tough texture necessitates tenderizing techniques like slow cooking, making them less ideal for quick meals but valuable for low-fat, high-protein diets. Culturally, tendon consumption holds significance in various traditions, often linked to beliefs in food-as-medicine principles. In and , tendons are prized for their purported benefits to joint and tendon health, aligning with the concept that consuming similar tissues strengthens corresponding body parts, as seen in offerings and restorative soups. Mexican , featuring tenderized beef including tendons, is a festive dish associated with celebrations, where the collagen-rich is thought to nourish and soothe in folk remedies for vitality. In broader folk medicine, tendon-based dishes are traditionally recommended for joint support, reflecting cross-cultural practices of using animal connective tissues to address musculoskeletal concerns. Modern processing of tendons focuses on extracting collagen peptides for supplements, often derived from bovine or porcine sources through enzymatic . A 2024 randomized controlled trial showed that daily doses of 10 grams of peptides alleviated symptoms of , improving pain and function without significant adverse effects. These supplements capitalize on the profile of tendons, providing bioavailable building blocks for maintenance. As of 2025, studies on peptides continue to explore their role in , with ongoing meta-analyses supporting efficacy in symptom relief.

Biomedical and Industrial Applications

In biomedical applications, tendons serve as valuable sources for allografts and xenografts in surgical repairs, particularly for reconstruction. Porcine tendon xenografts, such as decellularized digital extensor tendons, have been employed in (ACL) reconstruction, demonstrating positive safety and performance outcomes over five years in clinical studies involving 40 patients, with no graft failures reported. Similarly, porcine bone-patellar tendon-bone xenografts have shown long-term efficacy in human ACL reconstruction trials, providing an alternative to human-sourced grafts by reducing donor site morbidity. Tissue engineering leverages decellularized tendon matrices as scaffolds to promote regeneration, preserving the extracellular matrix's biomechanical properties while minimizing . These scaffolds facilitate , proliferation, and differentiation, supporting tendon repair in models of rupture and degeneration. For instance, decellularized tendon-derived scaffolds have been integrated with mesenchymal stem cells to enhance healing at tendon-bone interfaces, as reviewed in studies on their application for osteotendinous junction regeneration. In research, the rat Achilles tendon injury model is widely used to investigate tendon and therapeutic interventions due to its anatomical similarity to tendons and . This model, involving partial or complete transection, allows quantitative assessment of healing through biomechanical and histological analyses across multiple injury severities. Additionally, 3D techniques utilizing hydrogels enable the fabrication of tendon scaffolds mimicking the aligned fibrillar structure to support tenocyte viability and matrix deposition. Industrially, collagen extracted from tendons contributes to leather tanning processes by providing raw material for stabilization and enhancement of hides, often derived from trimming wastes to improve product durability. As of 2025, recycled from tannery waste has been increasingly used as a filling agent to enhance low-quality , promoting . In pharmaceuticals, tendon-derived is incorporated into delivery systems for hormones like insulin-like growth factor-1 (IGF-1), which stimulates synthesis and improves tendon repair outcomes in preclinical models. Bioinspired materials drawing from tendon's tensile properties—such as high strength and elasticity—inform robotic actuators, where fiber-reinforced composites replicate muscle-tendon units for enhanced compliance and load-bearing in . Recent advances include CRISPR-based gene editing targeting the scleraxis (Scx) gene to boost tenogenic differentiation and regeneration. In 2024 studies, CRISPR-Cas13 editing of macrophages modulated inflammatory responses in tendon injuries, promoting scarless healing in animal models. Overexpression of Scx via CRISPR in stem cells has further enhanced tendon lineage commitment, offering potential for scalable regenerative therapies.

References

  1. https://medlineplus.gov/ency/imagepages/19089.htm
  2. https://pmc.ncbi.nlm.nih.gov/articles/PMC4507431/
  3. https://newsinhealth.nih.gov/2014/06/protect-your-tendons
  4. https://www.ncbi.nlm.nih.gov/books/NBK513237/
  5. https://my.clevelandclinic.org/health/body/21738-tendon
  6. https://pmc.ncbi.nlm.nih.gov/articles/PMC3459684/
  7. https://pmc.ncbi.nlm.nih.gov/articles/PMC4650849/
  8. https://www.etymonline.com/word/tendon
  9. https://www.merriam-webster.com/dictionary/tendon
  10. https://africame.factsanddetails.com/article/entry-1191.html
  11. https://web.stanford.edu/class/history13/earlysciencelab/body/nervespages/nerves.html
  12. https://books.google.com/books/about/Galen_on_Anatomical_Procedures.html?id=m4OR73BI-7UC
  13. https://www.vesaliusfabrica.com/en/original-fabrica/the-art-of-the-fabrica.html
  14. https://books.google.com/books/about/Manual_of_Human_Histology.html?id=V6AEAAAAQAAJ
  15. https://www.ncbi.nlm.nih.gov/books/NBK519538/
  16. https://ajronline.org/doi/10.2214/AJR.22.27632
  17. https://radiopaedia.org/articles/patellar-tendon?lang=us
  18. https://www.ncbi.nlm.nih.gov/books/NBK537074/
  19. https://www.ncbi.nlm.nih.gov/books/NBK578171/
  20. https://pmc.ncbi.nlm.nih.gov/articles/PMC6173272/
  21. https://www.ncbi.nlm.nih.gov/books/NBK441844/
  22. https://www.bofas.org.uk/hyperbook/miscellaneous/physiology-of-tendons
  23. https://musculoskeletalkey.com/tendons-and-ligaments-2/
  24. https://pmc.ncbi.nlm.nih.gov/articles/PMC3769524/
  25. https://www.sciencedirect.com/science/article/pii/S174270611830045X
  26. https://pmc.ncbi.nlm.nih.gov/articles/PMC8432990/
  27. https://www.orthobullets.com/basic-science/9015/tendons
  28. https://www.ncbi.nlm.nih.gov/books/NBK10812/
  29. https://www.sciencedirect.com/topics/immunology-and-microbiology/tendon-receptor
  30. https://onlinelibrary.wiley.com/doi/full/10.1002/jor.24440
  31. https://pmc.ncbi.nlm.nih.gov/articles/PMC8112590/
  32. https://pmc.ncbi.nlm.nih.gov/articles/PMC2846778/
  33. https://pmc.ncbi.nlm.nih.gov/articles/PMC9583849/
  34. https://onlinelibrary.wiley.com/doi/10.1002/jor.22818
  35. https://pmc.ncbi.nlm.nih.gov/articles/PMC3676160/
  36. https://pmc.ncbi.nlm.nih.gov/articles/PMC10607611/
  37. https://www.sciencedirect.com/science/article/pii/S0021925820315404
  38. https://pmc.ncbi.nlm.nih.gov/articles/PMC12605242/
  39. https://musculoskeletalkey.com/mechanical-properties-of-ligament-and-tendon/
  40. https://pmc.ncbi.nlm.nih.gov/articles/PMC3244520/
  41. https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2017.00091/full
  42. https://pmc.ncbi.nlm.nih.gov/articles/PMC3727010/
  43. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0179976
  44. https://pmc.ncbi.nlm.nih.gov/articles/PMC1357215/
  45. https://www.sciencedirect.com/science/article/abs/pii/S0021929015003541
  46. https://journals.biologists.com/jeb/article/226/14/jeb244863/324867/Non-linear-properties-of-the-Achilles-tendon
  47. https://onlinelibrary.wiley.com/doi/10.1111/brv.13002
  48. https://pmc.ncbi.nlm.nih.gov/articles/PMC2375561/
  49. https://pmc.ncbi.nlm.nih.gov/articles/PMC7977083/
  50. https://pmc.ncbi.nlm.nih.gov/articles/PMC7769215/
  51. https://www.sciencedirect.com/topics/neuroscience/golgi-tendon-organ
  52. https://www.sciencedirect.com/topics/neuroscience/free-nerve-ending
  53. https://pmc.ncbi.nlm.nih.gov/articles/PMC4000254/
  54. https://pmc.ncbi.nlm.nih.gov/articles/PMC3721458/
  55. https://pmc.ncbi.nlm.nih.gov/articles/PMC4524513/
  56. https://pmc.ncbi.nlm.nih.gov/articles/PMC2714139/
  57. https://pmc.ncbi.nlm.nih.gov/articles/PMC7983167/
  58. https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/dvdy.10481
  59. https://doi.org/10.1016/j.matbio.2016.02.006
  60. https://pmc.ncbi.nlm.nih.gov/articles/PMC7769109/
  61. https://onlinelibrary.wiley.com/doi/10.1111/j.1469-7580.2012.01496.x
  62. https://elifesciences.org/articles/85873
  63. https://pmc.ncbi.nlm.nih.gov/articles/PMC3817943/
  64. https://pmc.ncbi.nlm.nih.gov/articles/PMC8077520/
  65. https://pmc.ncbi.nlm.nih.gov/articles/PMC2871999/
  66. https://journals.physiology.org/doi/full/10.1152/japplphysiol.00328.2016
  67. https://journals.biologists.com/jcs/article/118/7/1341/28724/Procollagen-trafficking-processing-and
  68. https://pmc.ncbi.nlm.nih.gov/articles/PMC3430002/
  69. https://www.sciencedirect.com/science/article/pii/S0945053X1500061X
  70. https://elifesciences.org/articles/55262
  71. https://asmedigitalcollection.asme.org/appliedmechanicsreviews/article/73/5/050802/1121891/Fibrillar-Collagen-A-Review-of-the-Mechanical
  72. https://www.mdpi.com/1422-0067/24/3/2370
  73. https://pmc.ncbi.nlm.nih.gov/articles/PMC1474228/
  74. https://pubmed.ncbi.nlm.nih.gov/28336820/
  75. https://pmc.ncbi.nlm.nih.gov/articles/PMC4637912/
  76. https://www.sciencedirect.com/science/article/pii/S0945053X19301702
  77. https://pmc.ncbi.nlm.nih.gov/articles/PMC6204628/
  78. https://www.mdpi.com/2079-9721/11/2/78
  79. https://link.springer.com/article/10.1007/s40279-024-02079-0
  80. https://pmc.ncbi.nlm.nih.gov/articles/PMC4241423/
  81. https://pmc.ncbi.nlm.nih.gov/articles/PMC1120980/
  82. https://medlineplus.gov/genetics/gene/col1a1/
  83. https://pmc.ncbi.nlm.nih.gov/articles/PMC4914372/
  84. https://pmc.ncbi.nlm.nih.gov/articles/PMC5136353/
  85. https://journals.ku.edu/jis/article/view/10117
  86. https://www.ncbi.nlm.nih.gov/books/NBK544324/
  87. https://www.ncbi.nlm.nih.gov/books/NBK448174/
  88. https://www.ncbi.nlm.nih.gov/books/NBK555501/
  89. https://www.ncbi.nlm.nih.gov/books/NBK430844/
  90. https://pmc.ncbi.nlm.nih.gov/articles/PMC4418182/
  91. https://pmc.ncbi.nlm.nih.gov/articles/PMC9396922/
  92. https://www.sciencedirect.com/science/article/pii/S1297319X24000071
  93. https://pmc.ncbi.nlm.nih.gov/articles/PMC8579915/
  94. https://pmc.ncbi.nlm.nih.gov/articles/PMC4519231/
  95. https://www.nejm.org/doi/full/10.1056/NEJMoa2108447
  96. https://www.frontiersin.org/journals/surgery/articles/10.3389/fsurg.2024.1483584/full
  97. https://pubmed.ncbi.nlm.nih.gov/9934421/
  98. https://pmc.ncbi.nlm.nih.gov/articles/PMC3826176/
  99. https://www.mayoclinic.org/diseases-conditions/tendinopathy/diagnosis-treatment/drc-20580691
  100. https://pubmed.ncbi.nlm.nih.gov/9763166/
  101. https://pmc.ncbi.nlm.nih.gov/articles/PMC3445127/
  102. https://ajronline.org/doi/10.2214/AJR.05.1047
  103. https://ormobility.com/physical-therapy-for-achilles-tendonitis/
  104. https://www.verywellhealth.com/the-alfredson-protocol-for-achilles-tendonitis-2696560
  105. https://pmc.ncbi.nlm.nih.gov/articles/PMC7739229/
  106. https://www.medcentral.com/pain/chronic/achilles-tendon-injuries
  107. https://www.orthoracle.com/library/achilles-debridement/
  108. https://nyulangone.org/conditions/achilles-injury/treatments/surgery-for-achilles-injury
  109. https://www.medicalnewstoday.com/articles/achilles-surgery
  110. https://pmc.ncbi.nlm.nih.gov/articles/PMC6339617/
  111. https://www.aafp.org/pubs/afp/issues/2010/1115/p1272.html
  112. https://www.spandidos-publications.com/10.3892/ijmm.2023.5273
  113. https://www.nature.com/articles/s41598-024-61399-3
  114. https://www.sciencedirect.com/science/article/abs/pii/S0305049198000224
  115. https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/equine-tendon
  116. https://www.frontiersin.org/journals/bioengineering-and-biotechnology/articles/10.3389/fbioe.2018.00140/full
  117. https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.25011
  118. https://www.nationalgeographic.com/animals/article/120315-crocodiles-bite-force-erickson-science-plos-one-strongest
  119. https://www.researchgate.net/publication/390024683_Comparative_mechanical_and_elastic_properties_of_the_doral_and_ventral_tendons_in_the_peduncle_of_harbor_porpoise_Phocoena_phocoena
  120. https://www.sciencedirect.com/science/article/pii/S0940960211801445
  121. https://pmc.ncbi.nlm.nih.gov/articles/PMC6920055/
  122. https://www.sciencedirect.com/science/article/pii/S1095643302002416
  123. https://evodevojournal.biomedcentral.com/articles/10.1186/s13227-015-0007-5
  124. https://anatomypubs.onlinelibrary.wiley.com/doi/full/10.1002/ar.24387
  125. https://pmc.ncbi.nlm.nih.gov/articles/PMC5503813/
  126. https://www.amnh.org/exhibitions/dinosaurs-ancient-fossils/trackways/who-was-that-dinosaur
  127. https://iamafoodblog.com/authentic-instant-pot-pho-recipe/
  128. https://www.chefs-resources.com/types-of-meat/offal-varieties/beef-tendons/
  129. https://www.mexicanplease.com/birria-de-res-recipe-beef-birria/
  130. https://link.springer.com/article/10.1007/s44187-025-00574-5
  131. https://www.mynetdiary.com/food/calories-in-beef-tendon-servin-51395139-0.html
  132. https://pmc.ncbi.nlm.nih.gov/articles/PMC10058045/
  133. http://www.westmeeteastclinic.com/jointhealth.php
  134. https://www.sciencedirect.com/science/article/pii/S2451865424001716
  135. https://josr-online.biomedcentral.com/articles/10.1186/s13018-023-04182-w
  136. https://pmc.ncbi.nlm.nih.gov/articles/PMC12455883/
  137. https://pmc.ncbi.nlm.nih.gov/articles/PMC10482801/
  138. https://pmc.ncbi.nlm.nih.gov/articles/PMC4736572/
  139. https://www.sciencedirect.com/science/article/pii/S209012322400033X
  140. https://pmc.ncbi.nlm.nih.gov/articles/PMC6499652/
  141. https://www.mdpi.com/2673-4117/5/3/98
  142. https://www.sciencedirect.com/science/article/abs/pii/S0959652615018053
  143. https://link.springer.com/article/10.1007/s10098-023-02552-w
  144. https://www.sciencedirect.com/science/article/pii/S2589004221009913
  145. https://preserve.lehigh.edu/_flysystem/fedora/2024-04/5a17e61684917bdf4c7c95d2da2ce185.pdf
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