The Achilles tendon, one of the tendons in the human body
Tendon - add - high mag.jpg
Micrograph of a piece of tendon; H&E stain
Anatomical terminology

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; both are made of collagen. Ligaments join one bone to bone, while tendons connect muscle to bone for a proper functioning of the body.


Histologically, tendons consist of dense regular connective tissue fascicles encased in dense irregular connective tissue sheaths. Normal healthy tendons are composed mostly of parallel arrays of collagen fibers closely packed together. They are anchored to bone by Sharpey's fibres. The dry mass of normal tendons, which makes up about 30% of their total mass, is composed of about 86% collagen, 2% elastin, 1–5% proteoglycans, and 0.2% inorganic components such as copper, manganese, and calcium.[1][2] 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 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.[1][3]

Collagen fibres coalesce into macroaggregates. After secretion from the cell, the 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 mm in length with a diameter of 50–300 μm, and finally into a tendon fibre with a diameter of 100–500 μm.[4] Fascicles are bound by the endotendineum, which is a delicate loose connective tissue containing thin collagen fibrils.[5][6] and elastic fibres.[7] Groups of fascicles are bounded by the epitenon. Filling the interstitia within the fascia where the tendon is located is the paratenon a fatty areolar tissue.[8]

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.[9] 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.[10] 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.[11] 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.[12]

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.[13]

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 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.[14]

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.[15]

Other Languages
Alemannisch: Sehne (Anatomie)
العربية: وتر (تشريح)
авар: Рихь
azərbaycanca: Vətər (anatomiya)
беларуская: Сухажылле
български: Сухожилие
català: Tendó
čeština: Šlacha
chiShona: Runda
Cymraeg: Tendon
dansk: Sene
eesti: Kõõlus
Ελληνικά: Τένοντας
español: Tendón
Esperanto: Tendeno
euskara: Tendoi
فارسی: زردپی
français: Tendon
Gaeilge: Teannán
galego: Tendón
한국어: 힘줄
Ido: Tendino
Bahasa Indonesia: Tendon
italiano: Tendine
עברית: גיד
Basa Jawa: Tendon
қазақша: Сіңір
Kreyòl ayisyen: Tandon
Кыргызча: Тарамыш
лакку: Хьхьа
Latina: Tendo
latviešu: Cīpsla
lietuvių: Sausgyslė
Bahasa Melayu: Tendon
Nederlands: Pees (anatomie)
norsk: Sene
norsk nynorsk: Sene
oʻzbekcha/ўзбекча: Pay
polski: Ścięgno
português: Tendão
română: Tendon
Runa Simi: Anku
русский: Сухожилие
Scots: Tenon
Simple English: Tendon
slovenčina: Šľacha
slovenščina: Kita (anatomija)
کوردی: ڕەفیسکە
suomi: Jänne
svenska: Sena
தமிழ்: தசைநாண்
Türkçe: Tendon
українська: Сухожилля
Tiếng Việt: Gân