# Tendon
A tendon (also called sinew) is a cord of dense fibrous connective tissue that attaches a muscle to bone and transmits the force generated by muscle contraction to the skeleton, enabling movement of the limbs and joints.[1](https://my.clevelandclinic.org/health/body/21738-tendon) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3676159/) Tendons are distinguished from ligaments, their close structural relatives, by anatomical location: tendons link muscle to bone, while ligaments link bone to bone.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3676159/) The human body contains thousands of tendons, and the [Achilles tendon](https://www.edgepedia.org/achilles-tendon), connecting the calf muscle to the heel bone, is the largest.[1](https://my.clevelandclinic.org/health/body/21738-tendon)
| Key fact | Detail |
|---|---|
| Function | Transmits tensile force from muscle to bone; stores elastic energy during locomotion[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3676159/) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6454883/) |
| Main constituent | Type I collagen, 65–80% of tendon dry mass and 95% of total collagen mass[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC5549180) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC10607611/) |
| Mechanical range | Human tendon and ligament tensile strength 4.4–660 MPa by location; maximum strain 18–30%; Young's modulus 0.2–1.5 GPa[6](https://onlinelibrary.wiley.com/doi/10.1155/2018/9868151) |
| Functional classes | Energy-storing tendons (e.g., Achilles) experience high in vivo strain; positional tendons (e.g., digital extensors) experience lower strain[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6454883/) |
| Main cell type | Tenocyte, a specialized fibroblast[7](https://www.sciencedirect.com/science/article/pii/S1877132718300733) |
| Healing time | Injury healing follows a phased pattern and can take up to two years to remodel back to a normal structure[7](https://www.sciencedirect.com/science/article/pii/S1877132718300733) |
| Disease burden | Upwards of two million cases of lower extremity tendinopathy occur in the U.S. per year[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC8719050/) |
## Structure and composition
Tendon is built on a hierarchical arrangement of collagen. The smallest unit, tropocollagen, is a triple-helix polypeptide of two α1 chains and one α2 chain synthesized inside tendon cells and secreted into the extracellular matrix, where the helices self-assemble through intermolecular crosslinks into parallel collagen fibrils 100 nm to 500 nm in diameter.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC5549180) Fibrils group into fibers, fibers into primary fiber bundles (subfascicles), then secondary bundles (fascicles), and finally tertiary bundles that form the whole tendon.[1](https://my.clevelandclinic.org/health/body/21738-tendon) Collagen fibers run parallel to the tendon axis, an arrangement that gives the tissue its tensile strength.[1](https://my.clevelandclinic.org/health/body/21738-tendon)
**Sheaths and coverings.** Fascicles are enclosed by a thin film of loose connective tissue called endotenon, and the whole tendon is wrapped in a connective tissue sheath called the epitenon, which is itself surrounded by the paratenon, a loose areolar tissue that lets the tendon move against neighboring structures.[9](https://www.ncbi.nlm.nih.gov/books/NBK222432/) • [1](https://my.clevelandclinic.org/health/body/21738-tendon) In some regions, such as the wrist, the paratenon forms a double-layered sheath lined with synovial cells that facilitates smooth gliding.[9](https://www.ncbi.nlm.nih.gov/books/NBK222432/) Some tendons of the hands and feet have a synovial sheath producing lubricating fluid.[1](https://my.clevelandclinic.org/health/body/21738-tendon)
Collagen accounts for 60–85% of tendon dry weight, and type I collagen makes up 95% of the total collagen mass, with types III and V representing most of the remainder.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC10607611/) Non-collagenous constituents, including proteoglycans such as decorin, biglycan, lumican, fibromodulin, lubricin, and aggrecan, the glycoprotein COMP, and elastin, organize the collagen network and contribute to the tissue's quasi-static and viscoelastic mechanical behavior.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC8719050/) The major cell type is the tenocyte, a specialized fibroblast.[7](https://www.sciencedirect.com/science/article/pii/S1877132718300733)
## Mechanical behavior
Tendon is highly resistant to tearing but not very stretchy, so it can be injured when strained and heals slowly.[1](https://my.clevelandclinic.org/health/body/21738-tendon) Measured properties vary widely by anatomical location: reported maximum tensile strength of human tendons and ligaments ranges from 4.4 to 660 MPa, maximum strain from 18 to 30%, and [Young's modulus](https://www.edgepedia.org/youngs-modulus) from 0.2 to 1.5 GPa.[6](https://onlinelibrary.wiley.com/doi/10.1155/2018/9868151) The tissue is strongly anisotropic; for human medial collateral ligament, the longitudinal tensile modulus of 332.15 ± 58.27 MPa contrasts with a transverse modulus of 11.02 ± 3.57 MPa.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC8719050/) When stretched, tendons show a characteristic non-linear stress–strain curve that arises from the crimped, parallel arrangement of collagen fibrils.[10](https://arxiv.org/abs/2103.04844) Loads can be large: the flexor tendons of the foot can handle more than eight times body weight.[1](https://my.clevelandclinic.org/health/body/21738-tendon)
**Functional specialization.** Tendons are often classified as energy-storing or positional. Energy-storing tendons, such as the Achilles tendon, store significant elastic energy during loading and typically experience high in vivo strain; positional tendons, such as the digital extensor tendons, function mainly in limb positioning and experience lower strain.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6454883/) Comparative studies report structural differences between these classes, with positional tendons generally showing lower cell density, larger collagen fibers, and larger crimp wavelength than energy-storing tendons.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC11982604/) Because individual tendons and ligaments have unique properties adapted to specific roles, simple anatomical definitions do not capture the full range; the extracellular matrix varies in accordance with function rather than falling into two discrete groups.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3676159/) • [12](https://pubmed.ncbi.nlm.nih.gov/17205554/)
## Attachments and specialized zones
Tendons manage stress at two junctions. The musculotendinous junction, where tendon meets muscle, is a frequent site of injury.[1](https://my.clevelandclinic.org/health/body/21738-tendon) At the bone end, the tendon attaches through the enthesis, a specialized insertion where collagen types I through III are expressed in a fibrocartilage transitional zone; collagen type II there withstands compressive load while types I and III dissipate bending of the collagen fibers.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC10607611/) Sharpey fibers, collagen fibers that extend into the bone, anchor the tendon.[1](https://my.clevelandclinic.org/health/body/21738-tendon) Where a tendon's path is altered around a bony structure or pulley, it experiences compression in addition to tension, and the tissue in that region is gradually transformed from linear bands of collagen fascicles into irregularly patterned fibrocartilage.[9](https://www.ncbi.nlm.nih.gov/books/NBK222432/)
## Injury, healing, and aging
Tendon injury is common. Tendinopathy, a degenerative condition characterized by pain, swelling, and tenderness, affects both athletic and nonathletic populations, with upwards of two million cases of lower extremity tendinopathy per year in the United States.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC8719050/) Healing after injury follows the same phased pattern as general wound healing but is incomplete, producing scar tissue, and remodeling back to a normal structure can take up to two years.[7](https://www.sciencedirect.com/science/article/pii/S1877132718300733) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC10607611/) Treatment options for tendinopathy include rest, ice, pain relievers, physical therapy with resistance exercises, and procedures such as steroid injection, shockwave therapy, dry needling, platelet-rich plasma injection, and surgery when the tendon has torn away from bone.[13](https://www.mayoclinic.org/diseases-conditions/tendinopathy/diagnosis-treatment/drc-20580691)
**Aging.** Aging progressively and often irreversibly changes tendon mechanical properties, driven by changes in cellularity, cell function, collagen turnover, fibril diameter and alignment, and crosslinking.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC10607611/) Aged tendons show decreased tensile strength, altered viscoelasticity, limited fiber sliding, and increased stiffness.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC10607611/) Nonenzymatic advanced glycation end product (AGE) crosslinks accumulate between collagen and sugars over years, increasing collagen rigidity; the process is accelerated by hyperglycemia and cellular senescence.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC10607611/) Tendon collagen has a long half-life, ranging from 2 months to 200 years by one estimate, which allows more time for AGE formation.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC10607611/)
## Evolution
Tendinous tissue is ancient. Phylogenetically, it first appears in the invertebrate chordate *Branchiostoma* as myosepta, a two-dimensional array of collagen fibers organized along two primary axes; in hagfish the first linear tendons appear, with unidirectional fiber orientation within the flat myoseptal sheet.[14](https://pubmed.ncbi.nlm.nih.gov/12485698/) Tendons also respond to load during life: under compressive load they form a fibrocartilaginous pad, and under severe stress, sesamoid bones, a capacity first documented in cartilaginous fish.[14](https://pubmed.ncbi.nlm.nih.gov/12485698/)
## References
1. [Tendon (Sinew): What It Is, Anatomy & Function, Cleveland Clinic](https://my.clevelandclinic.org/health/body/21738-tendon)
2. [Birch HL et al. Specialisation of extracellular matrix for function in tendons and ligaments. PMC3676159](https://pmc.ncbi.nlm.nih.gov/articles/PMC3676159/)
3. [Weiss JA, Gardiner JC. Load Transfer, Damage and Failure in Ligaments and Tendons. PMC6454883](https://pmc.ncbi.nlm.nih.gov/articles/PMC6454883/)
4. [Tendon injuries: Basic science and new repair proposals. PMC5549180](https://pmc.ncbi.nlm.nih.gov/articles/PMC5549180)
5. [Effect of Aging on Tendon Biology, Biomechanics and Implications for Treatment Approaches. PMC10607611](https://pmc.ncbi.nlm.nih.gov/articles/PMC10607611/)
6. [Alshomer F et al. Advances in Tendon and Ligament Tissue Engineering: Materials Perspective (2018)](https://onlinelibrary.wiley.com/doi/10.1155/2018/9868151)
7. [Tendon and ligament: basic science, injury and repair. Orthopaedics and Trauma 32(4), 2018](https://www.sciencedirect.com/science/article/pii/S1877132718300733)
8. [Eisner LE et al. The Role of the Non-Collagenous Extracellular Matrix in Tendon and Ligament Mechanical Behavior: A Review. PMC8719050](https://pmc.ncbi.nlm.nih.gov/articles/PMC8719050/)
9. [Tissue Mechanobiology. Musculoskeletal Disorders and the Workplace, NCBI Bookshelf NBK222432](https://www.ncbi.nlm.nih.gov/books/NBK222432/)
10. [A microstructural model of tendon failure. arXiv:2103.04844](https://arxiv.org/abs/2103.04844)
11. [Structural Variations of Tendons: A Systematic Search and Narrative Review. PMC11982604](https://pmc.ncbi.nlm.nih.gov/articles/PMC11982604/)
12. [Rumian AP, Wallace AL, Birch HL. Tendons and ligaments are anatomically distinct but overlap in molecular and morphological features. J Orthop Res 25(4), 2007](https://pubmed.ncbi.nlm.nih.gov/17205554/)
13. [Tendinopathy: Diagnosis and treatment, Mayo Clinic](https://www.mayoclinic.org/diseases-conditions/tendinopathy/diagnosis-treatment/drc-20580691)
14. [The evolution of tendon: morphology and material properties. PubMed 12485698](https://pubmed.ncbi.nlm.nih.gov/12485698/)
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*Topic: Encyclopedia › Life and health › Human health and medicine › Human structure and function › Musculoskeletal structures › Tendons and cartilage*
*Initially written Sep 17, 2026 · Reviewed: Sep 20, 2026; Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license