Ligament

What it is

A ligament (a tough band of tissue that holds bones together at a joint) allows the joint’s normal movements but limits how far they go¹. Ligaments give joints stability and also take part in proprioception, the sense of joint position and movement⁴. Like tendons, they are a dense, regular connective tissue, though their fibres do not all run parallel¹. They have few cells and few blood vessels⁵.

What ligaments are made of

Ligaments are mostly type I collagen (a thick, strong form of collagen), about 70% of their dry weight³. The rest includes fibres of elastin (a stretchy protein that lets tissue spring back after stretching), proteoglycans (large protein-and-sugar molecules that hold water) and small amounts of other collagens³. That collagen is arranged in levels, from fibrils (very thin strands of collagen) to fascicles (bundles of fibres lying side by side) to fibres to the whole tissue². This highly aligned build lets ligaments carry force from bone to bone and makes them among the body’s strongest tissues².

Where ligaments sit

Ligaments are classed by where they sit relative to the joint capsule (the sleeve of tough tissue that encloses a joint): outside it, fused into its wall, or inside it¹. Another scheme splits them into those inside the joint, such as the ACL, and those outside it, such as the knee’s MCL (medial collateral ligament)³. Most ligament research has been done on the ACL³.

Ligament material varies to suit each joint². The human knee MCL’s tensile strength (strength under a pulling force) was about 38.6 megapascals (a unit of force per area), but one shoulder ligament’s was 1 to 6².

How it responds to load

Ligaments resist pulling loads³. Because their fibres are so organised, their strength depends on direction: the human MCL is about 30 times stronger along its length than across it².

Why timing matters

Ligaments are viscoelastic: how they behave depends on time and on the loads that came before². Under a steady load they slowly lengthen, which is called creep². Held at a fixed length, the stress inside them slowly falls, which is called stress relaxation².

Use and disuse

A review describes ligaments and tendons as remodelling (steadily replacing old tissue with new) according to the motion and stress placed on them². The curve is lopsided: immobilisation (keeping a limb still) brings a rapid loss of properties and mass, while long-term exercise brings much smaller gains².

The curve is lopsided: immobilisation (keeping a limb still) brings a rapid loss of properties and mass, while long-term exercise brings much smaller gains.

Animal studies of immobilisation found different effects in different ligaments and species^2,6:

• In rabbits, a few weeks of immobilisation greatly stiffened the joint and weakened the MCL; after movement resumed, its properties took up to a year to return to near normal².
• In rabbits, immobilisation weakened the whole bone–MCL–bone unit, and its bone attachments showed bone loss and disrupted anchorage⁷.
• When rabbit knees were moved again, the MCL’s own material returned to normal, but the whole unit’s strength did not⁷.
• In a rabbit model, the immobilised ACL shrank to 74% of its normal cross-sectional area, though its failure stress did not drop significantly⁸.
• In rats, three weeks of immobilisation slightly loosened the whole knee but did not change the ACL’s stiffness or failure strength⁶. The authors pointed to the joint capsule instead⁶.

How it gets injured

An anatomy textbook defines a sprain as the stretching or tearing of a joint’s supporting ligaments¹. A review names repetition and overuse, eccentric (lengthening) work and quick cutting movements as frequent causes of ligament and tendon injury². Such injuries can upset the joint’s balance of mobility and stability, and the abnormal loading may damage other tissues².

How common it is

• Knee: ruptures, mainly of the ACL and MCL, are estimated at about 2 per 1,000 people a year in the general population². A 2022 review says 90% of knee ligament injuries involve one of these two⁵.
• Ankle: a 2018 International Ankle Consortium consensus statement calls sprain of the lateral ankle ligaments the most common musculoskeletal injury in sport and recreation⁹.
• Repeat ankle sprain: the Consortium reports that ankle sprains recur in as many as 80% of cases in high-risk sports¹⁰.

How injuries are graded

A review of ankle sprains counts many grading systems¹¹. Of the two most basic, one uses three grades based on which ligaments are damaged, and the other goes by how severe the damage is¹¹. MCL injuries, for example, are graded I to III by how far the inner side of the knee opens when stressed¹². A review notes that no MCL scheme has been validated in children and adolescents¹³.

How ligaments heal

Research using the MCL as a model divides healing into three overlapping phases²:

• Inflammation starts at once with a blood clot and lasts a few weeks².
• Repair follows: cells lay down type III collagen (a thinner, more flexible form of collagen) that bridges the torn ends and turns mostly to type I over about six weeks².
• Remodelling aligns and matures the collagen fibres, and can continue for years².

The result is scar, which is weaker and larger than normal ligament and stretches more under load^4,5. Laboratory studies of healing MCLs found stiffness recovered by 52 weeks, largely because the ligament grew to as much as 2½ times its normal cross-section². Its breaking load stayed lower and its material stayed poorer up to one year: more tissue of lesser quality².

A systematic review (a study that searches for all the research on a question in a set, checkable way and sums it up) covered seven studies of injured MCLs in rats and rabbits¹⁴. It found preliminary evidence that loading an injured MCL made it stronger and stiffer than unloading¹⁴. In three of those studies, the loaded ligaments were also looser¹⁴. The review found these changes were slow, with large, precise effects only by 14 weeks¹⁴.

Inside and outside the joint

A torn MCL can generally heal on its own, though remodelling takes years and it stays mechanically weaker than normal². A tear through the middle of the ACL has limited capacity to heal².

The ACL’s central part is poorly supplied with blood, which a review calls common in ligaments inside joints³. After an ACL tear, the blood clot that acts as a temporary scaffold in torn ligaments outside the joint is missing¹⁵. This is mainly because joint fluid keeps flowing and disperses the blood¹⁵. A 2022 review concludes the ACL’s poor healing has many causes, with no single theory explaining it⁵.

A 2001 MRI (magnetic resonance imaging, a scan that shows soft tissue in detail) study followed 48 knees after acute injury to the posterior cruciate ligament (PCL), and 75% showed a continuous ligament¹⁶. Its authors described this slack but continuous healing as rarely seen in the injured ACL¹⁶.

Not every study fits this picture. A systematic review of 9 low-quality studies (734 people) looked at complete ACL tears managed without surgery¹⁷. Every study reported some ligaments regaining fibre continuity on MRI¹⁷.

How training builds it

A review reports that exercise and training produced only marginal increases in the mechanical properties of ligaments such as the knee MCL². As of 2021, evidence that exercise during growth can enlarge the ACL and improve its properties came mainly from animal studies¹⁸. The only human evidence then was one small study of elite weightlifters with weaknesses in its methods¹⁸.

An MRI study that looked at one group once (a cross-sectional study) scanned 52 figure skaters and springboard divers who had trained since before puberty¹⁸. The ACL of their landing or take-off leg was on average 4.9% larger in cross-section than the other leg’s¹⁸. The difference was not linked to the age they began training or how many years they had trained¹⁸.

Training and injury rates

Other studies counted injuries rather than measuring the ligament. A meta-analysis (a study that pools the results of several earlier studies) pooled 18 studies of neuromuscular training (exercise aimed at how nerves and muscles work together to control movement)¹⁹. Across 27,231 female participants, the programmes lowered ACL injury risk from about 1 in 54 to 1 in 111¹⁹. The same analysis found larger reductions in middle- and high-school-aged athletes than in college or professional ones¹⁹.

Across the lifespan

Children and adolescents

A meta-analysis of 20 studies of 3- to 19-year-olds found joint hypermobility (joint laxity: looseness in the joints) in 32.5% of girls and 18.1% of boys²⁰. It was less common at older ages²⁰. The studies varied widely and showed publication bias²⁰. In rabbits, every immature bone–MCL–bone unit failed by pulling off the tibia (shinbone), pointing to the bone attachment maturing later than the ligament². Animal studies found ACL cells multiply more, and the ACL heals better, in animals that are not yet fully grown³.

Near full growth, a review notes, adolescents may fracture a growth plate (a layer of cartilage near the end of a growing bone, where the bone grows longer) at the knee end of the thigh bone instead of injuring the MCL¹³. In 135 children aged 5 to 12, a growth-plate fracture of the ankle was suspected despite normal X-rays²¹. MRI showed ligament injuries in 80% and growth-plate fractures in 3%²¹.

Sex hormones

Studies disagree on whether the menstrual cycle affects knee laxity^22,23. A 2025 review of 54 laboratory and clinical studies found, on meta-analysis, no effect of the cycle on knee laxity²². A 2017 meta-analysis of six studies found knee laxity 0.36 mm greater around ovulation than earlier in the cycle, of unclear clinical importance²³. The 2025 review concluded that pregnancy, the contraceptive pill and menopause may affect ligament properties, apparently in part through relaxin, a hormone best known from pregnancy²².

In a 2017 review, four of five studies of women not on hormonal contraception found the phase after ovulation least linked to ACL injury²³. It rated the strength of this evidence low²³. A 2024 review linked days 21 to 24, when blood relaxin peaks, with greater ACL injury risk²⁴. It judged the link plausibly causal but in need of further study²⁴.

Older adults

With age, ligaments show less organised collagen and more calcification (calcium deposits)²⁵. The make-up of their matrix (the material around the cells) and their cell activity also change²⁵. A 2026 review says these changes together affect how ligaments sense load, heal and perform mechanically²⁵. Studies of the ACL, two of them in cadavers (bodies donated for study), report:

• In cadaver knees, only 6% of cruciate ligaments were classed as normal, and degeneration scores rose with age, more strongly in the ACL³.
• In 27 pairs of cadaver knees, the stiffness and breaking load of the bone–ACL–bone unit fell significantly with the donor’s age²⁶.
• The ACL loses mechanoreceptors (nerve endings that sense stretch or movement) with age, which correlates with a decline in joint position sense³.
• The ACL’s stem cells (cells that can become other cell types) decrease in number with age³.

The 2026 review also reports that ACL injuries in people over 40 are increasing²⁵. It notes that comprehensive studies of how ageing affects ligament blood supply are lacking²⁵.

Connections

A 2022 review says 90% of knee ligament injuries involve the ACL or the MCL, and the ACL tear has its own page⁵. The posterior cruciate ligament has its own page. A review grades injuries to the outer side of the knee, the subject of the lateral collateral ligament page, by how far the joint opens²⁷.

At the ankle joint, a 2018 consensus statement calls sprain of the lateral ankle ligaments the most common musculoskeletal injury in sport and recreation⁹. The deltoid ligament and ankle syndesmosis have their own pages.

Ligaments share their dense, regular build with tendons, and they join the bones of a joint¹. A review says a ligament injury can lead to problems such as osteoarthritis, in which the cartilage lining the joint wears away².

Neuromuscular training, covered on the neuromuscular warm-ups page, was studied for ACL injury in female athletes¹⁹. A meta-analysis found poorer dynamic balance among the risk factors for ankle sprain in male athletes; balance training has its own page²⁸.

Sources

1. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, Korol O, Johnson JE, Womble M, DeSaix P (2022). Anatomy and Physiology 2e. OpenStax. Link
2. Jung HJ, Fisher MB, Woo SL (2009). Role of biomechanics in the understanding of normal, injured, and healing ligaments and tendons. Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology. doi:10.1186/1758-2555-1-9 PMID 19457264
3. McCarthy MM, Hannafin JA (2014). The Mature Athlete. Sports Health. doi:10.1177/1941738113485691 PMID 24427441
4. Hildebrand KA, Frank CB (1998). Scar formation and ligament healing. Canadian journal of surgery. Journal canadien de chirurgie. PMID 9854530
5. Georgiev GP, Telang M, Landzhov B, Olewnik Ł, Slavchev SA, LaPrade RF, Ruzik K, Tubbs RS (2022). The novel epiligament theory: differences in healing failure between the medial collateral and anterior cruciate ligaments. Journal of Experimental Orthopaedics. doi:10.1186/s40634-021-00440-0 PMID 35028759
6. Kaneguchi A, Ozawa J, Minamimoto K, Yamaoka K (2020). Three-week joint immobilization increases anterior-posterior laxity without alterations in mechanical properties of the anterior cruciate ligament in the rat knee. Clinical Biomechanics. doi:10.1016/j.clinbiomech.2020.104993 PMID 32298871
7. Woo SL, Gomez MA, Sites TJ, Newton PO, Orlando CA, Akeson WH (1987). The biomechanical and morphological changes in the medial collateral ligament of the rabbit after immobilization and remobilization. The Journal of Bone and Joint Surgery. American Volume. PMID 3667649
8. Newton PO, Woo SL, MacKenna DA, Akeson WH (1995). Immobilization of the knee joint alters the mechanical and ultrastructural properties of the rabbit anterior cruciate ligament. Journal of Orthopaedic Research. doi:10.1002/jor.1100130207 PMID 7722756
9. Delahunt E, Bleakley CM, Bossard DS, Caulfield BM, Docherty CL, Doherty C, Fourchet F, Fong DT, Hertel J, Hiller CE, Kaminski TW, McKeon PO, Refshauge KM, Remus A, Verhagen E, Vicenzino BT, Wikstrom EA, Gribble PA (2018). Clinical assessment of acute lateral ankle sprain injuries (ROAST): 2019 consensus statement and recommendations of the International Ankle Consortium.. British journal of sports medicine. doi:10.1136/bjsports-2017-098885 PMID 29886432
10. Gribble PA, Delahunt E, Bleakley CM, Caulfield B, Docherty CL, Fong DT, Fourchet F, Hertel J, Hiller CE, Kaminski TW, McKeon PO, Refshauge KM, van der Wees P, Vicenzino W, Wikstrom EA (2014). Selection criteria for patients with chronic ankle instability in controlled research: a position statement of the International Ankle Consortium.. Journal of athletic training. doi:10.4085/1062-6050-49.1.14 PMID 24377963
11. Fong DT, Chan YY, Mok KM, Yung PS, Chan KM (2009). Understanding acute ankle ligamentous sprain injury in sports.. Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology. doi:10.1186/1758-2555-1-14 PMID 19640309
12. Svantesson J, Piussi R, Weissglas E, Svantesson E, Horvath A, Börjesson E, Williams A, Prill R (2024). Shedding light on the non-operative treatment of the forgotten side of the knee: rehabilitation of medial collateral ligament injuries-a systematic review. BMJ Open Sport & Exercise Medicine. doi:10.1136/bmjsem-2023-001750 PMID 38933372
13. Pearce SS, Bangley RM, McHorse K, Dale K, Lopreiato NP, Schlecter J, Pennock A, VandenBerg C, Pierce L, Perkins C, Koehler R, Ellington M (2023). Pediatric and Adolescent Medial Collateral Ligament Injuries. Journal of the Pediatric Orthopaedic Society of North America. doi:10.55275/JPOSNA-2023-757 PMID 40432933
14. Bleakley C, Netterström-Wedin F (2023). Does mechanical loading restore ligament biomechanics after injury? A systematic review of studies using animal models. BMC Musculoskeletal Disorders. doi:10.1186/s12891-023-06653-x PMID 37349749
15. Kiapour AM, Murray MM (2014). Basic science of anterior cruciate ligament injury and repair.. Bone & joint research. doi:10.1302/2046-3758.32.2000241 PMID 24497504
16. Akisue T, Kurosaka M, Yoshiya S, Kuroda R, Mizuno K (2001). Evaluation of healing of the injured posterior cruciate ligament. Arthroscopy. doi:10.1053/jars.2001.21540 PMID 11239346
17. Pitsillides A, Stasinopoulos D, Giannakou K (2021). Healing potential of the anterior cruciate ligament in terms of fiber continuity after a complete rupture: A systematic review.. Journal of bodywork and movement therapies. doi:10.1016/j.jbmt.2021.06.003 PMID 34776148
18. Beaulieu ML, DeClercq MG, Rietberg NT, Li SH, Harker EC, Weber AE, Ashton-Miller JA, Wojtys EM (2021). The Anterior Cruciate Ligament Can Become Hypertrophied in Response to Mechanical Loading: A Magnetic Resonance Imaging Study in Elite Athletes. The American Journal of Sports Medicine. doi:10.1177/03635465211012354 PMID 34259598
19. Petushek EJ, Sugimoto D, Stoolmiller M, Smith G, Myer GD (2019). Evidence-Based Best-Practice Guidelines for Preventing Anterior Cruciate Ligament Injuries in Young Female Athletes: A Systematic Review and Meta-analysis.. The American journal of sports medicine. doi:10.1177/0363546518782460 PMID 30001501
20. Sobhani-Eraghi A, Motalebi M, Sarreshtehdari S, Molazem-Sanandaji B, Hasanlu Z (2020). Prevalence of joint hypermobility in children and adolescents: A systematic review and meta-analysis. Journal of Research in Medical Sciences. doi:10.4103/jrms.JRMS_983_19 PMID 33824669
21. Boutis K, Plint A, Stimec J, Miller E, Babyn P, Schuh S, Brison R, Lawton L, Narayanan UG (2016). Radiograph-Negative Lateral Ankle Injuries in Children. JAMA Pediatrics. doi:10.1001/jamapediatrics.2015.4114 PMID 26747077
22. Gilmer G, Crasta N, Tanaka MJ (2025). The Effect of Sex Hormones on Joint Ligament Properties: A Systematic Review and Meta-analysis. The American Journal of Sports Medicine. doi:10.1177/03635465241310145 PMID 39887996
23. Herzberg SD, Motu'apuaka ML, Lambert W, Fu R, Brady J, Guise JM (2017). The Effect of Menstrual Cycle and Contraceptives on ACL Injuries and Laxity: A Systematic Review and Meta-analysis. Orthopaedic Journal of Sports Medicine. doi:10.1177/2325967117718781 PMID 28795075
24. Parker EA, Duchman KR, Meyer AM, Wolf BR, Westermann RW (2024). Menstrual Cycle Hormone Relaxin and ACL Injuries in Female Athletes: A Systematic Review. The Iowa Orthopaedic Journal. PMID 38919370
25. Iwasaki N, Finding EJT, Wheeler-Jones CPD, Thorpe CT (2026). The role of tendon and ligament vasculature in ageing and injury. Biogerontology. doi:10.1007/s10522-026-10491-4 PMID 42622913
26. Woo SL, Hollis JM, Adams DJ, Lyon RM, Takai S (1991). Tensile properties of the human femur-anterior cruciate ligament-tibia complex. The American journal of sports medicine. doi:10.1177/036354659101900303 PMID 1867330
27. Figueroa F, Figueroa D, Putnis S, Guiloff R, Caro P, Espregueira-Mendes J (2021). Posterolateral corner knee injuries: a narrative review. EFORT Open Reviews. doi:10.1302/2058-5241.6.200096 PMID 34532075
28. Mason J, Kniewasser C, Hollander K, Zech A (2022). Intrinsic Risk Factors for Ankle Sprain Differ Between Male and Female Athletes: A Systematic Review and Meta-Analysis.. Sports Medicine - Open. doi:10.1186/s40798-022-00530-y PMID 36399159

Every source the library cites

Changes

• September 30, 2026 First published.