Tendon

What it is

A tendon passes force from a contracting muscle to the skeleton, which produces movement¹. It is dense, regular connective tissue (a family of body tissues), with collagen (a strong, rope-like protein) fibres in parallel bundles⁵. That layout makes it strong under pulling and resistant to stretching along the fibres⁵. Tendons also store elastic energy and give it back when a stretch is followed at once by a shortening².

A review describes tendon as having relatively few cells, mainly fibroblasts (tendon cells) lying along the collagen fibrils (very thin strands of collagen)¹. The fibrils are thought to be the main load-carrying structure, though exactly how force passes through a tendon is still largely unresolved¹. Compared with muscle, tendon has fewer cells for its dry mass, less blood supply and a lower metabolism³.

How it responds to load

From stretch to signal

A review describes how strain (stretch as a share of length) reaches tendon cells through proteins in their outer membrane³. This, with stretch-sensitive channels, switches on signals that raise collagen production³. A systematic review adds that how training changes tendons is still not fully understood².

Reviews disagree on what happens to collagen after exercise^1,6. A 2010 review reports that collagen formation rises, peaking at about 24 hours and staying raised for about 3 days⁶. A later review notes that one study found a rise in the patellar tendons of young men after kicking exercise¹. Several other studies, using the same method and exercise, could not robustly confirm it in adult human tendon¹.

A stable core and a changing surface

Carbon-14 dating indicated that the Achilles tendon core forms while a person grows taller, and is essentially not renewed afterwards, unlike muscle^1,7. One review suggests tendon collagen may form two pools: a large, stable one laid down while young, and a small, fast-changing one¹. A systematic review notes that training may therefore change only the tendon’s outer tissue².

Too little, too much, or none

A systematic review notes evidence that the size of the strain tendon cells feel, and how long it lasts, is the key signal for change². There may be a best dose: too little or too much strain may lead to breakdown rather than building². A review concluded that even 1–2 weeks of unloading a limb seems to reduce tendon stiffness significantly¹.

In adults, muscle size can change after 3 to 4 weeks of heavy resistance training³. A 2017 review found no reports of tendon changing that fast³. A review concludes that the two differ in how fast they adapt and which loading they respond to³. Plyometric (jump) loading, for example, seems a stronger stimulus for muscle than for tendon³.

An International Olympic Committee consensus statement (a report of what an expert panel agrees on) covers elite athletes⁸. It says emerging evidence points to poor load management, including rapid changes in load, as a major injury risk⁸. A review of 20 studies of elite male team-sport athletes linked a high ratio of recent to longer-term load with more injuries in most studies⁹. The studies worked out that ratio in varied ways, which the authors say limits firm recommendations⁹. Both sources cover injuries in general, not tendon injuries alone^8,9.

How it gets injured

Tendinopathy

A 2019 international consensus made tendinopathy the preferred term for lasting tendon pain and loss of function linked to loading¹⁰. A review notes that although tendons adapt to load, repetitive use often leads to injuries such as tendinopathy⁶. Tiny tears and material fatigue have been suggested as ways this happens⁶. Another review lists disorganised collagen, more small blood vessels and nerves, and more immune cells among its tissue features⁴.

How much inflammation is involved is still debated¹. Long-term tendinopathy is generally not dominated by it, though signs of it have been found early on¹. Some studies report that stiffness falls with tendinopathy, while another found no change¹. Whether any change comes from the condition or from less loading is unknown¹.

A meta-analysis pooled the wait-and-see groups of six randomised trials (which assign people to groups by chance), covering 518 people with tendinopathy¹¹. In those groups, pain and function improved without any intervention¹¹. Yet rotator cuff tendinopathy had not fully recovered at one year¹¹. Outer elbow, patellar and Achilles tendinopathy had not fully resolved within 12–16 weeks¹¹.

How common it is

Researchers counted new cases in 25 years of Hong Kong public-hospital records, covering 36,970 patients¹². Incidence (new cases over a set time) in those records was:

Group in the Hong Kong records	New cases per 100,000 people a year
10 tendinopathies together	about 15.512
Lateral epicondylitis (outer elbow), the commonest one	8.8412
7 non-traumatic tendon ruptures together	about 5.612

In a 5-year study at one hospital, Achilles, patellar and quadriceps tendon ruptures were injuries of middle age¹³. Rotator cuff tears and biceps tendon ruptures were commoner in the elderly¹³.

Wear before a rupture

One study took samples from 891 people whose tendons had ruptured spontaneously, mostly Achilles and biceps tendons¹⁴. None showed healthy structure, and 97% of the changes were degenerative (worn-down tissue)¹⁴. Such changes appeared in 34% of matched tendons from people who had died in accidents¹⁴. The authors concluded that, at least in a city population, degenerative changes are common after 35 and linked to rupture¹⁴. A review adds that this wear does not always cause symptoms¹⁵.

How it heals

Healing after an acute tendon injury is described in three overlapping phases: inflammatory, proliferative and remodelling (replacing old tissue with new)^15,16. Healed tendon does not regain its original mechanical properties¹⁶. After surgical repair of a torn Achilles tendon, cell activity during walking was about 6 times that of intact tendon at 3 months¹. It fell to 1.6 times by 12 months, suggesting it settles only sometime after 6–12 months¹. These timings come from the repaired Achilles tendon only¹.

How training builds it

What the reviews found

A meta-analysis pooled 61 studies of 763 people, mostly of the Achilles and patellar tendons and mostly resistance training². Loading produced moderate gains in stiffness and large gains in Young’s modulus, how stiff a material is in itself, regardless of its size or length². Cross-sectional area (the size of a slice cut straight across) rose a little². In 20 studies that measured all three, rising stiffness came mainly from a change in the tendon’s material, not from the tendon getting thicker².

The findings rest on the Achilles, patellar and quadriceps tendons and one thigh aponeurosis (a flat sheet of tendon)². The studies were small, and most people in them were untrained or recreationally active². The review’s three studies of well-trained athletes found no change in patellar tendon stiffness². An earlier meta-analysis drew on small studies, and 237 of its 264 people were male¹⁷.

How hard and how long

The two reviews disagree on intensity². The earlier one found stiffness gains depended on loading intensity¹⁷. The later one found no significant difference between high-intensity (70% of maximum or more) and low-intensity training². It suggests the percentage of maximum lifted may not reflect the strain the tendon actually feels². Sorted by strain, high-strain programmes produced large gains and low-strain ones no significant gains, though only three studies reported the strain involved².

A review reports that in healthy Achilles tendons, training at 90% of maximum effort raised stiffness more than training at 55%¹. The tendon also responded more to 6-second loading cycles than to 2-second ones¹.

On time, the earlier meta-analysis found larger effects for programmes of 12 weeks or more, though the difference was not significant¹⁷. The later review found notable stiffness changes between 4 and 8 weeks, with conflicting evidence on when thickness changes².

Which kind of training

In high-intensity resistance training, concentric (shortening), eccentric (lengthening), combined and isometric (joint-still) work all produced moderate-to-large gains in stiffness and Young’s modulus². The differences between these contraction types were not significant². Across training types, the review found:

Training type	Change in tendon stiffness
Resistance training	large increase2
Jump-based (plyometric) training	small increase2
Resistance plus aerobic training (two groups)	no change2

Across the lifespan

Growing up

A systematic review sums up that tendons in the legs grow thicker and stiffer through childhood and adolescence². Studies of the patellar and Achilles tendons suggest the material develops most before the adolescent growth spurt, while thickening carries on through adolescence³. In a season-long study of 44 athletes aged 12–17, those who developed patellar tendon pain had higher tendon strain than those who did not¹⁸. A review of 17 studies in under-18s found lower-limb tendinopathy in 8.2% to 33.3%, rising with age¹⁹.

Growing children also get injuries at an apophysis (a bony bump where a muscle attaches that grows from its own growth plate)²⁰. A Cochrane review (a systematic review done to set standards) describes these injuries in the legs as common²⁰. They occur most often below the knee (Osgood-Schlatter disease) and at the heel²⁰. Rapid growth and weight change are considered the main contributing factor²⁰. Many authors propose pull from tight tendons and muscles as the cause, but research indicates this is unlikely for the heel²⁰.

Adulthood and older age

The same systematic review says ageing is typically linked to lower tendon stiffness and Young’s modulus². Another review finds the picture mixed: studies report Young’s modulus either unchanged or reduced with age¹. In one study, people aged 66 had a lower Young’s modulus than people aged 26, but not once matched for activity¹. Studies also disagree on whether tendon size rises or stays the same with age¹. One study found patellar tendon cross-sectional area 12% lower mid-tendon in 26 older adults than in 11 young adults²¹.

In the Hong Kong records, people aged 40–59 had higher rates of most tendinopathies, but patellar and Achilles tendinopathy mainly affected younger people¹². Older tendons appear to stay responsive to load². In the 2022 meta-analysis, five groups aged 60 and over gained stiffness from resistance training, but less than other adults: small gains against large ones². The same meta-analysis found only one study of children².

Connections

A review names the rotator cuff, elbow, patellar, gluteal and Achilles tendons as common tendinopathy sites⁴. The library has entries on the Achilles tendon, patellar tendon, rotator cuff tendons, gluteal tendons and common extensor tendon.

Further entries cover the quadriceps, long head of biceps, distal biceps, distal triceps and pectoralis major tendons. Others cover the proximal hamstring, adductor longus and tibialis anterior tendons. An earlier review found tendon training studies only on the Achilles or patellar tendon¹⁷.

Tendon works with muscle, and the two adapt at different speeds³. Each training method compared above has a page: resistance training, eccentric training, isometric training and plyometric training².

Sources

1. Magnusson SP, Kjaer M (2019). The impact of loading, unloading, ageing and injury on the human tendon.. The Journal of Physiology. doi:10.1113/JP275450 PMID 29920664
2. Lazarczuk SL, Maniar N, Opar DA, Duhig SJ, Shield A, Barrett RS, Bourne MN (2022). Mechanical, Material and Morphological Adaptations of Healthy Lower Limb Tendons to Mechanical Loading: A Systematic Review and Meta-Analysis.. Sports Medicine. doi:10.1007/s40279-022-01695-y PMID 35657492
3. Mersmann F, Bohm S, Arampatzis A (2017). Imbalances in the Development of Muscle and Tendon as Risk Factor for Tendinopathies in Youth Athletes: A Review of Current Evidence and Concepts of Prevention.. Frontiers in Physiology. doi:10.3389/fphys.2017.00987 PMID 29249987
4. Millar NL, Silbernagel KG, Thorborg K, Kirwan PD, Galatz LM, Abrams GD, Murrell GAC, McInnes IB, Rodeo SA (2021). Tendinopathy.. Nature Reviews Disease Primers. doi:10.1038/s41572-020-00234-1 PMID 33414454
5. 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
6. Magnusson SP, Langberg H, Kjaer M (2010). The pathogenesis of tendinopathy: balancing the response to loading.. Nature Reviews Rheumatology. doi:10.1038/nrrheum.2010.43 PMID 20308995
7. Heinemeier KM, Schjerling P, Heinemeier J, Magnusson SP, Kjaer M (2013). Lack of tissue renewal in human adult Achilles tendon is revealed by nuclear bomb (14)C.. The FASEB Journal. doi:10.1096/fj.12-225599 PMID 23401563
8. Soligard T, Schwellnus M, Alonso JM, Bahr R, Clarsen B, Dijkstra HP, Gabbett T, Gleeson M, Hägglund M, Hutchinson MR, Janse van Rensburg C, Khan KM, Meeusen R, Orchard JW, Pluim BM, Raftery M, Budgett R, Engebretsen L (2016). How much is too much? (Part 1) International Olympic Committee consensus statement on load in sport and risk of injury.. British Journal of Sports Medicine. doi:10.1136/bjsports-2016-096581 PMID 27535989
9. Andrade R, Wik EH, Rebelo-Marques A, Blanch P, Whiteley R, Espregueira-Mendes J, Gabbett TJ (2020). Is the Acute: Chronic Workload Ratio (ACWR) Associated with Risk of Time-Loss Injury in Professional Team Sports? A Systematic Review of Methodology, Variables and Injury Risk in Practical Situations.. Sports Medicine. doi:10.1007/s40279-020-01308-6 PMID 32572824
10. Zwerver J, Mc Auliffe S, Rio EK, Scott A, Vicenzino BT, Weir A (2020). ICON 2019-international scientific tendinopathy symposium: building an ICONic tendon tower-launching a new era in clinical tendinopathy research.. British Journal of Sports Medicine. doi:10.1136/bjsports-2019-101214 PMID 31666219
11. Garzón M, Balasch-Bernat M, Cook C, Ezzatvar Y, Álvarez-Lliso Ó, Dueñas L, Lluch E (2024). How long does tendinopathy last if left untreated? Natural history of the main tendinopathies affecting the upper and lower limb: A systematic review and meta-analysis of randomized controlled trials.. Musculoskeletal science & practice. doi:10.1016/j.msksp.2024.103103 PMID 38879981
12. Lau KKL, Ng JP, Ling SKK, Ong MTY, Yung PSH, Lui PPY (2026). Incidence rates of tendinopathies and non-traumatic tendon ruptures in Hong Kong: a 25-year epidemiological study using big data from public hospitals.. Scientific reports. doi:10.1038/s41598-025-33465-x PMID 41526611
13. Clayton RA, Court-Brown CM (2008). The epidemiology of musculoskeletal tendinous and ligamentous injuries.. Injury. doi:10.1016/j.injury.2008.06.021 PMID 19036362
14. Kannus P, Józsa L (1991). Histopathological changes preceding spontaneous rupture of a tendon. A controlled study of 891 patients.. The Journal of Bone and Joint Surgery. American Volume. doi:10.2106/00004623-199173100-00009 PMID 1748700
15. Sharma P, Maffulli N (2006). Biology of tendon injury: healing, modeling and remodeling.. Journal of Musculoskeletal & Neuronal Interactions. PMID 16849830
16. Darrieutort-Laffite C, Blanchard F, Soslowsky LJ, Le Goff B (2024). Biology and physiology of tendon healing.. Joint Bone Spine. doi:10.1016/j.jbspin.2024.105696 PMID 38307405
17. Bohm S, Mersmann F, Arampatzis A (2015). Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults.. Sports Medicine - Open. doi:10.1186/s40798-015-0009-9 PMID 27747846
18. Mersmann F, Domroes T, Tsai MS, Pentidis N, Schroll A, Bohm S, Arampatzis A (2023). Longitudinal Evidence for High-Level Patellar Tendon Strain as a Risk Factor for Tendinopathy in Adolescent Athletes.. Sports Medicine - Open. doi:10.1186/s40798-023-00627-y PMID 37673828
19. Simpson M, Rio E, Cook J (2016). At What Age Do Children and Adolescents Develop Lower Limb Tendon Pathology or Tendinopathy? A Systematic Review and Meta-analysis.. Sports Medicine. doi:10.1007/s40279-015-0438-0 PMID 26626072
20. Williams CM, Krommes K, Paterson KL, Haines T, Caserta A, Thorborg K (2026). Non-surgical treatment for lower limb apophyseal injuries.. Cochrane Database of Systematic Reviews. doi:10.1002/14651858.CD015156.pub2 PMID 42454655
21. Carroll CC, Dickinson JM, Haus JM, Lee GA, Hollon CJ, Aagaard P, Magnusson SP, Trappe TA (2008). Influence of aging on the in vivo properties of human patellar tendon.. Journal of Applied Physiology. doi:10.1152/japplphysiol.00059.2008 PMID 18927271

Every source the library cites

Changes

• September 30, 2026 First published.