Tendon pain is one of the most frequent reasons athletes miss training. The Achilles tendon in runners, the patellar tendon in jumpers, "tennis elbow", shoulder problems in powerlifters — all of these are manifestations of one phenomenon. The editorial team examines why tendinitis develops, how it relates to training load and which drugs are able to increase the vulnerability of tendons.
Tendinitis or tendinopathy: what happens in the tendon
The word "tendinitis" means inflammation of a tendon, and that is exactly how the problem has long been named in everyday life and even in medical documents. However, histological studies of chronically painful tendons revealed mostly not classic inflammation but degenerative changes: disruption of the structure of collagen fibres, an increase in intercellular substance, proliferation of blood vessels.
In 2002, a group of authors led by K. Khan published in the BMJ an article with a telling title about the need to abandon the "tendinitis myth" (Khan et al., 2002). Since then, sports medicine has predominantly used the neutral term "tendinopathy", which describes pain and impaired tendon function without assumptions about the mechanism.
Modern data show that inflammatory processes at the cellular level do nonetheless take part, especially in the early stages, so the picture is more complex than simply "inflammation" or "degeneration". For the reader, something else is more important: chronic tendon pain rarely passes from anti-inflammatory tablets alone.
J. Cook and C. Purdam proposed a continuum model in which they distinguish reactive tendinopathy, "failed healing" and a degenerative stage (Cook & Purdam, 2009). The reactive phase arises in response to a sharp overload and is potentially reversible, while the degenerative one develops over a long time and recovers poorly.
Load: why tendons cannot keep up
A tendon is living tissue that adapts to load, but does so considerably more slowly than muscles. Collagen turnover in a tendon occurs slowly, and blood supply is limited. Studies show that the adaptation of tendons to changes in load requires a long time, whereas muscle strength can grow within just a few weeks (Magnusson & Kjaer, 2019).
That is why the most dangerous things for tendons are sharp changes: an increase in mileage, switching to a new type of surface, adding jumping exercises, returning to training after a break. For a tendon, not only volume matters but also the nature of the load — exercises that use it as a spring (jumps, sprints, changes of direction) are the most demanding.
Insufficient load is also harmful. After prolonged rest or immobilisation, a tendon loses stiffness and becomes more vulnerable. That is why both complete inactivity and a sudden jump in load equally increase the risk.
Finally, recovery between workouts matters. A tendon does not react to load immediately, and pain often intensifies the next morning. A series of hard workouts in a row without rest days leaves the tissue no time for remodelling.
Internal risk factors
Not all athletes with the same load develop tendinopathy. An important role is played by individual factors: age, biomechanics, body weight, previous injuries. With age, tendons become less elastic and their capacity to recover declines, so older amateurs are especially sensitive to a sharp increase in load.
Biomechanical features — excessive pronation of the foot, weakness of the hip muscles, limited mobility of the ankle joint — redistribute load and can overload individual tendons. Weakness of the muscles that "unload" the tendon also matters.
Metabolic conditions are also linked to tendinopathy: diabetes, obesity, disorders of lipid metabolism, gout. They change the structure of collagen and the blood supply of the tissue.
Previous tendinopathy is one of the strongest predictors of a repeat episode. Tissue that has already undergone changes does not recover fully, so the return to sport after a first episode must be especially gradual.
Pharmacology and tendons
The best-known group of drugs associated with tendon damage is the fluoroquinolone antibiotics (ciprofloxacin, levofloxacin and others). Regulators, in particular the FDA, added a boxed warning to their instructions about the risk of tendinitis and tendon rupture, primarily of the Achilles (FDA, 2016). The risk is higher in older people, with concurrent use of corticosteroids and after organ transplantation.
Corticosteroids are the second important group. Injections of glucocorticoids into the tendon area can quickly reduce pain, but a large systematic review in the Lancet showed that for some tendinopathies, in particular lateral epicondylitis, long-term outcomes are worse than with a wait-and-see approach or physiotherapy (Coombes et al., 2010). Systemic long-term use of corticosteroids also weakens connective tissue.
Anabolic steroids create a classic imbalance: muscle strength grows quickly, while tendons cannot keep up. A study among experienced strength athletes found that in users of anabolic steroids tendon ruptures, in particular of the upper limb, occurred more often than in those who did not use them (Kanayama et al., 2015). A direct effect of androgens on the structure of collagen is discussed separately.
Among other substances discussed in the context of tendon damage are statins and isotretinoin; the data here are less clear-cut. Non-steroidal anti-inflammatory drugs do not damage tendons directly but, by dampening pain, allow overload to be continued.
- fluoroquinolones — an official warning about tendinitis and ruptures;
- corticosteroids (injections and systemic) — pain relief at the cost of worse long-term outcomes in some cases;
- anabolic steroids — a gap between muscle strength and tendon strength;
- painkillers — masking the overload signal.
Typical locations in different sports
The location of tendinopathy is largely determined by the nature of the load. Runners most often suffer from Achilles tendinopathy, while volleyball and basketball players suffer from patellar tendinopathy, which is not for nothing called "jumper's knee".
In tennis and other racket sports, lateral epicondylopathy and shoulder tendinopathies are common. In strength sports, powerlifting and bodybuilding, the tendons of the rotator cuff, the long head of the biceps and also the elbow area are often affected.
Knowing "your" weak spots helps to work purposefully on prevention: to strengthen the corresponding muscles, control technique and dose the riskiest exercises.
| Sport | Typical location | Provoking movement |
|---|---|---|
| Running | Achilles tendon | Push-off, sharp increase in mileage |
| Volleyball, basketball | Patellar tendon | Jumps and landings |
| Tennis, squash | Wrist extensors (lateral epicondyle) | Backhand stroke |
| Swimming, throwing | Rotator cuff of the shoulder | Repetitive overhead movements |
| Strength sports | Biceps, rotator cuff, elbow | Presses, pulls with heavy weight |
Editorial conclusions
Tendinitis in athletes is in most cases a tendinopathy — a state in which disruption of tendon structure and pain are combined, rather than only classic inflammation.
The main cause is a mismatch between load and the tendon's ability to adapt: sharp jumps in volume, jumping exercises, a return after a break.
Fluoroquinolones, corticosteroids and anabolic steroids are able to increase the vulnerability of tendons, while painkillers mask overload.
We also recommend reading our materials on the prevention and diagnosis of tendinitis, on tendon ruptures in athletes and on collagen and supplements for ligaments.
References
- Khan KM, Cook JL, Kannus P, et al. Time to abandon the "tendinitis" myth. BMJ. 2002;324(7338):626–627.
- Cook JL, Purdam CR. Is tendon pathology a continuum? A pathology model to explain the clinical presentation of load-induced tendinopathy. Br J Sports Med. 2009;43(6):409–416.
- Magnusson SP, Kjaer M. The impact of loading, unloading, ageing and injury on the human tendon. J Physiol. 2019;597(5):1283–1298.
- Coombes BK, Bisset L, Vicenzino B. Efficacy and safety of corticosteroid injections and other injections for management of tendinopathy: a systematic review of randomised controlled trials. Lancet. 2010;376(9754):1751–1767.
- Kanayama G, DeLuca J, Meehan WP 3rd, et al. Ruptured tendons in anabolic-androgenic steroid users: a cross-sectional cohort study. Am J Sports Med. 2015;43(11):2638–2644.
- U.S. Food and Drug Administration. FDA Drug Safety Communication: FDA updates warnings for oral and injectable fluoroquinolone antibiotics due to disabling side effects. 2016.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
