Tendinitis, also written tendonitis, traditionally refers to inflammation of a tendon, the strong connective tissue that attaches muscle to bone. Tendons transmit the force generated by muscles so that joints and limbs can move. Tendinitis commonly produces pain, tenderness and sometimes swelling around the affected tendon, with discomfort often increasing when the tendon is loaded. The condition can occur in many anatomical locations, including the shoulder, elbow, wrist, hand, knee and foot.
The term is widely used clinically and in everyday language, although modern musculoskeletal terminology increasingly distinguishes acute inflammatory tendon disorders from broader tendinopathy. Tendinopathy encompasses tendon pain and impaired function arising through a combination of structural, cellular and mechanical changes, and many chronic tendon disorders do not show prominent inflammation. This distinction matters because the biological mechanisms and optimal management of acute tendon inflammation and chronic tendinopathy are not necessarily identical.
Tendons are composed predominantly of organised collagen fibres arranged to withstand tensile forces. They contain relatively little elastin compared with tissues that require greater elasticity, reflecting their role in transmitting muscular force rather than undergoing large stretches. Some tendons are surrounded by a synovial sheath that facilitates smooth movement, and inflammation of this sheath is called tenosynovitis.
Tendon tissue adapts to mechanical loading, becoming stronger and more tolerant when appropriately exposed to progressive stress, but excessive, sudden or repetitive loading can exceed its capacity for adaptation. Blood supply varies between tendons and anatomical regions, which can influence healing. Tendon pain is not determined solely by visible structural abnormalities because degenerative or disorganised tendon changes can exist without symptoms. Conversely, pain can be substantial even when imaging findings appear relatively modest. These factors make clinical assessment essential.
Tendon disorders commonly arise when loading changes faster than the tissue can adapt. A sudden increase in running, jumping, throwing, lifting or repetitive occupational activity can increase tendon stress. Poorly conditioned tissues may be particularly vulnerable when workload rises rapidly, while age-related changes can reduce some aspects of tendon capacity. Examples include Achilles tendon disorders, patellar tendon disorders, rotator cuff tendinopathy and lateral elbow tendon disorders.
The location often indicates which tendon is involved, but the diagnosis also depends on pain behaviour and physical examination. Pain is commonly provoked by loading the relevant muscle-tendon unit rather than by completely passive movement. Swelling or local warmth may occur, particularly in more inflammatory presentations. Tendon rupture is a different and more severe injury in which tendon fibres tear substantially or completely, often producing sudden pain, weakness and loss of function.
Tendinitis should be distinguished from muscle strain, ligament sprain, bursitis and joint disease. A muscle strain affects muscle fibres or the muscle-tendon unit, whereas a tendon disorder principally involves tendon tissue. A ligament sprain affects the connective tissues stabilising a joint rather than the structure attaching muscle to bone. Bursitis involves inflammation or irritation of a fluid-filled bursa and often produces local tenderness over a pressure point. Arthritis primarily concerns a joint and can cause intra-articular swelling and stiffness.
These conditions can coexist, and pain can make anatomical localisation difficult without examination. Tendinopathy is also broader than tendinitis because it includes chronic tendon pain and structural abnormalities that may involve failed adaptation and altered collagen organisation without acute inflammation being the dominant mechanism. Consequently, treating every persistent tendon problem as if it were simply an inflamed tissue can lead to inappropriate expectations about rest and anti-inflammatory treatment.
Diagnosis is usually clinical and based on the history, anatomical location, provoking activities and examination. A clinician may test the tendon by asking the person to contract the associated muscle or move against resistance. Ultrasound can demonstrate tendon structure, thickening, tears and other abnormalities, while MRI may be useful for complex or uncertain cases. However, imaging findings must be interpreted cautiously because structural tendon changes can occur in people without pain.
Imaging is therefore most useful when it answers a specific clinical question rather than being used automatically for every episode. Sudden loss of strength, a palpable gap, a snapping sensation or inability to perform a previously normal movement can raise concern about significant tendon rupture and may require urgent assessment. The diagnostic process should establish both the symptomatic structure and the severity of any tissue damage rather than simply identifying any abnormality visible on a scan.
Treatment depends on whether the condition is acute inflammatory tendinitis, chronic tendinopathy or a more substantial tear. For many non-ruptured tendon problems, relative reduction of aggravating activity followed by progressive rehabilitation is more useful than prolonged complete rest. Physiotherapy can prescribe graded loading exercises designed to restore tendon capacity and the strength of the associated muscle. The appropriate progression depends on the tendon, severity, duration and functional demands of the individual.
Pain during rehabilitation does not necessarily mean that the tendon is being damaged, but a large and sustained increase in symptoms may indicate that loading needs adjustment. Anti-inflammatory medicines can provide symptomatic relief in selected circumstances, although their role varies because chronic tendinopathy is not simply persistent acute inflammation. Steroid injections may be used selectively for some conditions but have potential limitations and risks, particularly around certain tendons.
Tendon recovery is influenced by mechanical loading, tissue capacity, general health and the nature of the injury. Tendons adapt relatively slowly compared with some other tissues, so symptoms can persist even after the initial trigger has been removed. A common pattern is repeated cycles of pain followed by complete rest, improvement, and then recurrence when the previous workload is resumed too quickly. Progressive rehabilitation attempts to break this cycle by increasing tissue capacity gradually.
Work and sport-specific loading is eventually important because a tendon that tolerates ordinary daily activity may still be unable to tolerate high-volume running, jumping or lifting. Warm-up, appropriate training progression and recovery periods can reduce excessive load fluctuations. Technique and equipment may matter in some sports, but there is rarely a single universal biomechanical defect responsible for tendinitis. Prevention is therefore generally based on sensible load management rather than eliminating movement.
Tendinitis is sometimes presented as a condition caused solely by inflammation and cured simply by rest, but that model is incomplete for persistent tendon pain. Chronic tendinopathy can involve collagen disorganisation, altered tendon structure, changes in cellular behaviour and impaired mechanical capacity, with inflammation not necessarily being the principal driver. Another misconception is that complete pain elimination must occur before rehabilitation begins.
In many cases, carefully controlled loading is part of the treatment process, provided serious injury has been excluded and the programme is appropriately tailored. Conversely, sudden severe pain accompanied by a snapping sensation or major loss of function should not be treated as ordinary tendinitis because tendon rupture may require specialist management. Distinguishing tendinitis, tendinopathy and tendon rupture is therefore important for understanding prognosis, selecting treatment and determining how quickly normal loading should be restored.
