Tendinopathy is an umbrella term used to classify the clinical presentation of pain or dysfunction occurring within a tendon which can be in the presence or absence of structural pathology. Clinically tendinopathy presents with localised pain that is dependent on loading as well as impaired function .
Tendons have the ability to change depending on which loads they are subjected to. Changes can occur not only at a structural level but also occur at a cortical level. Research in tendons and tendinopathy has mainly focused on the tendons themselves and not at what is happening at a spinal cord and brain level. Similarly, muscle strength in relation to tendinopathy has been more widely researched as compared to changes in motor control. Emerging literature is now investigating the cortical adaptations that occur in tendinopathy.
Adaptations at the Tendon Level
The pathophysiology of tendinopathy is not well understood. There are various hypothesis and models that attempt to describe the pathogenesis of tendinopathy, however, researchers and clinicians are still unsure as to precisely what is going on. Tendons are designed to adapt to changes in load. Load and capacity in tendons is a concept whereby a tendon's capacity only ever exceeds its load. When an excessive load is placed on a tendon, it has the capability of becoming dysfunctional.
In 2009, Cook and Purdam described the tendon continuum model, which stages tendinopathy according to changes in the tendons structure. The continuum model has three stages; the reactive tendon; tendon disrepair and degenerative tendinopathy. A tendon can be in multiple stages at the same time and depending on the intervention the tendon can move up and down the continuum.Pathological tendons are often thicker than their unaffected counterpart. This increase in AP diameter is possibly due to the tendon trying to maintain its loading ability. Docking and Cook, 2016, conducted a study that investigated patella and Achilles tendons. They found that pathological tendons, while having areas of disorganisation, had an increased amount of aligned fibrillar structure (the “healthy” part of a tendon) as compared with non-pathological tendons. This could potentially show that tendons with pathology, increase the amount of healthy tendon as an attempt to maintain its homeostasis and loading ability.
“Treat the doughnut not the hole”Considering that pathological tendons still have portions of the healthy tendon the rehabilitation principle in tendons “treat the doughnut, not the hole” has been suggested. This is where the goal in rehabilitation is to increase the loading capacity of the aligned fibrillar structure, the healthy part or "doughnut" rather than to try and regenerate the disorganised tissue, the hole.
Adaptations at the Brain
In tendinopathy, changes do not only occur in the periphery but also in the central nervous system. Every movement the body can make is represented within the primary motor cortex of the brain. Movements are a balance between excitatory and inhibitory stimuli. Rio et al 2015, showed that corticospinal excitability was elevated in people with patella tendinopathy. At the same time, it has been shown that there is cortical inhibition in tendinopathy. In layman’s terms, this is likened to a learner driver who would have a foot on both the accelerator and brake and press them at the same time. For a motor unit to have both excess excitation and excess inhibition, is an unhelpful movement strategy, and this can alter the motor control of the entire kinetic chain.These neuroplastic changes as part of a tendinopathy picture is an emerging concept. A new model of tendon rehabilitation called, tendon neuroplastic training, has been proposed as a more effective rehabilitation tool. This focuses on motor control rather than muscle strength on its own as a loading strategy to treat tendinopathy.
Pain and Tendinopathy
Tendon pain continues to puzzle the medical profession. Tendons presenting with pathological changes on imaging may not be painful. The warm-up phenomenon where tendons become less painful during activity also does not fit into a typical pain presentation either.
Potential contributors to nociception in tendons may be:
- Changes within the extracellular matrix, particularly increased prostaglandin production
- Increased vascularity
- Change in tenocyte structure and function
- Biochemical changes (cytokines, neuropeptides, neurotransmitters and metabolites)
- Changes in the ion channels within the cell membranes in tenocytes
None of these, however, fully explain the pain processes and it is likely that a combination of these and other cortical factors that result in tendon pain 
Pain related to tendinopathy pain is multifaceted and requires a comprehensive multimodal evaluation and management plan
- Rio E, Moseley L, Purdam C, Samiric T, Kidgell D, Pearce AJ, Jaberzadeh S, Cook J. The pain of tendinopathy: physiological or pathophysiological?. Sports medicine. 2014 Jan 1;44(1):9-23.
- Rio E, Kidgell D, Moseley GL, Gaida J, Docking S, Purdam C, Cook J. Tendon neuroplastic training: changing the way we think about tendon rehabilitation: a narrative review. British journal of sports medicine. 2016 Feb 1;50(4):209-15.
- Cook JL, Rio E, Purdam CR, Docking SI. Revisiting the continuum model of tendon pathology: what is its merit in clinical practice and research?. Br J Sports Med. 2016 Oct 1;50(19):1187-91.
- Cook JL, Purdam CR. Is tendon pathology a continuum? A pathology model to explain the clinical presentation of load-induced tendinopathy. British journal of sports medicine. 2009 Jun 1;43(6):409-16.
- Docking SI, Cook J. Pathological tendons maintain sufficient aligned fibrillar structure on ultrasound tissue characterization (UTC). Scandinavian journal of medicine & science in sports. 2016 Jun;26(6):675-83.
- Rio E, Kidgell D, Moseley GL, Cook J. Elevated corticospinal excitability in patellar tendinopathy compared with other anterior knee pain or no pain. Scandinavian journal of medicine & science in sports. 2016 Sep;26(9):1072-9.