Tendon Pain: How the Size of the Load Decides What a Tendon Does

01Tendinosis and tendinopathy

A degenerated tendon and a painful tendon are two different findings

Tendinosis and tendinopathy answer different questions, and holding them apart is what makes tendon pain intelligible. Tendinosis is a statement about tissue: disorganized collagen and an altered matrix, present in plenty of people who feel nothing. Tendinopathy is a statement about a person, and pain is part of it.

The dissociation runs hard in both directions. Investigations do not always match symptoms, and painless tendons can be catastrophically degenerated (Rio 2014). Someone can also carry disabling tendon pain with ordinary imaging. Pain and tissue pathology are not synonyms.

What an abnormal scan actually predicts

An abnormal scan of a painless tendon is not meaningless. Pooled across prospective studies, structural abnormality on ultrasound carried a relative risk of 4.97 for developing symptoms later, with a confidence interval of 3.20 to 7.73 (McAuliffe 2016). The risk reached 7.33 for the Achilles and 4.35 for the patellar tendon.

A fivefold risk is still not a sentence. Most abnormal tendons in those cohorts never became painful, and the reviewers concluded that factors beyond tissue structure drive tendinopathy. Findings in People Without Pain carries the wider data.

Structure alone also understates what the tissue has done. Ultrasound tissue characterization of 91 participants found that pathological tendons held significantly more aligned fibrillar structure than normal tendons (Docking 2016). They had thickened around the disorganized region and added sound tissue to carry the load.

That is why a distressing image does not sentence anyone to a distressing future, and a clean image does not dismiss genuine pain.

02Findings

What the research shows

From two controlled loading trials, a strain-controlled training study and four systematic reviews.

03The load signature

Tendon pain runs on load rather than on a clock

Tendon pain has a signature that sets it apart from most musculoskeletal complaints, and recognizing that signature is where careful care begins. Achilles tendinopathy is defined by localized pain and functional impairment related to mechanical loading (Traweger 2025). It reaches about 6 percent of the general population and up to 50 percent of elite endurance runners.

The pain sits directly over the tendon, is reproducible with palpation, and travels with a sense of weakness or giving way. It clusters around energy storage and release activities such as sprinting and jumping. It is rare at rest. Tendon pain is localized, persistent and specifically associated with tendon loading (Rio 2014).

The pattern across a day, not the moment of pain

Most tellingly, tendon pain often eases during activity after a warm-up, then flares several hours later rather than during the effort itself. Inflamed joints, discs and muscles tend to ache continuously, even when still. Tendons come and go, running on load rather than on a clock.

Pure tissue injury cannot fully explain a pain that softens the more you use the part and sharpens once you have stopped. That behavior is a clue, not a threat. Tendon pain is a report on load, not a live feed of tearing fibers.

Irritability is the clinical name for that behavior: how much load provokes symptoms, and how long they take to settle. Cook and Purdam built their model around it, noting that people present with varying degrees of pain, irritability and capacity to function (Cook 2009). A loading session is therefore judged over the following 24 hours, and Hurt Is Not Harm carries the trial evidence for that approach.

04The continuum

The continuum model explains why two painful tendons need different loads

Cook and Purdam proposed a continuum of tendon pathology because the available descriptions had run out of explanatory room. Overuse tendinopathy had been called degenerative or called failed healing. Neither account explained why some tendons recover with simple measures while others resist every treatment tried (Cook 2009).

The continuum replaced a single endpoint with a position. A tendon sits somewhere along a range of disorganization, and treatment can be placed rationally against where it sits rather than against a label. A swollen, highly reactive tendon and a long-standing degenerative one need different amounts of load.

The tissue changes track that progression. Proteoglycan accumulation, fluid accumulation with swelling and hypervascularization are the characteristic findings, and disorganization advances as altered cellular function drives the change in matrix structure (Traweger 2025).

The cell that reads the load

A tendon is not a passive rope. Tenocytes sit within an extracellular matrix of collagen fibers, proteoglycans and signaling molecules, and they sense mechanical strain continuously. In cultured human tendon cells, mechanical loading significantly raised messenger RNA for substance P, a peptide involved in nociception (Backman 2011).

Substance P then acted back on the cells that made it. Through the neurokinin-1 receptor it raised cell metabolism, viability and proliferation (Backman 2011). The tissue that carries the load also writes the chemistry that reports on it.

Reviewing the field, Rio and colleagues placed tenocytes, mechanosensitive and chemosensitive receptors, ion channels and central load and threat monitoring at the center of tendon pain (Rio 2014). The brain is continuously estimating current tendon load against current tendon capacity.

05Where the pain sits

Tendon pain stays focal, and the nervous system still amplifies it

Tendon pain usually stays pinpoint precise. The complaint localizes to the tendon and often to one part of it, and localized pain related to loading is the defining clinical feature (Traweger 2025). This sets tendon pain apart from complaints that broaden and blur as they persist.

The precision does not mean the nervous system stays out of it. A systematic review of 16 studies in persistent rotator cuff, lateral elbow and patellar tendinopathy found lowered pressure pain thresholds over the painful tendon and at sites remote from it (Plinsinga 2015). Remote lowering points to sensitization of the nervous system rather than of the tendon.

No qualifying Achilles studies existed at the time of that search, and the authors caution against carrying an upper-limb finding straight down to the lower limb. Central Sensitization sets out the mechanism in full.

A focal complaint and an amplified system can coexist, and that combination changes the starting dose. A sensitized person needs a smaller first load, because the same kilogram is read through a louder amplifier.

06Dose in strain units

A tendon adapts to the size of the load, not the style of the exercise

Tendon is the tissue where dose can be measured in its own units, and the strain experiments settle the question. Arampatzis and colleagues trained 11 men for 14 weeks, four days a week. One leg worked at 2.97 percent tendon strain and the other at 4.72 percent, at matched frequency and matched volume (Arampatzis 2010).

Only the high-strain leg changed. It gained tendon stiffness and elastic modulus, and its strain at a given force fell. The low-strain leg did the same work and gained nothing. The authors concluded that strain must exceed what habitual activity already delivers before adaptation is triggered at all.

The meta-analysis and the clinic agree

Pooling 27 studies, 37 interventions and 264 healthy participants, tendon stiffness rose with a standardized mean difference of 0.70 and cross-sectional area with 0.24. Stiffness adaptation depended significantly on loading intensity and did not depend on muscle contraction type (Bohm 2015).

The clinical trials reach the same verdict from the other side. In 58 patients with chronic midportion Achilles tendinopathy, one group was randomized to eccentric training and one to heavy slow resistance. Both improved on function and pain, and both held at 52 weeks (Beyer 2015).

What did differ was how people got on with the work. Session compliance was 92 percent for heavy slow resistance and 78 percent for eccentric training. A dose nobody completes is not a dose.

07When the dose is wrong

Too little load and the wrong kind of relief both leave a tendon sore

A loading program that does not match the tendon in front of it fails at a measurable rate. Isolated eccentric training became the dominant conservative strategy, and up to 45 percent of patients may not respond to it (Malliaras 2013). Across 32 studies the reviewers found little evidence for isolating the eccentric component.

The relief that removes the signal behaves differently from the load that raises capacity. In 39 men with patellar tendinopathy given corticosteroid injection, eccentric decline squats or heavy slow resistance, all three arms improved across 12 weeks (Kongsgaard 2009). By the half-year mark the injection group had deteriorated while the loading groups held.

The injection had changed the tissue all the same, cutting tendon swelling by 13 percent and vascularization by 52 percent. Heavy slow resistance cut swelling by 12 percent and vascularization by 45 percent, and it alone raised collagen network turnover.

Why imaging is a poor progress report

Change on the scan is not the thing being treated. Across the Achilles studies reviewed, most found no association between improved imaging and improved symptoms, including every high-quality study (Malliaras 2013). The one mechanism consistently linked to clinical improvement in both tendons was improved neuromuscular performance.

That is the argument against a plan built on stretching alone, isolated strengthening alone, or the radiology report. The body often needs measured challenge rather than endless protection.

08Claims removed from this page

Three claims from the earlier version were removed

The statement that nerve endings in tendon carry very small receptive fields, offered as the reason tendon pain stays focal, could not be matched to a published source and came off. So did the claim that tendinopathy rarely widens into diffuse pain, because pressure pain thresholds fall at sites remote from the tendon (Plinsinga 2015).

Lactic acid accumulation, listed among the peripheral mediators, came off for the same reason as the receptive fields. The neovascularization claim survived and now carries numbers. Vascularity fell by 45 to 52 percent in the patellar trial (Kongsgaard 2009), and in the Achilles that change does not track how a person feels.

09Graded loading first

Graded loading is the intelligent first step for a painful tendon

Because structural severity and pain severity are only loosely related, the useful assessment runs on two tracks at once. The peripheral track covers tissue loading, current capacity and mechanical adaptation. The central track covers threat perception, motor control, injury history and what a person believes about their pain.

Progressive loading builds tendon capacity much as increasing weights in a gym build adaptation, and the strain data give that statement its units: 4.72 percent worked and 2.97 percent did not (Arampatzis 2010). Treatment is often protracted and failure rates are high (Traweger 2025), which argues for getting the dose right.

Conservative first is the intelligent choice in either direction. Either graded loading restores capacity and spares someone a life altering intervention, or it does not, and the worse possibilities have been ruled out carefully before anything invasive is considered. When Conservative Care Stops sets the thresholds for escalating.

The general rule stated here carries into the sites where it gets complicated. Lateral Hip Pain Reconsidered takes the gluteal tendons, where compression at the greater trochanter changes what counts as a safe load. Proximal Hamstring Pain takes the ischial attachment and the graded return that follows.

Confidence, not fear, is the therapeutic ingredient, and tendons reward it.

10The model on tendon pain

What the Unified Model of Tone claims about tendon pain

Everything above is established science, including the trial that found no difference between two loading programs. What follows is our model's reading, stated as ours rather than drawn from the papers cited.

Tone is the integrated organization through which the body’s interacting processes relate to one another at a given moment. Our model states the dosing rule directly. Too little input is not registered, matched input is integrated, and excessive input becomes defense, noise or damage. The correct dose is what the system can use to enter a more adaptive state.

Tendon is where that rule stops being a metaphor. Identical volume delivered to two legs of the same men produced adaptation at 4.72 percent strain and nothing at 2.97 percent (Arampatzis 2010). The threshold sat above what daily walking already supplied.

Why the loading trials keep tying

The same rule reads the null results without embarrassment. Eccentric training and heavy slow resistance tied at 52 weeks in 58 Achilles patients (Beyer 2015). Pooled adaptation depended on loading intensity rather than contraction type (Bohm 2015). Two literatures, one answer: the dose is the input, and the exercise is only its delivery.

The 45 percent who do not respond to isolated eccentric loading read the same way (Malliaras 2013). A trial that gives one predetermined protocol to everyone averages a well-matched dose against a mismatched one, in a sample never sorted by capacity. Our model expects that average to land near zero, and it does.

The prediction

From that follows a claim the tendon literature does not yet make. The most informative variable is not the starting number but the ability to change appropriately and return. In flexible adaptation the tendon meets a demand, changes state, resolves it, and returns with more capacity than it began with.

So we predict that four measures recorded together in the same people will share one underlying factor. The four are decline squat pain, tendon stiffness under load, pressure pain threshold at a site away from the painful tendon, and time to return to baseline after a fixed loading bout.

We predict the direction of change as well. Under a dose matched to capacity, people who begin stiff and people who begin lax both move toward the middle, and the spread narrows. This is a claim about how tendon pain is organized rather than a claim about what treatment does.

If decline squat pain, tendon stiffness under load, remote pressure pain threshold and time to return to baseline after a loading bout are shown to move together, the unification claim is confirmed.

11The tone reading

How tendon pain expresses tone

Every topic in this library expresses all of tone. In tendon pain three aspects carry the signature, because the same tissue can be measured in strain, in stiffness and in pressure pain threshold at the same time.

The remaining foundations run through tendon pain as well. Set point: habitual activity sets the strain a tendon treats as ordinary, and adaptation begins only above it. Constraint: guarding narrows the movements available, and the unloaded tendon loses the capacity it was protecting. Prediction: the brain weighs the next repetition against remembered capacity, which is why pain often precedes the effort. Input quality: three seconds under load built more tendon than one second at equal volume, so the shape of the signal counts. Coupling: tendon stiffness, muscle strength and movement strategy shift together, which is why neuromuscular performance tracked recovery. These are readings of one organization rather than separate systems, which is the core claim of the Unified Model of Tone.

12Across the library

How this page relates to the rest of the library

The dosing rule stated here is the general case, and the pages around it take the sites where it gets harder.

13Frequently asked

Questions patients ask about tendon pain

14The sources

References

12 primary sources, each linked to its record. Figures quoted on this page were checked against the published abstract.