What is the root cause of tendonitis?

The five whys, run on a sore patellar tendon

What is the root cause of tendonitis is a question that borrows a phrase from engineering and quality control, where root cause means the point in a chain of causes at which a fix would prevent the failure recurring rather than patching its symptom, and the honest page answers it with the engineer's own tool, the five whys, asked in sequence of a sore patellar tendon in a lifter until the chain stops producing new answers. Why does the tendon hurt? Because the tendon's matrix has disorganised: its collagen fibres have separated, its cells have proliferated and changed character, water and ground substance have accumulated, new vessels and nerves have grown in, and the cells and the ingrown nerves are signalling pain under load, which is the pathology the word tendonitis points at and which is better called tendinopathy because inflammation in the ordinary sense is a small part of it. Why has the matrix disorganised? Because the load placed on the tendon exceeded its capacity to adapt: every tendon has a capacity, the load it can carry repeatedly while remodelling its matrix in the positive window after each session, and when the load exceeds that capacity the breakdown half of the turnover curve outruns the synthesis half, session after session, and the matrix is degraded faster than it is rebuilt. Why did the load exceed the capacity? Because the load rose faster than a tendon adapts, and here the chain reaches the fact that explains most tendonitis: a tendon adapts to new load over months, through slow changes in its stiffness, cross-linking and cross-sectional area driven by sparse cells with a poor blood supply, while the muscle pulling on it adapts in weeks, so that a programme the muscle has grown into is still a programme the tendon is growing into, and the gap between the two is where the overload lives. Why did the load rise faster than the tendon adapts? Because something in the programme moved faster than months: weekly volume jumped, intensity rose, a new exercise or range arrived, jumping or sprinting work was added, training resumed after a lay-off at the old numbers with the muscle remembering them and the tendon having lost them; or because the tendon's capacity was lower than the lifter assumed, through age, which slows adaptation from the forties, through weak muscles around the joint that leave the tendon carrying load the quadriceps or calf should have shared, or through poor recovery, sleep and feeding. Why did the programme move faster than the tendon? Because it was written for the muscle, by a lifter or a coach who measured progress in the muscle's weeks and had no reason to think about the tendon's months until the tendon spoke. The chain stops here, because asking why again produces no new answer: the root cause of load-related tendonitis is a rate mismatch between the load applied and the tendon's rate of adaptation, and the fix that prevents recurrence is a programme that progresses at the tendon's pace, with heavy slow loading to raise capacity, increases small enough and spaced widely enough for the tendon to follow, and recovery days in which the positive half of the turnover curve runs.

The accelerators, the chain that ends in the body, and the link that is never there

The accelerators belong on the chain as modifiers rather than roots, because each makes the same mismatch injure sooner without being the mismatch itself. Age slows tendon adaptation and widens the gap. Smoking slows every connective tissue repair. Poor sleep and under-eating starve the positive window of the turnover curve. Fluoroquinolone antibiotics weaken tendon directly and carry a regulator's warning. A corticosteroid injection into the tendon weakens it. Body weight raises the load on every bodyweight movement. A lifter who removes the accelerators has not fixed the root but has bought the tendon time to adapt, and a lifter who fixes the programme and keeps smoking has fixed the root and left a brake on. In a minority of cases the five whys end somewhere else entirely, and the honest page marks where the chain forks. If the tendon pain is in two or more sites at once, or arrived with no change in load anywhere in a three-month look-back, or comes with swelling, warmth, morning stiffness lasting an hour or more, or pain at rest and at night, then the answer to why did the load exceed the capacity is not that the load rose but that the capacity fell, for a reason inside the body: raised blood sugar, including undiagnosed diabetes, which stiffens and weakens tendon through glycation; raised cholesterol; inflammatory arthritis or gout, which inflame tendon sheaths and attachments directly; the menopause transition, which changes tendon as it changes other collagen tissues; a recent fluoroquinolone course; or a statin started in the preceding months. That chain's root cause is a systemic driver, the fix is medical, and the person to run the whys is a doctor, who will find most of them treatable and will see the tendon pain settle when they are. The link that is never there, on either chain, is the one the shelf inserts: a collagen shortfall. At no why does the answer become because the tendon lacked collagen, since a fed body carrying ordinary protein has glycine, proline and lysine in surplus, the tendon's cells make collagen from that pool whenever the load signal tells them to, and the problem in tendinopathy is the signal outrunning the cell's capacity to respond, not the cell running short of material; the collagen pre-loading protocol is a timed-substrate adjunct beside the loading that fixes the root, with a marker trial and one small outcome trial, and no collagen claim exists for tendons in the UK. The worked example is stated on its true terms: two capsules, 1200mg hydrolysed type 1 marine collagen with 12mg vitamin C at 15% of its reference value plus zinc, 120 for £16.99 at 28p a day, lab tested and UK made, below the 2.5 to 15g research band, carrying vitamin C's collagen formation sentence for cartilage and bones and no tendon claim. What is the root cause of tendonitis: a programme that moved at the muscle's pace through a tendon that adapts at its own, or, in the minority, a body whose tendons a doctor should look at, and never a shortage of collagen.

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