What causes tendonitis?

The one cause: load rising faster than the tendon can adapt

What causes tendonitis has a one-line answer and a longer list, and the honest page gives the line first because the list only makes sense as variations on it: tendon pain is caused by load exceeding the tendon's current capacity, and most often by load rising faster than the tendon can adapt to it. The mechanism is a mismatch of clocks. Muscle responds to training in weeks, strength climbing quickly through neural adaptation and then hypertrophy, and a lifter running a good programme adds weight to the bar month on month; tendon and ligament respond to the same training on a timescale of months, stiffening and thickening slowly as the collagen matrix remodels, the connective tissue adaptation literature's finding that the slow tissue lags the fast one. So the tendon that is loaded by a muscle three months stronger than it was is being asked for capacity it has not built, and the tissue's response to repeated load beyond capacity is the failed-healing change the biopsies show, disorganised collagen, increased ground substance, new vessels and nerves, and pain. That is the cause, and it explains the pattern every lifter recognises: tendon pain arrives after the good months, not the bad ones, after the block where the squat went up fast, after the return from a layoff when the muscle memory came back quicker than the tendon did, after the new sport or the new job that loaded a tendon in a way the training never had. It also explains why the established management is load and not rest, since the tendon's problem is capacity, and capacity is built by progressive loading over the months the tendon needs, guided by pain during and after the session rather than by avoiding it, the tendinopathy rehabilitation literature's core, while rest lowers the load without raising the capacity and leaves the gap intact for the return.

The accelerators, the thing that is not a cause, and where collagen sits

The accelerators are everything that widens the gap between load and capacity faster than a steady programme would, and they fall into training, body and drug groups. Training accelerators: a sudden jump in volume, frequency or intensity, the extra session, the new block, the return from holiday at the old weights; a new exercise or a deeper range that loads the tendon at an angle or length it has not adapted to, the first months of deep squats, of dips, of overhead work, of hill running; compression positions, in which the tendon is pressed against bone under load, the Achilles in deep dorsiflexion, the hamstring tendon at the sit bone in deep hip flexion, the rotator cuff overhead, since compression plus tension is harder on a tendon than tension alone; and poor recovery, short sleep and sessions spaced too closely for the slow tissue to remodel between them. Body accelerators: age, since tendon stiffens, loses water and adapts more slowly across the decades; the years after menopause, when the fall in oestrogen changes tendon behaviour and tendon problems become commoner; body weight, which is load on every lower-limb tendon at every step; and smoking, which impairs blood flow and healing across tendon, ligament, bone and skin. Drug accelerators, printed because they are documented and often missed: corticosteroids, oral or injected near a tendon, weaken tendon tissue; and the fluoroquinolone class of antibiotics carries a documented association with tendinopathy and rupture, most often the Achilles, higher in older adults and with steroids alongside, recognised in the regulator's safety communications, so a lifter who develops tendon pain during or after a course tells whoever prescribed it. Then the thing that is not a cause: a collagen shortfall in the diet. Tendon pain is a load and adaptation problem, not a nutritional deficiency, and no tendon problem in the literature is caused by eating too little collagen, so a reader who believes their tendonitis is a sign they need collagen has the causation backwards, and collagen's place is the one it holds on every tendon page of this site, an adjunct to the loading programme as research, around fifteen grams with roughly 50mg of vitamin C an hour before loading having doubled a collagen synthesis marker in a small trial, a marker result with no outcome and no claim, discussed with the physiotherapist and never bought instead of the assessment and the programme. The worked example is stated as that adjunct: two capsules, 1200mg hydrolysed type 1 marine collagen with 12mg vitamin C plus zinc, 120 for £16.99 at 28p a day, lab tested and UK made, a dose below the protocol's grams and taken for vitamin C's collagen formation sentence, fixing none of the causes above. What causes tendonitis: load outrunning a slow tissue, accelerated by jumps, ranges, compression, poor recovery, age, smoking and two drug classes, and not by the collagen aisle.

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