Achilles Tendinopathy: How It Develops and the Loading Protocol That Works

Achilles pain runs on a script. Stiff for the first steps out of bed, easier once warm, louder again the next morning, and you keep training because it goes away by the second kilometer. That is a tendon short of capacity. Rest does not give capacity back. Twelve weeks of heavy, slow, progressive calf loading is the treatment with the best evidence behind it, and for most people it works.

What is going wrong inside the tendon

Every push-off runs through the Achilles. It connects the two calf muscles, gastrocnemius and soleus, to the heel bone, and pain sits in one of two places: the midportion, 2-6 cm above the heel bone, which is the runner's version, or the insertion, where tendon meets bone.

The old name was tendinitis. The "itis" was the problem. A chronically sore Achilles is not on fire; it is disorganized collagen, extra water-binding ground substance between the fibers, and small vessels and nerves growing in where they do not belong, which is why anti-inflammatories and two weeks on the couch keep disappointing people.

Cook and Purdam gave the process a shape with the continuum model of tendon pathology: a reactive tendon after a load spike, dysrepair when the spikes keep coming, degeneration when parts of the structure stop responding, and the early stages settle when load is managed. Degenerate parts may never look normal on a scan again. That matters less than it sounds. The healthy tissue around them carries the load once you train it.

One more fact sets expectations. Danish researchers dated the collagen in 28 human Achilles tendons with carbon-14 from nuclear bomb tests and found the tendon core is laid down during growth and essentially not renewed after about age 17, so you are not growing a new tendon in 12 weeks. What you change is how stiff the existing material is, how much force the muscle above it produces, and how the nervous system reads load through it.

Running loads this structure like nothing else: among 785 male former elite athletes in Finland, middle- and long-distance runners had 31 times the odds of Achilles tendinopathy before age 45 compared with matched controls. Those were elite careers, recalled at a median age of 69, so the number marks what years of repeated push-off can do rather than a recreational runner's odds.

How it develops: a spike the calf could not absorb

Start with the forces. When researchers measured Achilles loading in 11 adults averaging 74 kg, peak tendon force was 2.3 kN at 9 km/h and 2.6 kN at 12.6 km/h, around 3 to 3.5 times bodyweight per step, and it rises with speed. Those were measured directly, and the authors note they land below the 5 to 7 times bodyweight that modelled estimates report. At a cadence of 170 that is 85 loads per leg per minute. About 3,800 per Achilles in a 45-minute run. The cadence calculator turns your steps per minute into stride length, so you can see what a longer stride adds per step.

Most of that force comes from the soleus, the flat muscle under the gastrocnemius that works with the knee bent, and runners who do calf work at all tend to do straight-leg raises and leave the soleus untrained. Bent-knee calf strength is the gap in almost every case.

The injury is a mismatch: load rose faster than the tendon could adapt. The usual triggers:

• A hill block or a speed block after a flat, easy winter. Both raise force per step, and tendon adapts over months where lungs adapt over weeks. Tissue capacity is the quality nobody measures until it fails.
• A jump in weekly volume. Three runs a week to five is a two-thirds increase in tendon loads, and the reaction tends to arrive the next morning, not mid-run, which is why runners blame the wrong session.
• Lower-drop shoes. Less heel under you means more ankle range per stride and more Achilles load until the tendon catches up.
• A calf that was already weak. Among 566 healthy adults aged 20-81 doing single-leg heel rises to fatigue on a 10-degree incline, the median was 24 reps for men and 21 for women, so test both legs. As a rough rule, if the sore side quits ten reps before the good one, or short of 20, the calf is part of the story.
• Anything that shrinks the margin. Age. A previous episode, bad sleep, a stressful month. Fluoroquinolone antibiotics and cortisone injected near the tendon weaken it outright.

Why rest, stretching and quick fixes fail

Rest works on the symptom and against the cause. Three weeks off and the morning stiffness fades because the reactive tendon calmed down, but calf and tendon lost capacity while you waited, so the first hill back delivers the same load to a weaker system. That is the relapse loop. And coming back from injury means rebuilding what the rest took away.

Aggressive calf stretching is the second reflex, and for insertional cases it is a mistake: deep dorsiflexion compresses the tendon against the heel bone, the mechanism that hurt it. For midportion pain it does nothing for the tendon's ability to carry force.

Isometrics were sold as an instant painkiller after patellar tendon research showed 45-second heavy holds could dull pain. The Achilles did not cooperate. In a trial of 16 patients, five 45-second holds at 70% of max left pain unchanged in those who had any, 4.2 before and 4.9 after; a quasi-randomized trial in 91 patients found no immediate pain reduction on a hop test after isometric holds in either ankle position, isotonic work or rest; and Silbernagel's group is blunt: no current evidence supports isometrics as better than any other exercise for Achilles tendinopathy. Use holds as a starting load when everything else hurts and expect them to numb nothing. Shockwave, orthotics, taping and injections sit at the margins too. Cortisone near the Achilles adds rupture risk to a tendon already struggling.

Before you load anything: a sudden pop or a kicked-in-the-calf feeling, or an ankle that cannot push off, is a rupture pattern and needs emergency care the same day, not this protocol. Heat, swelling or night pain that does not settle needs a clinician too. This page describes tendinopathy, the gradual kind with morning stiffness. If your pain started in one moment rather than over weeks, get it looked at before you start.

The loading protocol that works

The evidence starts in Umeå in 1998. Håkan Alfredson took 15 recreational athletes with chronic Achilles tendinosis, all unable to run despite conventional treatment, through 12 weeks of heavy eccentric calf loading, and all 15 returned to full running at their pre-injury level. A comparison group of 15 treated conventionally all went to surgery. The dose is the famous part: 3 sets of 15 heel drops with a straight knee and 3 with a bent knee, twice a day, 180 reps a day, 7 days a week, with 5 kg added to a backpack each time the drops stop hurting. Non-disabling pain during the exercise is allowed.

Eighty-four days without a day off is a grind. So in 2015 a Copenhagen group tested a gym alternative: Beyer randomized 58 patients with midportion pain of more than three months to Alfredson's twice-daily drops or heavy slow resistance three times a week, and both groups improved the same at 12 weeks and at one year. Satisfaction at 12 weeks was 100% with heavy slow resistance against 80% with the drops; compliance was 92% against 78%. Their heavy slow protocol runs three lifts: seated calf raise, standing calf raise and heel raise on a leg press, 3 seconds up and 3 seconds down, from a 15-rep max in week 1 to a 6-rep max in weeks 9-12.

Why heavy? Because load magnitude is the active ingredient: a meta-analysis of 27 loading studies in healthy adults found tendon stiffness gains depended on how heavy the training was, splitting at 70% of maximum, and not on whether the work was eccentric, concentric or isometric (Bohm 2015). Eccentric-only work delivered heavy load. It was never a special stimulus.

Pick by logistics. With a gym, run heavy slow resistance, three sessions of about 25 minutes; with a staircase and no gym, run Alfredson off the bottom step. The protocol that fails is the one you quit in week four. A 1RM calculator turns a set of 12 into the 6-rep load you will need by week 9.

The 12-week protocol at a glance

Rows 1-6 are the default track, heavy slow resistance; row 7 replaces rows 2-6 if you have no gym, row 8 modifies either track when the pain sits at the bone, and rows 9-11 layer on top. RM is the heaviest load you can move for that many clean reps with the last one hard. "3 + 3 s" is three seconds up, three seconds down. No bounce.

Pain rules, red flags and the insertional exception

Pain-free tendon rehab does not exist, and chasing it is how people do nothing for six months. Karin Silbernagel's pain-monitoring model gives the numbers: on a 0-10 scale, 0-2 is safe, 3-5 is acceptable, above 5 is too much. Pain during and right after a session may reach 5, it should be back to baseline by the next morning, and it must not climb from one week to the next; in the protocols built on the model, load goes up when the previous step stayed at 5 or under. Break a rule and you step back one row for a week. You do not stop.

The rules were tested under real training. Silbernagel randomized 38 patients to keep running and jumping under the model or to drop all tendon-loading sport for six weeks, and at 12 months both groups had improved the same, with no sign the runners who kept going had paid for it.

One more thing needs a clinician rather than a table: no change after 12 weeks of honest loading, which means imaging and a sports physio before round two. The rupture and swelling red flags are above, before the protocol, where you meet them before you start.

Insertional pain runs the same protocol with one change. No compression. Anything that folds the ankle into deep dorsiflexion under load presses the tendon against the heel bone, so the heel never drops below the step, calf stretches are out, and uphill running waits. Jonsson tested this in 27 patients with 34 painful tendons who had hurt for an average of 26 months, the same 3 x 15 twice-daily eccentric dose stopped at floor level, and at four months 67% were satisfied and back to their previous activity. Common practice among clinicians is a heel lift of roughly 10-15 mm in both shoes for the first few weeks, which takes some compression out of every step. The heavy lifts stay. Only the range changes. This is the one place our own ankle mobility guide does not apply: chasing end-range dorsiflexion is right for a squat and wrong for a painful insertion, so park the mobility work on that side until the tendon settles, then rebuild the range.

Keep running, then rebuild the top end

The instinct is to stop until it stops hurting. The evidence says keep going inside the rules, flat, easy, shorter: pull hills, speed, plyometrics and the long run at once, hold total kilometers at or under what you ran before the flare, and apply the pain-monitoring model to every run. A weekly volume planner maps the 4-week block once kilometers go back in, with the down week already placed.

Rebuild the top end one variable at a time, in this order: easy volume, strides, hills, intervals, then plyometrics. New variable in, louder tendon the next morning: repeat that week.

Set the calendar honestly. A meta-analysis of 24 loading studies mapped the pace of it, gains in pain and function showing up as early as 2 weeks and peaking around the 12-week mark, about 21 points on the VISA-A, and Silbernagel's group writes that full recovery can take a year or longer. Five years after exercise-only treatment, 80% of patients had fully recovered and 20% still had symptoms. The patients most afraid of loading the tendon recovered the least calf function. Fear costs more than load does.

Then keep paying. Heavy calf work stays in your week after the pain is gone, one or two sessions of 3 x 6-8, seated and standing, alongside the rest of your strength work as a runner. The relapse pattern is predictable. The calf work gets dropped the month the tendon feels fine.

How Movement Rebels fits

Achilles rehab fails on logistics more than biology. The calf session is the first thing cut from a busy week. The hill loop sneaks back in week three because the tendon felt fine on Tuesday. The Movement Rebels coach carries the schedule: it reads your Garmin or Apple Health runs, pace, cadence and per-km splits, and the strength sessions you log in the calendar, so the three heavy calf sessions count toward your week's load instead of sitting outside the plan. Write "Achilles rehab, week 4, mornings 3/10" in your notes for the coach and rate soreness in the morning check-in. When soreness climbs two mornings running, the morning brief flags it, and one message in chat has the coach move Thursday to flat easy running and leave the calf lifts alone. Sunday's plan is built from the last 14 activities rather than the plan you wrote before the tendon complained.

Start with the week-0 row tonight. Three sets of 15, both feet, flat floor, three seconds down.