Achilles Tendonitis: Why Your Heel Hurts When You Run

You lace up your shoes, take your first few steps out the door, and your heel feels stiff and sore, like the back of your ankle needs oiling. A mile in, it loosens up and you almost forget about it. Then you stop, cool down, and the ache comes roaring back — sometimes worse than when you started.

That pattern — stiff at the start, better mid-run, sore afterward — is the calling card of Achilles tendonitis, a common overuse injury in American runners. Rest it completely and it often comes back the moment you run again; push through the pain and an irritated tendon can turn into a partially torn one. Most cases heal without surgery, but on the tendon's schedule, not yours.

Meet the strongest tendon in your body

The Achilles tendon connects your calf muscles to your heel bone. It is the thickest, strongest tendon you have: every running stride sends forces several times your body weight through it — well over 1,000 pounds per step for a 180-pound runner, thousands of times per run.

Tendons heal more slowly than muscles because their blood supply is poorer — especially in the middle of the Achilles, about 2 to 6 centimeters above the heel, where most cases develop. That is a big reason recovery takes weeks or months, not days.

Doctors now usually call it Achilles tendinopathy, because in chronic cases the problem is not simple inflammation but degeneration: collagen fibers become disorganized, like a frayed rope, after repeated overload without enough recovery. That changes the treatment. Anti-inflammatory pills may dull pain, but they do not reorganize frayed collagen. Gradual, correct load does.

Two types, two different problems

Location changes the plan, so note where yours hurts:

Mid-portion tendinopathy causes pain and sometimes a tender, slightly swollen lump a few inches above the heel. This is the classic runner's version, driven by training load.

Insertional tendinopathy hurts right where the tendon meets the heel bone, often with a bony bump that shoes press on. Deep calf stretches that compress the tendon against the bone can aggravate this type, so confirm which type you have — a sports medicine doctor or physical therapist can tell in minutes — before starting rehab.

Why it happens: usually too much, too soon

Achilles tendonitis rarely comes from one bad step. It comes from a load the tendon was not ready for:

• A sudden jump in mileage or intensity. Adding hill repeats or jumping from 15 to 30 miles a week in one go is the most common trigger. The 10-percent rule — raising weekly mileage by no more than about 10 percent — exists largely because of injuries like this.
• A change of surface or shoes. Switching to trails, hills, minimalist shoes, or a lower heel drop changes how the tendon is loaded with every step.
• Tight and weak calves. Tight calves raise tendon strain; weak calves leave the tendon absorbing load the muscle should handle. Desk workers who sit all day then run hard in the evening get the worst of both.
• Age and body weight. Tendons lose elasticity with age, which is why cases cluster in runners in their 30s, 40s, and 50s, often people returning to running or training for a first marathon or Turkey Trot.
• Skipping rest days. Tendons adapt during recovery, not during the run. Back-to-back hard days deny the tendon its rebuilding window.

The rehab that actually works: progressive loading

For years the advice was rest, ice, and ibuprofen. For a degenerating tendon, complete rest is often counterproductive: an unloaded tendon gets weaker, so pain returns as soon as you run again. The modern approach, backed by decades of research including the Alfredson eccentric program, is progressive tendon loading — strengthening calf and tendon gradually until they can handle running again.

1. Calm it down first

In the painful early phase, cut running sharply or pause briefly, but keep moving with walking, cycling, or swimming. A small heel lift takes pressure off it, and ice for 10 to 15 minutes after activity eases soreness. Masking pain to keep running hard is how tendinopathy worsens.

2. Eccentric heel drops: the cornerstone exercise

The most studied rehab exercise is the eccentric heel drop: stand on a step with heels hanging off the edge, rise on both feet, then lower slowly — over about three seconds — on the sore leg alone. The lowering, not the lifting, is the treatment.

The classic protocol is 3 sets of 15 repetitions, twice a day, for about 12 weeks. Some soreness is expected; sharp pain is not. For insertional pain, do it on flat ground rather than off a step, so the tendon is not compressed against the bone. Most runners improve noticeably within 6 to 8 weeks.

3. Build strength, then running capacity

Once heel drops are comfortable, progress to seated and standing calf raises, single-leg balance work, and eventually hopping drills. Then return to running with a walk-run program on flat, soft surfaces, adding time gradually only if pain stays mild and settles within 24 hours. Soreness gone by the next morning means the load was right; pain lingering into the next day means you did too much.

4. Fix the setup

Replace worn-out shoes, be cautious with a sudden switch to low-drop or minimalist footwear, and reintroduce hills and speed work slowly. Gentle daily calf stretching helps, but aggressive stretching of an irritated insertional tendon can aggravate it.

What about the other treatments?

Shockwave therapy has reasonable support for chronic cases that have not responded to exercise, and night splints can help morning stiffness. Cortisone injections near the Achilles are generally discouraged because they can weaken the tendon and, rarely, contribute to rupture. None of these replaces progressive loading — at best, they add to it.

Red flags: when it is not just tendonitis

See a doctor promptly if you notice any of these:

• A sudden "pop" in the back of the ankle, followed by weakness pushing off or standing on tiptoe — a possible rupture needing urgent evaluation.
• Rapid swelling, redness, or fever around the ankle.
• Pain that does not improve after 3 months of proper rehab, or a lump that grows quickly.
• Numbness, foot color changes, or pain starting after a new medication, such as fluoroquinolone antibiotics, which raise tendon injury risk.

A tendon that has been painful and overloaded for months is more vulnerable than a healthy one — another reason to rehab properly instead of running through the pain.

The bottom line

Achilles tendonitis rewards patience and punishes impatience. It is not fixed by a week of total rest or by gritting your teeth through a training plan. It is fixed by gradually increasing load, over weeks and months, until the collagen remodels and the tendon is stronger than before.

If your heel aches at the start of every run, back off the mileage, start the heel drops, strengthen your calves, and rebuild on flat ground. Most runners who follow that plan are back to full training within 3 to 6 months — and they stay back.