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Achilles tendon
The thickest and strongest tendon in the body, carrying several times body weight at every running stride.
Latin name: Tendo calcaneus. In plain English, the thick cord at the back of the ankle. It is filed here under joints, tendons and fascia.
Reference information, not a diagnosis. Where a symptom is described below, it is there so you can describe your own more precisely to a clinician — not so you can name what you have.
Where it is
The Achilles is the merged tendon of the two calf muscles, running down the back of the lower leg to the heel bone. Its fibres spiral as they descend, which is thought to help it store elastic energy. Between the tendon and the heel bone at its insertion sits the retrocalcaneal bursa; another bursa lies between the tendon and the skin.
The mid-portion of the tendon, roughly two to six centimetres above the heel, is where most tendinopathy and most ruptures occur. This region has a comparatively poor blood supply, and it is also the part that experiences the highest tensile and compressive loads during running.
| Feature | Detail |
|---|---|
| Formed by | The tendons of gastrocnemius and soleus merging above the ankle |
| Attachment | The posterior surface of the calcaneus, the heel bone |
| Blood supply | Relatively sparse in the mid-portion compared with the ends, which is one reason that region is slow to heal |
| Job | Transmits calf force to the heel, and stores and returns elastic energy during walking and running |
What it does
It is a spring as much as a cable. During running the tendon lengthens as the foot lands and recoils as you push off, returning a substantial part of the energy of each stride and reducing the work the muscles have to do. That elastic behaviour is why running feels so much more efficient than repeatedly hopping from a standstill.
Tendinopathy here is a load problem, not an inflammation problem in the classic sense. The tissue changes in structure under repeated overload, and it responds to progressive loading rather than to rest alone. Complete rest tends to make tendons weaker and more sensitive, which is why current management is built around graded strengthening supervised by a clinician.
Movements that load it
| Exercise | Category | Muscles the exercise library lists |
|---|---|---|
| Jump rope | Conditioning | Calves, shoulders, heart |
| Tempo running (12 km/h) | Cardio | Legs, heart, lungs |
| Easy running (8 km/h) | Cardio | Legs, heart, lungs |
| Stair climbing | Cardio | Quads, glutes, calves, heart |
| Brisk walking (5.5 km/h) | Cardio | Legs, heart |
Calf raises loaded heavily and slowly are the mainstay of Achilles rehabilitation, and hopping and skipping load the tendon elastically once it tolerates that. The key principle is that load has to increase gradually: most Achilles problems in runners follow a sudden change — more mileage, more speed work, hills, or a change in shoes that alters heel height.
Keeping its range
Calf stretches with a straight and bent knee reach the two muscles feeding the tendon. Be careful with insertional pain right at the heel bone: stretching into deep dorsiflexion compresses the tendon against the calcaneus and often aggravates it, which is one of the clearest examples of a stretch being the wrong tool.
When it goes wrong
| Problem | What it typically feels like | Usual setting |
|---|---|---|
| Mid-portion Achilles tendinopathy | Pain and stiffness in the tendon a few centimetres above the heel, worst on the first steps in the morning | A rapid increase in running volume or intensity |
| Insertional Achilles tendinopathy | Pain right where the tendon meets the heel bone, aggravated by stretching and by shoes pressing there | Compression at the insertion; managed differently from the mid-portion type |
| Achilles rupture | A sudden snap or the feeling of being kicked in the back of the ankle, with weakness pushing off | Often in recreational sport in mid-life; needs urgent assessment |
A sudden snap at the back of the ankle with an inability to push off or rise onto the toes needs same-day medical assessment. Ruptures are missed surprisingly often because people can still walk.
Nothing on this page is a treatment plan. If a problem is limiting what you do, is not improving, or is getting worse, the right next step is an assessment by a doctor or a physiotherapist who can examine you.
Sources: descriptive anatomy from Gray's Anatomy of the Human Body, 20th US edition (1918), which is in the public domain; Latin names follow Terminologia Anatomica, published by the Federative International Programme for Anatomical Terminology. Nerve root levels are the conventional ranges taught from those sources and vary between individuals and between textbooks. Exercise cross-links use the TempoLife exercise library. Checked 2026-09-06.
Frequently asked questions
Why does Achilles pain hurt most in the morning?
Morning stiffness that eases with a few minutes of walking is a hallmark of tendinopathy. It reflects how the tendon behaves after a long period without loading rather than any damage occurring overnight.
Should I rest a sore Achilles?
Complete rest tends not to help. Tendons respond to graded loading, and current management is built around progressive strengthening. What to do and how fast is a decision worth taking with a clinician rather than from a website.
Do certain antibiotics affect the Achilles?
Fluoroquinolone antibiotics carry a recognised warning about tendon problems including rupture, particularly in older people and those on corticosteroids. If you are on one and develop tendon pain, contact the prescriber rather than pushing through.
Nearby in this reference
Shoulder joint
The ball and socket at the top of the arm
Knee joint
The hinge between thigh and shin
Hip joint
The ball and socket where the leg meets the pelvis
Lumbar spine
The five vertebrae of the lower back
Plantar fascia
The thick band along the sole of the foot
Deltoid
The cap of the shoulder
All 34 body parts · Exercise library · Body map
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