Achilles tendon

Function

How the tendon joins muscle to heel

The Achilles tendon links the calf muscles to the calcaneus (heel bone)^1,2. A tendon is a tough cord of tissue that passes force from a working muscle to bone, and that pull makes the body move³. The Achilles is described as the body’s biggest and strongest tendon⁹. The muscles at the back of the lower leg mainly produce plantar flexion, which means pointing the foot down¹.

Tendons are made of a kind of connective tissue called dense regular connective tissue¹. In it, strands of collagen, a strong rope-like protein, are packed side by side in bundles¹. That layout makes the tissue strong under tension and hard to stretch along the fibres¹.

How three strands make one tendon

The tendon is built from three strands of fibre, each from a different part of the calf muscles². The gastrocnemius, the calf muscle nearest the skin, has two parts called heads, inner and outer, that both run from thigh bone to heel bone^2,10,11. Fibres come from the soleus, a flat muscle lying deeper, and from both heads of the gastrocnemius^1,2. Each strand is called a subtendon². In 106 legs from 53 male donors, the three subtendons made up these shares of the tendon at the heel²:

Subtendon from	Share of the tendon at the heel
Outer gastrocnemius head	About 44%2
Inner gastrocnemius head	About 28%2
Soleus	About 28%2

On the way down to the heel, the subtendons wind around each other². One meta-analysis put the average twist at about 46.5 degrees¹². The amount of twist varied from one person to the next¹².

How the tendon stores and returns energy

The tendon also acts like a spring, storing energy and giving it back^4,5. In one study, the tendon gave back 7.8 to 11.3 joules of stored elastic energy during push-off in treadmill walking and running⁴. In ten men hopping on one leg, the tendon’s recoil returned about 38 joules per hop⁵. That was around 16% of the total work of each hop⁵. In the same hopping study, Achilles stiffness, meaning the force it takes to stretch the tendon by a set amount, varied between the ten men⁵.

How load changes with the task

One study of 11 people estimated peak tendon load in kilonewtons⁴:

Task	Estimated peak tendon load
Walking	About 2.0 kilonewtons4
Running slowly	About 2.3 kilonewtons4
Running fast	About 2.6 kilonewtons4

In this study, peak stretch went from about 4.0 to 4.9% of the tendon’s length⁴. Speed had a significant effect on force⁴.

A study of eight healthy young adults estimated it in multiples of body weight⁶:

Task	Estimated peak tendon load
Seated heel raise	About half of body weight6
Forward hop on one leg	7.3 times body weight6

Breakdown

What tendinopathy is and where it sits

The Dutch guideline on Achilles tendinopathy states that, by current consensus, tendinopathy means local pain in a tendon that comes with activities that load it¹³. The guideline groups Achilles tendinopathy by where it sits¹³. Insertional tendinopathy lies within 2 cm of the heel attachment¹³. Midportion tendinopathy lies higher up, more than 2 cm above it¹³.

Who develops tendinopathy

In one cohort study of 3,379 recreational runners, 4.2% developed Achilles tendinopathy¹⁴. Among the new cases, about 64% were midportion and 28% insertional¹⁴. The rest involved both regions¹⁴.

In 2009, a Dutch study counted how many of 57,725 registered patients saw their family doctor for midportion Achilles problems¹⁵. The rate was 1.85 per 1,000 registered patients¹⁵. Among adults aged 21 to 60, the figure was 2.35 per 1,000¹⁵.

Another study looked at 58 patients with Achilles tendon pain who met criteria for surgery¹⁶. Of these, 31% had no direct link to sport or vigorous physical activity¹⁶.

What raises the odds is not well settled

One systematic review found limited evidence for nine factors that may raise the odds of tendinopathy⁹. These included earlier tendinopathy or a leg fracture⁹. Another was weaker strength in the muscles that point the foot down, tested at a fixed speed⁹. Twenty-six other suggested factors showed no link, including being overweight, foot posture when standing still, and activity level⁹.

In the runner study, tendinopathy in the past year was the strongest factor linked to a new case¹⁴. Its odds ratio, how many times higher the odds of an outcome are in one group than another, was near 6.5¹⁴.

How often the tendon ruptures

An acute rupture is a sudden tearing of the tendon^17,18. Sudden tears may be grouped as a range: tiny tears, partial tears where the tendon is partly broken, interstitial tears inside the tendon, and complete ruptures^17,18. A meta-analysis pooled 28 studies of rupture based on registries or representative samples⁷. Overall, it found 15.7 ruptures per 100,000 person-years, which could be 100,000 people followed for one year⁷. Rupture is not spread evenly across people or over time⁷:

Group	What the pooled studies found
Over time	6.1 in 1979 and 31.1 by 2021, an average rise of about 2.7% a year7
By sex	Males had about three times the rate of females7
By age	Rates peaked at ages 30 to 49 in males and 40 to 49 in females7
By activity	About 68% of ruptures were linked to sport7

Recovery

How healthy tendon adapts to load

Healthy tendon adapts to repeated load¹⁹. A meta-analysis of exercise studies in healthy adults aged 18 to 50, each lasting at least eight weeks, found that loading made tendons stiffer¹⁹. Their Young’s modulus, the stiffness of the material itself, also rose¹⁹. Cross-sectional area, the size of a slice cut straight across the tendon, grew by a smaller amount¹⁹. Gains in stiffness depended on how heavy the load was, not on the type of muscle contraction¹⁹.

Most of those programmes ran for 8 to 14 weeks¹⁹. Programmes of 12 weeks or longer showed larger effects, though the difference was not statistically significant¹⁹.

What happens when load is taken away

A review concluded that keeping a limb still lowers the tendon’s mechanical properties in animals, and that human data largely mirror this³. Even one to two weeks of unloading a limb seems to reduce tendon stiffness, without changing its thickness³. In one study of five healthy people, one leg was kept off the ground for four weeks²⁰. Afterwards, that leg’s Achilles tendon had a Young’s modulus about 10% lower²⁰.

Even one to two weeks of unloading a limb seems to reduce tendon stiffness, without changing its thickness.

Why tendon heals slowly

A review describes tendon as tissue with a poor blood supply that heals poorly compared with bone or skin⁸. Carbon dating of Achilles samples suggested the tendon core forms while a person is growing taller^3,21. After that, the core appeared to be essentially not replaced, unlike muscle, which is renewed continuously^3,21.

After a sudden tendon injury, healing is described in three stages in a row²²:

1. Inflammation²².
2. A stage of new tissue growth²².
3. A remodelling stage²².

A review of tendon healing states that healed tendon does not get back its original mechanical properties²².

A review of tendon healing states that healed tendon does not get back its original mechanical properties.

How long the effects last

In one study, on average 14 years after surgical repair, the tendon was about 12 mm, or 6%, longer than on the other leg²³. In that study, a longer tendon went with smaller calf muscles and lasting loss of strength²³. In another study, calf strength and endurance deficits remained seven years after rupture²⁴. Apart from heel-rise height, calf performance did not improve further after the two-year mark²⁴.

The Dutch guideline says recovery from tendinopathy can also be slow, taking months to years¹³. It states that most people recover, but symptoms may persist at least up to ten years in 23 to 37%¹³.

Connections

The calf muscles it serves

The Achilles tendon joins the soleus and both heads of the gastrocnemius to the heel^1,2. Together these three are called the triceps surae, and they do most of the work of pointing the foot down¹¹. In 48 healthy people pushing the foot down as hard as possible, none of the three knee angles tested singled out one calf muscle alone²⁵.

What studies of the calf raise show

The calf raise is one exercise that loads this tendon⁶.

• In one study of resistance-trained men, the authors suggested that calf raises can recruit the soleus and both gastrocnemius heads¹¹.
• In its 22 trained men, activation of these muscles did not differ significantly between the calf raise and the leg press¹¹.
• A separate study of 33 people looked at a standing heel raise with the toes turned out¹⁰. The inner head of the gastrocnemius was more active than the outer head¹⁰.
• A narrative review found support for the calf-raise test as a way to assess the tendon and calf muscles as one unit²⁶.

Other muscles that work in the heel raise

Other lower-leg muscles take part in the heel raise too²⁷. The tibialis posterior and flexor digitorum longus are two of the four deep muscles at the back of the leg¹. They lie beneath the calf muscles that end in the Achilles tendon¹. The fibularis (peroneus) longus instead is one of two muscles on the outer side of the leg, with the fibularis brevis¹. Within the front and back muscle groups of the lower leg, the muscles toward either side turn the foot in or out and rotate it¹.

In one study of 14 healthy men, these muscles’ activity in heel raises differed between the foot straight, turned in and turned out²⁷:

Muscle	Where it lies	Most active in heel raises with the foot
Tibialis posterior	Deep, back of the leg	Turned in 30 degrees1,27
Flexor digitorum longus	Deep, back of the leg	Turned in 30 degrees1,27
Fibularis longus	Outer side of the leg	Turned out 30 degrees1,27

How the tendon links to the arch of the foot

The plantar fascia is a band of tissue in the sole of the foot that runs from the heel bone to the toes^1,28. It is a main support of the foot’s long arch^1,28. The Achilles tendon and plantar fascia both help spread load through the foot, absorb shock and hold up the arch during push-off²⁸.

One study measured 19 feet from donors²⁸. The Achilles tendon’s cross-sectional area at the heel went strongly with that of the plantar fascia along its heel attachment²⁸. The authors read this as a sign of a working link between the two²⁸. The same study could not confirm full fibre continuity between the two²⁸.

A small muscle beside it

The plantaris is a calf muscle that runs at a slant between the gastrocnemius and the soleus¹. Some people have two, and no plantaris was found in about 7% of donor bodies¹. In a review of 200 ankle MRI (magnetic resonance imaging) scans, its tendon was directly related to the Achilles attachment in 42.5% of cases²⁹.

Sources

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Every source the library cites

Changes

• October 22, 2026 Links to the new Achilles tendon rupture page
• September 29, 2026 Revised: study sizes and populations now stated in the key points and load section; linked to the calf raise page.
• September 28, 2026 Revised. Restructured for easier reading (key points, subheadings, tables, a figure and a chart), and several sentences narrowed to match their sources more exactly.
• September 28, 2026 First published.