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Anatomy of the Achilles Tendon

Achilles Tendon- Interesting Facts

It is the longest and thickest tendon in the body.
Taller and heavier people have thicker tendons than their smaller contemporaries.
The tendon is significantly bigger in your dominant leg if you play sports regularly.
It gets bigger as you get towards 50 years old.

Where is the Achilles tendon?

Well it’s the long cord that attaches your calf muscles to the heel at the back of your lower leg.

It comes in all shapes and sizes – from long and thin to short and thick but they all do the same job – transfer forces from the calf muscles to the foot and vice versa.
I say muscles because you have two muscle groups in the calf and they both perform slightly differing functions.

Gastrocnemius Muscle

Gastrocenmius muscle gives the calf its distinct shape.
This muscle itself has two distinct parts (heads) to it medial and lateral.
These heads are attached to either side of the back of the knee and actually cross the knee joint to be attached to the lower end of the thigh bone (femur) by strong flat tendons.
The medial head is larger than the lateral which can sometimes be absent although I don’t think I’ve ever seen it. 
The terms medial and lateral are explained in Medical Terms Explained.

Soleus Muscle

The two heads come together about half way down the lower leg and blend with a sheet of fibrous tissue (aponeurosis) which joins onto the tendon of the third muscle –Soleus.
Soleus is attached to the upper ends of two lower leg bones namely the tibia and fibula and so is deep to the gastrocnemius muscle.
From here its fibres pass posteriorly to its tendon which blends with the gastrocnemius and other fibres extend further down the leg to attach directly to the Achilles tendon almost to its lower end.

Achilles tendon (or more correctly tendocalcaneus) is the thickest and strongest human tendon.
It begins near the middle of the lower leg and reaches up to 15cm long before attaching to the heel bone (calcaneus).

This diagram shows the back of the leg with the gastrocnemius muscle cut away to show the deeper Soleus muscle in its entirety.

The Gastrocnemius muscle sits over the upper part of the Soleus.
There is another muscle which is small and this called Plantaris (this can be absent in many people).
Plantaris is not shown in the diagrams but it starts on the lateral side of the femur and passes obliquely down the leg to attach to the medial side of the Achilles tendon or directly onto the medial aspect of the heel bone (calcaneus).

Structure Of Achilles Tendon

The tendon consists of bundles of connective tissue fibres called type-I collagen fibres.
Collagen is the main structural protein in the body which provides the strength to the tendon. 
Type-I Collagen is present in bone, tendons, cartilage and scar tissue and is a very strong protein when it is linked to adjacent protein molecules.
ankle MRI
The tendon ranges in thickness between 4mm to over 7mm. Taller people have thicker tendons than shorter people.
There are differences between the average thickness of the Japanese population compared to western populations.
Heavier people have tendons with an increase in cross-sectional area (slightly thicker and wider) to help deal the the increased forces in walking and running.
Take a look at the MRI photograph on the right. I will explain what you are looking at.
Imagine cutting through the right ankle joint parallel to the floor and lifting the leg and upper part of the ankle away.
you would be looking down on the lower part of the sub-talar joint – the Calcaneum bone.
At the back of the heel you see the black structure – this is the Achilles tendon.

The tendon has a high tensile strength and is able to stretch up to 4% without any damage to the structure of the fibres. However if the stretch is 8% then the tendon ruptures.

During running the Achilles tendon can sustain peak forces of around 6 to 8 times bodyweight and it can apparently handle forces up to 12.5 times bodyweight.
By my calculations this is over a ton for someone weighing just under 13 stones!!

The fibres of the tendon twist by anything from 30° to 150° prior to its insertion in the Calcaneum, with most of the rotation occurring in the last 5-6 cm.
This rotation is thought to aid in elastic recoil of the tendon which allows it to store energy and then release it back to the foot to aid walking and running. Imagine pulling on both ends of a length of thick wool or yarn. As you pull there is some unwinding of the loose spiral in the wool and it lengthens slightly. When you release your pull is springs back to its original length. Obviously the collagen fibres in the Achilles are not as loosely arranged but you can now perhaps understand how this phenomenon works.
This ‘untwisting’ of the tendon allows greater instantaneous muscle power and speed of reaction than could be generated by the calf muscles acting alone.

paratenon stripping

The tendon is covered by a tissue known as the paratenon. This is a sheath of synovial cells which produce a kind of lubricant. The inner layer is in contact with the tendon itself, and the outer layer is continuous with the other surrounding tissues and their rich blood supply.
See picure on the left from which shows how the inflamed parartenon can be stripped away from the underlying tendon.
The paratenon can become inflamed and enlarged, usually in runners because of the repetitive nature of running with each foot hitting the ground the same way every stride 300-400 times a mile. This thickening in the paratenon produces the ‘creaking’ sensation of crepitus which can be felt over a swelling in the tendon. This swelling can be from the paratenon or from degenerative changes in the tendon itself.

This degeneration and resultant thickening is why the tendon has a greater cross-sectional area in the over 50′s.
There is also a response to training in the tendon because it has been shown that the tendons of the dominant leg in sportspeople have a significantly greater cross-sectional area than people who do not take part in sport.

The blood supply to the Achilles tendon comes mainly from vessels derived from the posterior tibial artery around where the muscles join up with the tendon. Branches from this area pass down the tendon and through the paratenon to supply the tendon itself. There is also a blood supply which supplies the tendon as it inserts into the Calcaneum and branches pass upwards from here. Where these two supplies meet is known as the “watershed zone”. This is an area 2-6 cm proximal to the Calcaneum, in which the blood supply is less abundant. This is the region where most degeneration and therefore rupture of the Achilles tendon occurs. This ‘dodgy’ blood supply gets even sparser with age which is why age is a factor in Achilles tendon ruptures.

You now know:-

  • a fairly detailed anatomy of the Tendo-Achilles
  • how it links the calf muscles to the Calcaneum.
  • how the tendon is constructed
  • how its’ spiral construction aids propulsion
  • its’ role in transferring forces from the foot to the leg and vice versa.
  • blood supply to the Achilles
  • what the ‘watershed area’ is
  • significance of the ‘watershed area’ in degeneration

Other Achilles Related Posts

Achilles Tendon Rupture
Surgical Repair of Achilles Tendon Rupture
Ruptured Achilles Tendon and Non-Surgical Treatment
David Beckham's Achilles Tendon Rupture
Calf Exercises - Achilles Tendon Stretches
Function of the Achilles Heel Tendon

6 Responses to “Anatomy of the Achilles Tendon”

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  4. Roger Werne Says:

    I have recently had achilles tendon surgery to remove a nodule of scare tissue from within my right achilles tendon. I am 66 years old and compete in masters track and field, the pole vault. The scar tissure resulted from many years of repeated injury and rehab. to the tendon. I am also a PhD structural engineer with a specialty in solid mechanics and I began to study the achilles tendon as a structural element. It appears from my reading that most injuries occur in the narrow part of the tendon about 5 cm above the Calcaneal Tuberosity because this is also where the tendon is narrowist and where I suspect the stresses are highest. Since the tendon structure is orthotropic i.e like a piece of wood, strong parallel to the grain and weak normal to it, the details of the “grain structure” of the tendon are very important in understanding how the tendon actually carries load in this narrowed region. If the tendon fibers were all parallel in this region then the flaring of the tendon above and below would likely cause tensile stresses to occur normal to the fibers. I might then argue that if the tensile stresses normal to the fiber are large enough and the material between the fiber weak enough under repeated stress, i.e. fatigue loading, then the fiber might separate causing a small lesion which could induce the growth of scar tissue if it did not heal properly. Once scar tissue exists in the region it creates a stress concentration which further diminishes the strength of the area and can accelerate the injury process upon further loading. Having said all of that, it is my understanding the the tendon fibers in this narrow region may not be parallel but spiraled as they progress from the Calcaneal Tuberosity upward. Where can I find a detailed 3D topology of how the achilles tendon fibers are oriented as they progress from the Calcaneal Tuberosity to the Gastrocnemius muscle? My thesis is that repetitive stresses normal to the tendon fibers that are caused by the geometrical layout of the tendon fibers can cause lesions and scar tissue that can lead to a chromic condition. Thanks, Roger Werne, PhD

  5. Coach outlet Says:

    Hi, many thanks so a lot for these ideas!…

  6. Ian Constable Says:

    @Roger Werne
    Hi Roger thanks for the interesting comment. I have no knowledge of detailed 3D topology but will keep my eye out. An interesting article in the American Journal of Roentgenology may be of interest to you. It uses MRI to study degenerative changes in Tendons and has pictures detailing the variations in lesions that they found in the tendons. Here is the link

    Please keep me up-to-date with anything that you uncover. Ian

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