WHAT IS THE ACHILLES TENDON?

Achilles tendon is heel cord, also known as the calcaneal tendon. The Achilles tendon connects the calf muscles to the calcaneal tuberosity. However, its structure is more complicated than it seems. Interestingly, the Achilles tendon is the thickest and strongest tendon in your body, and at the same time it is at the highest risk of injury. Such a paradox.

Marble sculpture of Achilles dying, created by Filippo Albacini in 1825. The statue depicts Achilles reclining on a rock, with an arrow piercing his heel, symbolizing his only vulnerability. His face shows agony, and he wears a classical Greek-style helmet. The sculpture is displayed in a dimly lit museum setting, with paintings in the background.

MYTHOLOGICAL ROOTS

Perhaps you are wondering why the tendon is called a name of a mythical hero? There are two versions available for information. The first of them says that Achilles was immersed in the waters of Styx as a child by his mother - the sea goddess, the nymph Tethys. Thanks to this, his body was to be immortal and resistant to any blows. Unfortunately, the heel that his mother held him by during the bath was not submerged, which made its his weakest point. The second version says that during the Trojan War, Achilles was stabbed in the heel by a poisoned arrow of Paris and was thus killed.

STRUCTURE

The Achilles tendon, though seemingly simple, is one of the most anatomically intriguing tendons. It averages 15 cm in length and up to 2.5 cm in diameter in cross section. Its shape and thickness are not uniform: higher up, it is flat, thin, and wide, then it transitions into an oval and narrow shape (about 4 cm above its attachment to the calcaneus), before widening and flattening again at its insertion. This attachment point can vary in height and surface area, making it a unique characteristic of each individual.
The tendon is composed of collagen fibers, which are not highly elastic and not very stretchy. It has relatively poor blood supply and is poorly innervated at certain levels, which contributes to its long healing time, especially in cases of injuries like ruptures.

It consists of the terminal fibers of the gastrocnemius muscle and the soleus muscle, which lies slightly deeper than the former. These two muscles together form the triceps surae. In 93% of cases, the plantaris muscle, running along the lateral edge, can be an accompanying muscle that supports the function of the Achilles tendon.
Based on this, one could say that the Achilles tendon consists of two layers that eventually merge into one. This is partially true—at a certain level, the terminal fibers of the gastrocnemius muscle connect with those of the soleus muscle through aponeuroses that fuse together.

APONEUROSIS

is a type of the deep fascia, in the form of a sheet of pearly-white fibrous tissue, regular and poorly supplied with blood tissue  that attaches sheet-like muscles needing a wide area of attachment. Their primary function is to join muscles and the body parts they act upon, whether it be bone or other muscles.

The gastrocnemius muscle (1) is a superficial muscle located at the back of the calf. It consists of two heads: lateral and medial. Together with the soleus muscle, it forms the triceps surae, which in turn creates the Achilles tendon (3). The primary function of the gastrocnemius muscle is plantar flexion of the foot at the ankle joint and flexion of the leg at the knee joint. It is also responsible for fast leg movements, such as jumping and sprinting, as it is composed of fast-twitch muscle fibers, unlike the soleus muscle.
The soleus muscle (2) gets its name from a fish—the sole, a flat and broad marine fish whose shape resembles the muscle (its Polish name references a stingray). Like the gastrocnemius, it is also a superficial muscle, but it lies beneath the gastrocnemius. Together, they form the triceps surae and the Achilles tendon. Its main function is plantar flexion of the foot. It also plays a crucial role in maintaining an upright posture—without its constant tension while standing, the body would fall forward. Due to its slow-twitch muscle fibers, it is designed for long-term endurance work. Additionally, in a standing position, it helps pump blood back to the heart.

The plantaris muscle is a superficial muscle running between the gastrocnemius and soleus muscles. 8–12% of people lack this muscle. While it does not directly contribute to the Achilles tendon, it supports its function. It assists in plantar flexion of the foot and flexion of the knee. The plantaris muscle may provide proprioceptive feedback to the central nervous system about foot positioning. Its motor function is minimal, but its long tendon is often used for surgical grafting in reconstructive procedures.

BURSAE

BURSA

is a thin, small fluid-filled sac located at the point of friction between the bone and the surrounding soft tissue: skin, muscle, ligament or tendon. The bursa lies between the bone and the opposite surface (for example, the skin) like a tiny balloon of fluid wedged between two surfaces.

Between the anterior part of the tendon and its insertion, there is the bursa of calcaneal tendon. It prevents excessive friction. However, this is not the only bursa in this place - the second one is between the tendon attachment to the calcaneus and the skin and is called the subcutaneous calcaneal bursa. It protects the tendon attachment. As the tendon works, the bursae move between the layers of tissue. This can be seen especially in the case of the Achilles tendon bursa, which is hidden between the tendon and the calcaneus. When you bend your foot up (dorsiflexion), the bursa moves upward slightly. When you bend your foot downward (plantarflexion), the bursa moves downward. The bursa also has its protection in the form of adipose tissue located in the area of the Achilles tendon insertion, creating a specific, triangular shape. This is called Karger fat pad, which protects the bursa against excessive pressure and from moving sideways.

1. the bursa of calcaneal tendon | 2. the subcutaneous calcaneal bursa | 3. the calcaneus | 4. the Achilles tendon

KARGER'S FAT PAD

KARGER'S FAT PAD

(also known as the precalcaneal fat pad or preAchilles fat pad) is a lipomatous structure which normally appears lucent (fat density) on radiographs and has relatively well-defined (Kager triangle). It is located in the anterior part of the  flexor hallucis longus muscle, the superior cortex of the calcaneus inferiorly, and the Achilles tendon posteriorly.

Have you ever wondered why the sides of the Achilles tendon are empty and can be caught with your fingers through the skin? This is most evident when looking at the foot from behind. It looks like there is empty space there, but it is not!

The main function of Karger's fat pad is minimalization of the pressure in the subcutaneous calcaneal bursa during the bend the foot up and the protection of the joint capsule against overload and breakage. This structure also prevents other collateral structures from shifting, which minimizes injuries.

TORSIONED

The Achilles tendon can be said to have a four-bundle structure.This means that the tendon is made up of four tendon bundles, which are the ends of the muscles: three of them come from the gastrocnemius muscle and one from the soleus muscle. Importantly, these bundles are not arranged parallel to each other, but are twisted in a characteristic way, which affects the function of the tendon. How are they twisted? Differently. Because it is not the same for every human being, although there are certain types of fiber twist. They reach 90° at the insertion to the calcaneus. This twisting of the bundles gives the tendon high strength, as it is able to take a load 3.9 times greater than the body weight while walking and 7.7 times greater than the body weight while running. Unfortunately, such a structure also affects his injuries, which are sometimes difficult to heal and recovalence takes a long time.

FUNCTIONS

The intricate and advanced construction of the Achilles tendon allows you to stand, walk, run, jump, twist your feet to the side, load them on the lateral and medial edges. The main function of the tendon, however, is to transmit to the foot (through the calcaneus) the force generated by the calf muscles, and the function of these in turn is plantar flexion of the foot. So the Achilles tendon is actively involved in bending the foot down. The calf muscles then tighten, pulling the tendon, which pulls the heel bone up. 93% of this force goes to the tendon. For any FOOT MOVEMENT to occur, the muscle strength is transmitted through the tendons to the bones. This is an amazing cooperation.

LIKE A SPRING...

The calcaneal tendon is involved in almost every physical activity of the foot. Plantar flexion movement is needed with each step as you move. The tendon stores some kind of energy that is spent on foot activity. This is called elastic energy. The Achilles tendon is like a spring that, while stretching, must "pull" to release the stored energy, and this is precisely what happens during the plantar flexion: energy is released. However, in order for the tendon to accumulate energy and give it back, it is necessary that it is sufficiently stiff, i.e. resistant to high forces, and at the same time flexible, so that it returns to its shape and elastic to release energy.

STIFFNESS

is the resistance of a tendon to high forces that can stretch it, i.e. increase its length.

FLEXIBILITY

is the ability of a tendon to elongate and return to the same state it had before it stretched without structural damage. The Achilles tendon can increase in length by up to 4% without being damaged. At this value, the fibers return to their shape without any problem. Above 4% - microcracks of collagen fibers occur, and in the range of 8-10% - the tendon breaks.

Both of these features are necessary for the tendons. Thanks to their stiffness, the tendons do not break, they also maintain some stabilization between the muscle and the bone. However, their flexibility means that they do not deform, changing the structure, which would translate into their functions. The lower the stiffness, the greater the flexibility and elasticity to be able to accumulate energy more efficiently and then release it. However, too much flexibility and resilience causes the energy to be released too quickly, which is so great that it breaks the fibers - they break. There must be appropriate proportions between stiffness and flexibility, because they affect the efficiency of the movement: as little energy loss as possible, consistent work between the muscle and the bone to optimize the efficiency and economy of movement.

...AND LIKE THE RUBBER

The Achilles tendon is also like the rubber connecting the calf to the foot. In a healthy person whose feet are fully functional, the rubber stretches and pulls. This is how the foot moves up and down, which allows walking, running, jumping, etc. The tendon itself is of the appropriate structure, length and width. The opposite is true for children with clubfoot. Individual parts of the calf muscles have a slightly different structure, they are fibrotic, smaller and shorter. The Achilles tendon is also thicker, wider and short. Pathological ANATOMY OF THE CLUBFOOT affects all functions of the foot. Achilles tendon is shortened. They can be compared to a tightly stretched rubber. It is wider, thicker. It thus has greater stiffness. It is difficult to stretch them, even if you put a lot of force into it. It is inflexible, even though the structure of the Achilles tendon itself contains many collagen fibers, which are densely packed. Unfortunately, it is not efficient enough to restore the tendon to its original form, i.e. to the one before the defect, just by stretching. Due to this anomaly in the structure of the tendon, the foot is stopped in the only position - plantar flexion without the possibility of upward movement.

THE ISSUE OF AN EXCESSIVELY SHORTENED ACHILLES TENDON IN CHILDREN WITH CLUBFOOT
CAN ONLY BE RESOLVED THROUGH PERCUTANEOUS ACHILLES TENDON TENOTOMY.
THIS PROCEDURE RESTORES THE FOOT’S NORMAL FUNCTION AND RANGE OF MOTION, ALLOWING MOVEMENT BOTH UPWARD AND DOWNWARD.
AS A RESULT, THE TENDON REGAINS ITS ABILITY TO FUNCTION PROPERLY DURING WALKING, RUNNING, JUMPING, AND OTHER FOOT MOVEMENTS.

UNFORTUNATELY, DUE TO THE NATURE OF THE DEFORMITY, THE ATROPHIC CALF MUSCLES,
WHICH ARE NOT STRETCHED USING A FOOT ABDUCTION BRACE,
CONTINUE TO PULL ON THE ACHILLES TENDON.
THIS CAUSES IT TO BECOME SHORT AND INEFFICIENT, LEADING TO A RECURRENCE OF CLUBFOOT.

MEDICAL RESEARCH & MEDICAL SOURCES
  1. Szaro P. et al.: „Fascicles of the adult human Achilles tendon – an anatomical study.”
  2. Blitz N.M. et al.: „Anatomical aspects of the gastrocnemius aponeurosis and its insertion: a cadaveric study.”
  3. Bertolotto M. et al.: „High resolution ultrasound anatomy of normal Achilles tendon.”
  4. Rong H.: „The structure of human Achilles tendon and its biomechanical significance.”
  5. Strocchi R. et al.: „Human Achilles tendon: morphological and morphometric variations as a function of age.”
  6. Czyrny Z.: „Ultrasonografia ścięgna Achillesa – anatomia i patologie.”
  7. Van Gils C.C. et al.: „Torsion of the human Achilles tendon.”
  8. Edama M. et al.: „The twisted structure of the human Achilles tendon.”
  9. Maganaris C. et al.: „Biomechanics of the Achilles tendon.”
  10. Doral M.N. et al.: „Functional anatomy of the Achilles tendon.”
  11. Wren T.A. et al.: „Mechanical properties of the human achilles tendon.”
  12. Muraoka T. et al.: „Elastic properties of human Achilles tendon are correlated to muscle strength.”
  13. Spang C.: „The plantaris tendon in relation to the Achilles tendon in midportion Achilles tendinopathy.”
  14. Pękala P.A. et al.: „The Achilles tendon and the retrocalcaneal bursa. An anatomical and radiological study.”
  15. Theobald P. et al.: „The functional anatomy of Kager’s fat pad in relation to retrocalcaneal problems and other hindfoot disorders”
  16. Dalmau-Pastor M. et al.: „Anatomy of the triceps surae: a pictorial essay.”
  17. Ponseti I.V.: „Congenital Clubfoot. Fundamentals of treatment.” (2nd edition)
  18. Lin T.W. et al.: „Biomechanics of tendon injury and repair.”
PHOTOS & GRAPHICS
  1. Mythical Achilles by Filippo Albacini, 1825
  2. Achilles
  3. AnatomyLearning
  4. Dalmau-Pastor M. et al.: „Anatomy of the triceps surae: a pictorial essay.”
  5. Climbing Rope