The Achilles tendon (AT) is the largest tendon in the human body, it consists of sub-tendons from 3 muscles of the triceps surae (soleus, lateral gastrocnemius, medial gastrocnemius). These sub-tendons together form the tendinous part of the AT and twist as they descend in the insertion at the calcaneal bone. This twisted structure of the AT enables it to handle the functional loads applied to the tendon. It also balances resisting tension and allowing compliance as the function of tendons is more complex than just transmitting force from muscle to bone. They store and release energy, protect muscle from stretch damage, allow favourable muscle output and enhance muscle performance.
A study from Poland published in Scandinavian Journal of Medicine & Science in Sports investigated the twisted structure of the AT and its individual sub-tendons. Specimens of the AT, with preserved calcaneal bone and a fragment of the triceps surae muscle, were obtained from 53 fresh-frozen, male cadavers (n=106 lower limbs).
The proximal junction between the medial and lateral heads of the gastrocnemius muscle (MHG, medial head of gastrocnemius; LHG, lateral head of gastrocnemius). From Ballal et al. (2014)
The twisted structure of the AT has been described as a clockwise rotation (lateral) in the left limb and counterclockwise rotation (lateral) in the right limb (as seen from proximal to distal end).
This study confirms that the three types of the AT with differently twisting subtendons:
Type I—least twisted, in which the part of the AT originating from the gastrocnemius muscle is attached to the posterior two-thirds of the calcaneal tuberosity, and the SOL is attached to the posterior one-third of the calcaneal tuberosity. The most prevalent, found in 52% of the subjects from 4 cadaveric studies (n=341).
Type II—moderately twisted, in which the part of the AT originating from the gastrocnemius muscle is attached to the posterior half of the calcaneal tuberosity, and the SOL is attached to the other half of the calcaneal tuberosity. The second most prevalent, found in 40% of the subjects.
Type III—extremely twisted, in which the part of the AT originating from the gastrocnemius muscle is attached to The Achilles tendon (AT) consists of fascicles that originate from the medial head of the gastrocnemius (MG), lateral head of the gastrocnemius (LG), and soleus muscle (Sol). These fascicles are known to have a twisted structure. Least, found in 8% of the subjects.
Types of the left Achilles tendon twist (top: three-dimensional model, below: cadaveric pictures: dorsal view). ANT, anterior; POST, posterior; L, lateral side; M, medial side; GL, the subtendon originating from the lateral head of gastrocnemius muscle; GM, the subtendon originating from the medial head of gastrocnemius muscle; SOL, the subtendon originating from the soleus muscle. From Pekala et al. (2017)
The authors also found that the torsion angle of the AT’s superficial fibers (medial gastroc) is significantly smaller than that of the fibers lying deeper in the tendon (lateral gastroc and soleus). The greatest degree of AT torsion is observed in the most anterior (deepest) part of the tendon (lateral gastroc).
The authors concluded that types of AT torsion may potentially alter the biomechanical properties of the tendon, thus possibly influencing the pathophysiologic mechanisms leading to the development of various tendinopathies.
Another study from Belgium showed that the AT displaces non-uniformly with a higher displacement found in the deep layer of the tendon. Adding to this, a non-uniform regional strain behavior was found in the Achilles tendon during passive elongation, with the highest strain in the superficial layer.
The 3 types of twist of the left Achilles tendon, posterosuperior view. The lower schema: Transverse cross-section through the left Achilles tendon, 1 cm above the tuber calcanei. A, anterior; L, lateral; LG, fibers from the lateral head of the gastrocnemius; M, medial; MG, fibers from the medial head of the gastrocnemius; P, posterior; Sol, fibers from the soleus muscle. From Edama et al. (2015)