High horizontal force and lateral force transmission induced by the twist of the Achilles tendon.

The Achilles tendon-the largest and strongest of the many tendons in the human body-consists of subtendons arising from each head of the triceps surae. These subtendons run helically and intertwine to form a twisted three-dimensional structure. More than a century has passed since the twisted structure of the Achilles tendon was first described, and numerous studies have investigated its functional morphology. However, it remains unclear how the presence or degree of twist affects the mechanical function of the lower limbs. Here, using finite element models of the Achilles tendon with and without twist, we show that the presence and degree of twist can modulate the forces transmitted from muscle to skeleton in a complex manner. In contrast to the model without twist, the twisted models clearly exhibit anterolaterally directed horizontal forces at the distal end under a simple proximal tensile load. The lateral and anterior components reach approximately 101 N and 63 N, respectively. Furthermore, the twisted structure induces inter-subtendon force transmission, and the magnitude of the transmitted force increases with increasing degree of twist. These results indicate that the uniaxial contractile force generated by the triceps surae is transformed into triaxial forces by the twisted Achilles tendon and is redistributed in a complex three-dimensional manner through inter-subtendon interactions.