The Achilles tendon, also known as the calcaneal tendon, is the thickest and strongest tendon in the human body. It is located at the back of the lower leg, connecting the calf muscles to the heel bone (calcaneus). This structure is fundamental for human locomotion.
Muscles Driving the Action
The primary muscles that converge to form the Achilles tendon are the gastrocnemius and the soleus, collectively referred to as the triceps surae. The gastrocnemius has two heads, originating from the medial and lateral condyles of the femur, above the knee joint. Deep to the gastrocnemius, the soleus muscle originates from the posterior surfaces of the tibia and fibula.
These two powerful calf muscles merge distally to form the common Achilles tendon, which then inserts into the middle part of the posterior surface of the calcaneus (heel bone). When the gastrocnemius and soleus muscles contract, they pull on the Achilles tendon, initiating movement in the foot and ankle.
Key Movements Enabled by the Achilles Tendon
The Achilles tendon facilitates a range of fundamental actions, with its primary function being plantarflexion, which is the movement of pointing the toes downwards or pushing the sole of the foot away from the leg. This action is responsible for approximately 93% of the plantarflexion force.
During walking, the Achilles tendon is particularly active in the push-off phase, propelling the body forward. It also plays a significant role in running, jumping, and activities like standing on tiptoes. For instance, in young individuals, the tendon can withstand forces up to 12 times their body weight during running.
The Biomechanics of Force Transmission
The Achilles tendon efficiently transmits force from the calf muscles to the heel bone, acting as a strong and somewhat elastic structure. It consists mainly of type I collagen fibers, which account for approximately 70-80% of its dry weight. These fibers are arranged in a hierarchical structure, providing tensile strength and resistance to deformation.
This tendon also plays a role in energy storage and release during dynamic movements, such as walking and running. As the tendon stretches, it stores elastic energy, which is then released as it recoils, contributing to forward propulsion. The fibers of the Achilles tendon spiral about 90 degrees as they descend, with gastrocnemius fibers attaching more laterally and soleus fibers more medially on the calcaneus. This unique arrangement allows for concentrated stress and efficient power transfer, enabling the tendon to withstand significant loads, sometimes up to ten times the body’s weight during intense activities.