The calcaneal tendon, also known as the tendon of Achilles, is a posterior leg tendon — a fibrous connective tissue that joins muscles in the back of the leg. It is formed when the soleus muscle tendon joins with the gastrocnemius tendon.
The gastrocnemius (calf) and soleus muscles are part of the superficial posterior compartment group, which also contains the popliteus and plantaris muscles. This group is located at the back of the lower leg, close to its surface. The calcaneal tendon is also part of this group.
The purpose of the calcaneal tendon is to attach the soleus muscles and calf muscles (plantaris and gastrocnemius) to the heel bone, scientifically known as the calcaneus. The plantaris muscle tendon, if present, assists in this process by inserting into the calcaneal tendon's medial border (the side closest to the midline of the body).
The calcaneal tendon is the strongest and thickest tendon in the body. It is able to withhold a stress of about four times a person's body weight when walking and its narrowest part (above its insertion) is four centimeters thick. The tendon starts in the middle of the calf and extends approximately 15 centimeters in length, down to the heel, and passes vertically down, behind the ankle.