The Achilles tendon (or heel cord) is the thick strap that can be felt running down the back of the calf into the heel. The tendon is made up of many bundles or fibres of a strong material called collagen, which is the body’s main tissue building block. It is attached to, and worked by, the large muscles that make up the calf (gastrocnemius and soleus). When the Achilles tendon pulls on the heel it makes us go up on tiptoe, or pushes us forwards when walking or running. If this tendon is not working it is difficult to walk and the ankle feels weak.