The "Achilles heel" is a term that came into popularity in the 19th century to represent a singular point of vulnerability. Although the phrase originates from Greek and Roman mythology, the metaphor can be interpreted quite literally.
Tendons are bands of connective tissue that connect muscles to bones.
The Achilles tendon is the tendon that connects the heel bone to the gastrocnemius and soleus muscles (two calf muscles).
Although many tendons are protected by a synovial sheath, the Achilles tendon is protected by a paratenon, which makes it slightly more vulnerable to injury.
A short Achilles tendon causes the patient to walk on the balls of the feet (tip-toes).
Stretch the Achilles tendon by pressing your foot against the wall at approximately knee level. Make sure you're directly facing the wall, and make sure your other foot is perpendicular to the wall and flat on the ground.
Surgical treatment may be a better option if physical therapy doesn't achieve the results necessary. An orthopedic surgeon can lengthen the Achilles tendon through a procedure called gastrocnemius recession.