Achilles tendon

The Achilles tendons are a pair of tendons that join muscles in the calf to bones in the heel. They are the thickest tendons in the body and are long, reaching half way up the calf. It is named after the mythical Greek warrior Achilles, whose heel was his sole vulnerable point as his mother held him by it while dipping him in the River Styx in an attempt to bestow him with invulnerability. 
It is a common site for tendonitis. It can also rupture under severe stress, such as that typical of explosive acceleration of the feet, such as sprinting.
Achilles tendon at Wikipedia
Category:Anatomy