The Achilles is the thick tendon from the lower calf to the heel. As you might have seen quite recently, David Haye reportedly ruptured his Achilles tendon in the boxing match against Tony Bellew. The relevance of this example is the similarities between boxing and running. You repetitively push off from your foot, activating your calf muscle (gastrocnemius) and adding load to the Achilles tendon. Hence why David Haye didn't stand a chance.