What is Helbing's sign?
Helbing's sign was named after the German physician (Carl Helbing) who first wrote about it in the early 1900s. It is a curving inward of the Achilles' tendon. The Achilles' tendon attaches the calf muscle to the large heel bone called the calcaneus.

Helbing's sign occurs most often when the arch of the foot drops down such as occurs with flat feet. The curving of the Achilles' can be best seen from behind when the person is standing.