it’s a patella tendon strap as someone else mentioned.
As someone else mentioned it’s just a patella strap
Something like this may assist; https://www.amazon.com/Pro-Tec-Athletics-Patellar-Tendon-Strap/dp/B004AOGA56
You can check other brands but this is what I'm referring to:
http://www.amazon.com/Pro-Tec-Athletics-Patellar-Tendon-Strap/dp/B004AOGA56
They're very helpful
Consider getting one of these.
http://www.amazon.com/Pro-Tec-Athletics-Patellar-Tendon-Strap/dp/B004AOGA56