What is the name of an instrument used to demonstrate knee-jerk?

The instrument commonly used to demonstrate the knee-jerk reflex is called a reflex hammer. It is a small, handheld device typically made of rubber and metal, designed to strike tendons gently to elicit reflex responses. The most common reflex tested is the patellar reflex, which involves the knee-jerk response when the patellar tendon is tapped.