What unit of lenght represents the strengh of a punch?

About those machines which tests the strengh of a punch, which unit of lengh is utilized? I'm talking about those machines that comes up with a punching bag, and when you punch them, they register the strengh of your punch.|||Strength would be a unit of force not length. Perhaps they could measure the distance (in meters, millimeters, centimeters, feet, or inches) that a spring is compressed which could be converted to force using Hooke's Law.