Fluidforms writes:

The Cassius Punching Bag contains a matrix of 9×7 force sensors that measure the force exerted by a blow to the punching bag. The sensors are plugged into an Arduino board that sends the values over USB to a computer using the MIDI protocol. The computer is running a Processing sketch that receives the MIDI packets and writes the current state of the sensors to a folder in a “|” separated format. An OpenGL render then reads these text files and graphically represents the current state of the punches to the bag since the last reset.

Arduino punching bag with homemade sensors including video, code, and schematic – Link.

(Thanks, Byron!)