we have a project about taekwondo wireless score board..we use rf and we don't know what sensor we gonna be put in the taekwondo vest inorder to trigger and display it in Big 7-Segment using PIC18f877

 

A piezo sensor - http://en.wikipedia.org/wiki/Piezoelectric_sensor - would seem to work best. You might even be able to give some indication of the force of the blow.

The hard part is reading all those sensors and sending the data. That will take a microprocessor and some radio equipment, plus a battery for power. Maybe that could go in a pouch that was in a place not likely to get hit and not likely to be rolled on when taking a fall.