[xavi]

Hello Nicola, welcome to the forum

1.- Cheap solution is to measure first the reaction time on take off, and then measure run:
a) take off: no platform. photocell connected to push button, athlete is with the foot just behind the photocell. When you press the button, athlete see the green light changing on Chronopic, and then he runs. When photocell is touched it ends.
b) run: athlete on contact platform has to run to photocell. (no reaction time).

2.- More expensive solution is to have button and two photocells and measure all together.

In my opinion, solution 1 it's ok.

Cheers.