Just gave it another thought... If I can find a way to know when the swimmer reached to the end of the pull I can measure the time and culculate the velocity but, how can I know when the swimmer reached to the end? Well, I need to see pulse on the ecc sensor but, there is pulse every time the swimmer move his head to breathe. I see the pulse because gravity changes axis. If I would find a way to ignore it, that's it.