I wondered how I got those bruises on my forearms when I had to practice standing starts using my road bike the other day. I have a fixie set up for the road when I can't get to the track, but that day I decided to work in my starts on a bike ride with my wife. I did not feel the contact during the ride, but the next day I saw the bruises and couldn't remember doing anything that would cause them. I postulated what the cause was, and you guys just confirmed it!