“The peak of the saddle shall be a minimum of 5 cm to the rear of a vertical planepassing through the bottom bracket spindle. This restriction shall not be applied to thebicycle ridden by a rider in a sprint event on track (flying 200 m, flying lap, sprint,team sprint, keirin, 500 metres and 1 kilometre); however, in no circumstances shallthe peak of the saddle extend in front of a vertical line passing through the bottombracket spindle.The peak of the saddle can be moved forward until the vertical line passing throughthe bottom bracket spindle where that is necessary for morphological reasons. Bymorphological reasons should be understood everything to do with the size and limblength of the rider.Any rider who, for these reasons, considers that he needs to use a bicycle of lesserdimensions than those given shall inform the commissaires' panel to that effect at thetime of the bike check.Only one exemption for morphological reasons may be requested; either the peak ofthe saddle can be moved forward or the handlebar extensions can be movedforward, in accordance with Article 1.3.023”
is the full article on the saddle setback. So track sprinter can go past the 5 cm limit but not have the front of the saddle ahead of the bottom bracket center.
http://www.uci.ch/mm/Document/News/R...NG_English.PDF is the full UCI equipment rule guide with explanations.