 The start of a race is marked by a white line 5 cm wide . In all races that are not run in lanes the start line must be curved , so that all the athletes start the same distance from the finish . Starting blocks may be used for all races up to and including 400 m ( including the first leg of the 4 × 100 m and 4 × 400 m ) and may not be used for any other race . No part of the starting block may overlap the start line or extend into another lane . 
