For large international meets they are assigned based on FINA's seeding rules: http://www.fina.org/content/sw-3-seeding-heats-semi-finals-and-finals http://www

For large international meets they are assigned based on FINA's seeding rules: http://www.fina.org/content/sw-3-seeding-heats-semi-finals-and-finals http://www.fina.org/content/sw-3-seeding-heats-semi-finals-a... Finals are seeded 4,5,3,6,2,7,1,8 fastest-to-slowest. Semifinals are circle seeded (two heats where b is the second heat) 4-b, 4-a, 5-b, 5-a, 3-b, 3-a etc. Heats are circle seeded for the last (fastest) three heats, and then regular-seeded for the remaining heats. Basically if the pool had a current, you'd expect to see more 'upsets' from the lanes with less current, or from the lanes with a positive current in the case of the 50m.