This stroke is the only one in which the competitors do not start on the starting blocks. Already in the water, the swimmers kick off with their feet, propelling themselves backwards and letting go of the pool wall. They swim on their backs powering themselves with alternative circular strokes of their arms, whilst kicking in turn with their legs.In Olympic competition, the back crawl, or backstroke, takes place over distances of 100 and 200 m as well as in relay medley.