In theory, a perfect, straight line will be made down the backside of a swimmer from their head to their feet. The body should be on a horizontal plane under the water, with the legs kicking straight from the thighs and hips, not the knees. A great deal of flexibility is usually needed to reach the goal of a perfect streamline, particularly flexibility of the shoulders.
Theoretically, a perfectly straight line could be drawn down a swimmer's back from head to toe. The body should remain level underwater with leg kicks initiated from the thighs and hips rather than the knees. Achieving this ideal streamlined position requires significant flexibility, particularly around the shoulders.