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.
In theory, a flawless, straight line will be drawn down the swimmer's back from their head to their feet. The body should remain on a horizontal plane underwater, with leg kicks executed straight from the thighs and hips, not the knees. A considerable amount of flexibility is typically required to achieve perfect streamline, especially in shoulder flexibility.