Sport - Swim
Centerline
Crossing the centerline is a common problem that’s difficult to notice while swimming with a proper head position. Here’s how to check your stroke.
Right (The top swimmer in the photo above): To maintain an efficient stroke, the hand should stay to the outside of the centerline, right in line with the shoulder.
Wrong (The bottom swimmer in the photo above): The hand should not cross over the centerline. This not only slows down your stroke, it also strains the shoulder with every pull.
Documentation / Reference