If you frequently read this webpage you know about the frequency of shoulder pain in swimmers. However, swim coaches, trainers, therapists, etc. do not know which strokes stress the shoulder. The novice would think fly or free would be most stressful to the shoulder, but the amount of shoulder stress per stroke depends on the structures causing pain and can vary depending on the specific injury.
Backstroke is likely the most painful stroke in swimmers with shoulder pain. Elite swimmers backstrokers roll their bodies allowing their hand to enter next to their body with their elbow next to their side.
Backstrokers who do not roll their shoulders properly will still have the same catch, but the lack of rotation greatly stresses the anterior aspect of the shoulder. This in combination of full shoulder flexion and internal rotation can lead to instability or secondary shoulder impingement. Therefore having proper shoulder strength and stability is necessary to prevent their injuries.