Thoracic outlet syndrome (TOS) is a condition where the nerves or arteries that go to the arm and hand get squeezed between soft tissues near the side of the neck. Swimming can be a helpful part of therapy for TOS patients who can tolerate it. However, you should avoid doing the full breaststroke and the backstroke. These strokes put extra pressure on the nerves and vessels near the thoracic outlet. When your symptoms are under control, you may choose to try these strokes again. But if your TOS symptoms start to come back, you should stop.