Surprise risk: Shoulder strains don't just strike poolaholics who swim six to eight miles a day. This inflammation of the rotator cuff, which makes the joint feel like it's on fire, can also afflict weekend freestylers who aren't doing the stroke properly, says Matthews, who used to teach swimming and aquatic fitness.
Ouch! Factor: 4 if you don't switch your stroke or your style, and 5+ if your strain becomes a tear (pay attention to the pain and back off before that happens).
Swim safely: While swimming freestyle, make sure that all five fingers dive under the water together, with your palm facing down, Matthews says. If your thumb is going in first, you're over-rotating your shoulder. She adds that because we rotate our shoulders inward all day long while typing and driving, for instance, we all need to make sure we're stretching the opposing muscles—and this is especially important for swimmers. "Any move where the thumb points up and back can help," says Matthews—like this Pilates-inspired standing pose.