Avoid: Sore, aching shoulders
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 Jessica Matthews, an exercise physiologist for the American Council on Exercise, who also used to teach swimming and aquatic fitness.
Do it right: When 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.
Read more: Two professional coaches help you get reacquainted with your inner mermaid