Ever see a flabby dolphin or a weak-looking competitive swimmer? We didn't think so. That's because swimming is a great way to increase muscular strength and muscle tone -- especially compared to several other aerobic exercises.
Take running, for example. When a jogger takes few laps around the track, that jogger is only moving his or her body through air. A swimmer, on the other hand, is propelling himself through water -- a substance about twelve times as dense as air [source: Yeager]. That means that every kick and every arm stroke becomes a resistance exercise -- and it's well known that resistance exercises are the best way to build muscle tone and strength.
There's yet another bonus of a watery workout: Swimming has also been shown to improve bone strength -- especially in post-menopausal women [Source: Huang, et al.].