Don't remain below the surface when you're alone.
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If you swim for fitness or as part of a weight-loss program, you'll naturally want to know how to maximize your workout efficiency and burn as many calories as possible. To that end, swimming below the water's surface may intuitively seem like an efficient way to burn extra calories. However, underwater swimming carries risks that may not be worth the few extra calories you burn.
Increase Your Resistance
When you swim the backstroke, front crawl or butterfly strokes, your arms typically travel above the water during the recovery phase. Water, however, offers 1,000 times more resistance than the air above the water line. As a result, swimming any of these three strokes below the surface requires more effort - and burns more calories - than swimming them in the standard fashion. Your arms and legs remain below the surface during a standard breaststroke swim, so moving underwater should make little difference with respect to the number of calories you'll burn.
Save Your Breath
Swimming underwater may help you burn more calories in a single front crawl stroke when compared to swimming on the surface, but being surrounded by water also creates a problem. Due to Mother Nature's failure to supply humans with gills, you can't breathe underwater. Obviously, this will limit the time you can spend below the surface.
Additionally, depriving your body of oxygen during a workout is counterproductive because oxygen is the raw material your body requires to fuel your hard-working muscles. If you're trying to do an extended workout in a bid to lose weight, swimming below the surface is likely to do more harm than good to your fitness effort. Meanwhile, swimming on the surface for 10 minutes will burn about 60 calories when you do the breaststroke, 80 if you perform the backstroke, 100 when you perform the front crawl and 150 if you prefer the butterfly.
Consider Your Safety
Swimming below the water's surface for extended periods is not recommended, particularly when you're alone. Holding your breath for too long can cause you to pass out, and if you do so underwater, you risk drowning. At minimum, you must have a trained lifeguard present if you attempt a prolonged underwater swim. However, military trainer and former Navy SEAL Stew Smith notes that underwater swimmers have died even when lifeguards were present.
Try Scuba Diving
If you really want to swim underwater, an obvious solution is to learn scuba diving. Scuba divers are generally interested in exploring the underwater environment - they don't typically take the trouble of donning a suit, fins and mask, and then strapping on an oxygen tank, just to lose weight. But scuba diving does provide an efficient workout that can burn up to 900 calories per hour. Learn the sport from a certified instructor and, like underwater swimming, dive with others for safety.