According to the Health Fitness Instructor Handbook, one of the reference manuals for the American College of Sports Medicine, the amount of calories burned during swimming depends "on the velocity of movement and the stroke being used, but it is also influenced by the skill of the swimmer. A skilled swimmer requires less energy to move through the water, so that person has to swim a greater distance than an unskilled person to achieve the same caloric expenditure."
The handbook says that for poor swimmers "the energy cost of simply treading water can be as high as 7.5 calories per kilogram of body weight per minute. Elite swimmers use this same number of calories per minute to swim at 36 miles per minute whereas an unskilled swimmer might require twice that energy expenditure to maintain the same velocity."
On the surface, it would seem to be more advantageous to have mediocre swimming skills to burn more calories but it backfires on you because you tire faster than you can rack up calories. In other words, you burn more calories when you are a novice swimmer but you cannot swim very long or very fast. The more skilled you become, the less calories you burn but you still burn more calories in the long run because you can swim longer and faster before you become fatigued.
If you are wondering why women burn less than men, the Fitness Leader's Handbook, another reference manual, says that because of women's "greater buoyancy associated with higher body fatness, women expend fewer calories per mile than men, independent of skill level". People with more fat have an easier time staying afloat. That in turn means fewer calories are burned. This is good news if you are a competitive swimmer so you can reserve your calories to fuel your speed and distance.
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