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by Dr David Costill

February 4, 1993

Body composition affects performance

Soon after I began doing research on athletes, I became convinced that champions were genetically gifted and made of "special stuff." As a distance runner for nearly 20 years, I was satisfied with my middle of the pack finishes in marathon races because I knew that I had mostly fast-twitch muscle fibers that were best suited for sprinting and a cardiovascular system that was designed for events lasting 2 to 3 minutes rather than 2 to 3 hours. Our studies with runners such as Frank Shorter, Alberto Salazar, and Bill Rodgers made it clear that they had done a good job of choosing their parents. When I returned to swimming in 1982, I shifted my sports- related research from running to swimming, expecting to find that the best swimmers also had the best physiology. Surprisingly, that was not the case!

What we found were many swimmers with exceptional strength and endurance who were only average performers, and a few individuals with average strength and endurance who were outstanding swimmers. That is, the best swimmers are exceptional because of factors other than their physiology talents, namely skill. In many cases the best swimmers are the ones with the "best swimming mechanics." The qualities that contribute to swimming skill are termed "biomechanical" because they involve factors responsible for resistance of the body's movement through water (e.g., drag) and the efficient application of muscular force for propulsion. These factors of swimming biomechanics range from the shape and composition of your body to the sculling actions of your arm/hand action during swimming. It would be impossible to detail all of these factors in this column. Nevertheless, let me illustrate one factor with which we must all contend, but which may be only partly under our control: body shape and composition.

Having a body that is shaped like a javelin may be considered by some to be the "perfect body" for swimming, since it offers almost no resistance to movement through the water. But the fat content of the body may be of equal importance, because it determines your buoyancy and the amount of energy you must expend to simply stay on the surface of the water. Being on the lean side means that you must work harder to stay high on the water. Though having a lot of body fat means that you float well, the added body mass adds resistance to your movement through the water.

What's the ideal body fat? That's hard to say, since the percentage of fat in elite swimmers may vary from 6% to 25% of body weight. Women, in general, have a higher body percentage of body fat than men, giving them a buoyancy advantage. Female fat tends to be disproportionately distributed in the lower half of the body, giving a bit more lift to the legs which in turn reduces body drag. As a result, the amount of energy (calories per kg of body weight) needed for a woman to swim at the same pace as a thin man is significantly less.

So, why not make all male swimmers fatter? Since men tend to store a large amount of their fat above the waist, putting on more fat would shift their buoyancy forward, making their legs sink - which increases drag. This is easily demonstrated by the fact that most men can swim faster with a float between their legs than without it, whereas women experience little or no improvement when they add flotation to their legs. Thus, if I were put to the task of recommending the "ideal fat level" for Masters swimmers, I'd play it safe and suggest that the range for men be from 10% to 20% and from 15% to 25% for women.

Although there are a number of ways for you to determine your body fat percentage, you should be aware that there is no perfect method. The simplest method involves the measurement of the thickness of fat stored under your skin with specially designed calipers. By sampling the thickness of skinfolds at selected sites on the body, it is possible to estimate the body's overall fat content. Since we all store fat in different areas of the body, this method cannot be considered perfect, but it will provide you with a point of reference and a general idea of your body composition.

Being too thin is usually not a major problem for most Masters swimmers (including me), but you should realize that dieting to lose weight may not result in large improvements in your swimming performance. To the contrary, losing fat generally means that you must use more energy each day than you are eating, putting you in a negative caloric balance. In addition to burning body fat, dieting also causes the body to use some of its protein for energy, which can result in a sizable breakdown of muscle protein. So, while you may lose some excess body fat by restricting your food intake, you can also lose muscle tissue, leading to a decline of muscle strength and swimming performance. Consequently, if you intend to lose weight to improve your swimming, make sure your dieting is done during periods of the year when your training can be at relatively low intensity and when you don't intend to compete.

Body composition is only one of the considerations for optimal swimming performance. In future columns we plan to address such issues as (1) the value of strength training, (2) learning better swimming technique, (3) interval training to gain endurance, strength, and speed, and (4) quantifying swimming training (how much is enough) for fitness and competition.

Dr. David Costill is the director of Ball State University's Human Performance Laboratory and is current chairman of the USMS Sports Medicine, Health and Safety Committee. A Masters swimmer for the past 12 years, he has won numerous national freestyle and individual medley championships.


  • Health and Nutrition


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  • Sports Medicine