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by Scott McDonald

December 25, 2019

Body mass index and swim technique are typically the biggest determining factors

Getting older for many people sometimes means less exercise and more sitting. Even when dietary patterns stay the same, a sedentary lifestyle could lead to a lower metabolism rate, and the weight begins packing on the body.

Body weight can play a role in sports training, both good and bad. For swimming, buoyancy, technique, and body mass index factor in more than body weight.

“Your technique (drag reduction, effectiveness of propulsion) is by far the best predictor of performance,” says Terry Heggy, a U.S. Masters Swimming-certified Level 3 coach. “Focusing on weight loss without correcting stroke flaws is not going to significantly increase your swim speed.”

Heggy, coach of Team Sopris Masters in Glenwood Springs, Colo., says once a swimmer gets streamlined and propulsive, then trimming the pounds will help with speed.

Body weight itself can be deceiving. For example, take two people of the same gender, height, and build. One of them becomes extremely fit and develops big shoulders, large back muscles and strong, powerful leg muscles, and will outweigh the person without this muscle development. The muscular person might be listed as “obese” on standard height/weight charts, but these classifications don’t always accurately measure athletes.

Then there are those who weigh more than the average person of similar build because they are taking in more calories than they are burning, which adds fat.

Whether density comes from muscle or fat, more density can create more drag, or resistance, in the water. Density increases with a higher muscle-fat ratio and decreases with greater inflated lung volume.

So is a thinner person, or someone with a lower BMI always going to have faster splits than someone who carries more weight?

“A person’s drag profile (the surface area facing the water in the direction of motion) is probably the most important factor in swimming speed. Therefore, a swimmer with larger girth will face more resistance than someone who is thin,” Heggy says. “However, a thin swimmer with crummy form can have a larger drag profile by wobbling from side to side, kicking too deeply, or swimming with poor posture. 

“In other words, a fat guy with a good stroke will beat a skinny guy who thrashes.”

Heggy adds that some cold-water and long-distance swimmers choose to keep extra weight on for the purposes of insulation and fuel storage during their long and difficult swims.

So, when you walk into a swim meet and observe the athletes, it’s not always a clear-cut answer as to who will be the best swimmers.

“Based on casual observation at Masters meets, I’d say that you can’t predict a swimmer’s performance based on body weight (especially in upper age groups),” Heggy says. “But that’s because in a technical sport like swimming, the athlete’s training background is far more important than his or her current BMI.”


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