RIO DE JANEIRO — The old swimming adage is that breaststrokers are born, not made. You often can look at a swimmer standing still and tell whether they are a natural fit for the most esoteric of the four strokes.

Start from the ground up: Most successful breaststrokers have duck feet that point outward, making it all the easier to perform the frog kick that goes with the stroke. Flexible ankles are a plus. So is a long torso.

But what if there is one other rare physical trait that lends itself to excellence in breaststroke? And what if that trait is a birth defect?

It sounds crazy, and it might be nothing more than coincidence. But two of the three American Olympic breaststrokers here were born with a chest wall deformity, and a third elite American breaststroker has the same physical abnormality.

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Kevin Cordes, who was a finalist in the 100 breast earlier this week and finished eighth in the 200 breast final Wednesday night, has a condition known as pectus carinatum. It is characterized by a protrusion of the sternum and ribs.

Cordes is, essentially, pigeon-chested. He had surgery in third grade to mitigate the appearance of it, leaving him more numb in the lower sternum.

Teammate Cody Miller, the bronze medalist in the 100 breast, has the inverse effect: pectus excavatum. That’s a sunken chest, where the ribs and sternum have grown inward.

Sam Tierney, an All-American at Missouri who was a semifinalist in both the 100 and 200 breast at Olympic trials, also was born with pectus excavatum. Tierney had surgery in high school to expand his chest, wherein doctors inserted a metal bar for two years.

So that’s 66 percent of the American Olympic breaststroke crew, and 37.5 percent of the top eight male breaststrokers in the country, if you go by their top times over the last year. Research estimates that 0.2 percent of live births have the same condition as Miller and Tierney, and 0.006 percent have Cordes’ condition.

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Those are striking percentages. Again, it could be mere coincidence. But as Cordes said when I pointed it out, “That’s crazy.”

I asked all three athletes if there could be a correlation – if perhaps the chest shape contributes to the physics of successful breaststroke. Cordes said he didn’t know. Miller was doubtful. Tierney was more open to the idea.

For all three, the chest-wall issues were (or are) a cardiovascular challenge. Miller and Tierney said their lung capacity and stamina were inhibited. Tierney’s surgery alleviated that issue, while Miller (who has not had a corrective surgery) has simply overcome it.

“I think I’d be a better breaststroker with a normal chest,” Miller said.

Certainly so, from an aerobic standpoint. But what about stroke mechanics?

I’m particularly thinking about the pull, which brings water into the chest as the hands come together beneath the body. Could there be a benefit in a slightly misshapen chest creating something of a spout for that displaced water to go?

Tierney pointed out that he, like many swimmers and particularly breaststrokers, has poor posture. His shoulders come forward and chest is down, a common physical trait in swimmers due to greater muscle mass on the front of the torso – it essentially pulls the shoulder into a slouch. That’s also a common posture profile of someone with pectus excavatum.

“That’s what breaststroke pull is,” Tierney said. “Your shoulders are coming forward while your chest is in.”

Miller, a student of the stroke who has observed Cordes’ technique for years, sees another physiological characteristic that might help his American teammate.

“His ability to hold his body line in water has more to do with his scoliosis,” Miller said. “You can very easily see his arched back. That helps his body line, which allows him to bring his heels up and have curvature of his torso.”

That’s part of what makes breaststroke different – it is an undulation stroke that is all about rhythm and mechanics and flow, and less about pure speed and power.

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