Cody Miller was born with a condition that diminished his lung capacity. Lung capacity is of huge importance to Olympic swimmers. In fact, it’s been theorized that one of the reasons Michael Phelps is so good is because his lung capacity is 12 litres, or double the average man’s. But Cody Miller, who was born with a deformity called pectus excavatum, doesn’t let that stop him. What is pectus excavatum?

Here’s what you need to know.

When Cody Miller qualified for the Olympics, he shared a story on Imgur about the deformity that caused his lung capacity to be diminished. Here’s a photo he shared of what his chest looks like:

Miller explained that although he was born with pectus excavatum, the deformity did not begin appearing until he was about 10 years old. The condition causes his sternum to cave in, leading to stress on his respiratory system. His lung capacity is reduced anywhere from 12 to 20 percent as a result.

He wrote:

I struggled with my appearance from a young age. I was a kid who was afraid to take off his shirt in gym class… people thought I was weird. At swim meets, I walked around the pool deck awkwardly while people stared and pointed at me. I was weird and abnormal… However, I’ve realized this: No one is 100% satisfied with the way they look. Everyone has some thing about themselves they dislike. And that’s OK! Professional athletes, models… everyone has their own insecurities! I’ve embraced the fact that I have a giant hole in my chest! It’s OK!

Pectus excavatum causes the ribs and sternum to grow abnormally. Sometimes the congenital deformity is just cosmetic, but sometimes it can harm cardiac and respiratory health. Doctors aren’t sure what causes it, but about 37 percent of people with it have a first-degree family member who also has it. It can also be connected to syndromes like Marfan syndrome and other connective disorders. Some people have surgery to correct pectus excavatum. One type of surgery is very invasive and involves inserting a bar beneath the sternum to hold it in place. Another surgery, called the Nuss procedure, also involves slipping a bar beneath the sternum, but it’s less invasive and the bar stays in place for about two to five years. Miller opted not to have any surgery for his condition.