We all know exercise does a body good.

But the authors of a new book called "The Science of Fitness" take us deep inside our body's cells to uncover exactly why exercise makes us healthy. In a reveal that sounds like something out of "Star Wars," it turns out that tiny bacteria-like dynamos embedded in our cells create an inner force — or energy.

Celebrated cyclist and Medina resident Greg LeMond teamed up with one of his biggest fans, Dr. Mark Hom, a Virginia-based radiologist and cycling enthusiast, to write the book.

In recent interviews, LeMond and Hom described how high-intensity interval training propelled the cyclist to become the only American to win the Tour de France and how it can help everyday people boost energy levels, slow down aging and prevent disease.

Q: Why did you decide to write this book?

LeMond: Cycling is a very demanding sport. It requires a lot of different types of intensities. It's a sport that when you race professionally, you race about 100 races a year. Training is a critical part of it, and I've always been one who didn't want to be told what to do. I wanted to know why I had to do it. So I've always been really into understanding the scientific part of it.

Hom: Basically, my wife and I were turning 50. We were both very healthy but then we were reaching that stage in our lives when we kind of stopped exercising. We both realized: Do we really want to slide into mediocrity and just get old and fat? I tried to find one book that would explain for both of us the most efficient way to train. But there really was no book that explained to my satisfaction what was underlying exercise and fitness. As a doctor, I always wanted to know why something worked. So I did my own literature research and it would always come back to the mitochondria.

Q: What do mitochondria have to do with fitness?

Hom: They convert your body fat and your food, using oxygen, into energy that your muscles, your heart and your brain can actually use. Mitochondria are related to ancient bacteria. They have their own DNA. It's a symbiotic relationship. Do you remember the "Star Wars" episode where Qui-Gon is explaining to Anakin about the "midi-chlorian"? So George Lucas came up with this mechanism to explain why certain characters had the force and some didn't, or why some had more of the force than others. These midi-chlorians were symbiotic creatures that live inside your cells and make your energy. He was borrowing that from mitochondrial biology.

What happens when you exercise with high intensity like what Greg advocates, it causes [a] trigger in your cell. The cell is [like] a complex little city. Like a city, it depends on power plants. Let's say you have a high energy demand and the city starts having brownouts. There's a mechanism, almost like a government, that allocates resources to make more power plants.

So there's a signal for when you start having low energy from working out really intensely. Your cell adapts by telling the mitochondria to start multiplying. So when you train properly, you'll multiply your mitochondria, which will make more energy, which will allow you to have more endurance and more performance.

Q: Greg, you were diagnosed with mitochondrial myopathy 20 years ago, following a hunting accident. How has that affected your training? Do you still ride?

LeMond: Yeah. I do have to do shorter, more strength stuff. I do some 30-second sprints. But I do find if I ride too much or exercise too much, then I don't improve and my lead levels in my blood go up dramatically. So I keep my rides to under an hour and then I feel really good.

Q: In the book, you say that one hour of exercise can do more for overall health and disease prevention than any medicine. How much exercise is enough to boost our mitochondria?

Hom: Greg says he gets a lot of athletes coming to him for advice. He says a lot of amateur athletes in particular are overdoing it. They're overtraining.

Recent studies have shown that ultramarathon runners — you would think they'd be super healthy — actually they're causing [damage]. For some, they're causing accelerated coronary artery disease. There's a certain point where you can definitely train too much.

LeMond: Anything you do, it has to be intense. It used to be believed that the more you train, the more mitochondria you get. But it's more about the intensity. The body adapts to the stress load. Between 30 minutes to one hour [of exercise] is plenty, four days a week. That would be the ideal thing for most people.

Q: So what kind of exercise should we do to benefit our mitochondria?

Hom: I'm not a fitness coach or trainer. But like I said, the mitochondrial multiplication trigger is when you sap your energy — when you go down to that low energy state by doing high-intensity interval training.

I do what's called a superset countdown technique. Supersets are when you do not just one exercise but two opposing exercises. So I do pullups, which are good for your back and biceps, and pushups, which are good for your pectorals and triceps. You can do one right after the other because you don't have to recover in between because they're different muscle groups. If you go from one exercise to the other right away, it keeps your heart rate up and gives you an aerobic benefit.

The advantage is it builds the right kind of muscle fiber. So you get the right combination of strength and endurance. When I first started this, I would do 12 pullups and 24 pushups. I would count down to 10 pullups and 20 pushups. Then eight pullups, 16 pushups. So now I can do 35 pull­ups in a row!

Q: So Greg, when you hop on a bike these days, where do you like to go?

LeMond: I live in Medina so everywhere from my house is good. But there's some riding over in Wisconsin, near Durand, and in the Viroqua area — that's probably some of the best riding in the country. Truly, it's unbelievable riding. It's no traffic. Hills. It's gorgeous!

Q: By the way, Mark, you're a cyclist and a doctor. What's the scientific reason why we never forget how to ride a bike once we learn?

Hom: The brain is very plastic — meaning it can change. The way it changes is it makes new connections. When you learn to ride a bicycle, your brain is making connections. The motorsensory coordination parts of your brain are making new connections. Once those connections are made, they'll last your whole life.

Allie Shah • 612-673-4488