Bonnie Tsui has a commute—of sorts. Before dawn, five days a week, she leaves her house in Berkeley, in the San Francisco Bay Area, and drives for an hour to the seaside community of Pacifica. There, she dons a wetsuit, grabs her surfboard, and strides into the ocean as the sun is rising.

It’s a ritual that has Tsui doing something she loves; she’s 48 and has been surfing for the past 18 years. As you might imagine, it also gives her a strenuous workout. All of which helped make her well equipped to write her new book, On Muscle: The Stuff That Moves Us and Why It Matters (Algonquin, April 22).

Whereas Tsui explored her lifelong passion for swimming in her previous book, Why We Swim (2020), On Muscle looks more broadly at the science behind movement itself. How exactly do muscles work? What differentiates certain muscles? What’s behind muscle memory? “Muscles deserve more consideration than we give them,” Tsui writes. “Muscles are smarter than we think. They have different personalities. They remember things.”

For the book, Tsui travels to the University of Texas at Austin to visit Jan Todd, profiled by Sports Illustrated as the “World’s Strongest Woman.” At the University of California, San Francisco, she observes the dissection of a human cadaver: “I want to actually see muscle for the first time.”

On Muscle is also a cultural examination of her subject. She writes about standards of muscular beauty, questioning, for one, the absurdly top-heavy depictions of American superheroes. The book’s graceful illustrations of various muscles are hers.

At the heart of the book is an affecting account of the author’s relationship with her father. An exercise fanatic, he led Tsui and her brother in workout routines at their makeshift home gym on Long Island. Together, they soaked up martial arts movies. Her paternal grandfather died of a heart attack at age 64, so, writes Tsui, “my father has been preoccupied with outrunning death ever since.” She seldom sees her father these days. After her parents divorced, he moved back to his native China. “When I reflect on why I wanted to write a book about muscle,” she writes, “I realize that a lot of it has to do with a longing for my dad. I found myself wanting to write about things I can talk to him about.”

In her words, Tsui is a “mortal of precisely average size and strength.” She laughs easily, her playful nature reflected in her breezy writing. I spoke with Tsui in a park near her home in Berkeley, where she lives with her husband and two sons. Our conversation has been edited for length and clarity.

A common way of praising a book is to say that you couldn’t put it down. And yet, reading your book, I felt I did need to put it down. I was very conscious of sitting on my butt and feeling like I needed to get up and stretch or do some pushups. Did you realize how much you might change people’s reading habits?

I’m extremely glad that it made you get up and walk around. We all sit too much, don’t we? It’s really funny to think that the experience of reading the book had to be interrupted. I love that.

What’s the most unusual thing you discovered about muscles in researching the book?

One of the things I didn’t know about muscle is that it’s an endocrine tissue. You think about it as a tangible thing that’s for locomotion, right? It’s mechanical. But the fact that it’s an endocrine tissue and sends signaling molecules all over your body—I thought that was so cool. And it went such a long way to explaining that brain-body connection, and what movement does for your brain. Brain structures like the hippocampus get bigger with exercise. Bulking up your muscles literally bulks up your brain. I mean, that’s just so great. We humans are built to move, and so when we don’t, lousy shit happens.

Do you have a regular exercise routine?

I still swim a lot, but now surfing is taking up a bigger part of my time—in part because it’s just really fun to learn something and adapt to it. Exercise as play is a theme in the book. I highly subscribe to it, and want everyone to, because that’s what’s going to keep you doing the thing, because you love it, and it brings you joy. But I will say that all of the aches and pains of aging have been alleviated in great part by lifting weights. I had been struggling with this nagging shoulder, from a lifetime of swimming and then paddling. Now my shoulder feels amazing. I just spent a week surfing in Costa Rica, and I feel better than I’ve ever felt in my surfing life, which is crazy.

You started surfing at 30. Does that mean there’s hope for the rest of us?

Surfing is one of those things that you can’t be good at it in a year—it takes so much time to actually not feel like a total idiot every time you go out there. And so to put in the time and feel that there is progress being made is obviously important.

I think of surfing as a rare sport in which you’re responding to nature, with only a board between you and the water.

What’s cool about surfing is that the ocean is constantly changing, and you have to use your muscles to figure out how to work with the water in a way that promotes this feeling of unity and flow.

You describe yourself as a lifelong athlete. What’s the most grueling sport or exercise you’ve ever done?

Probably crew, in college [on the Charles River, while at Harvard]. It was so hard. Sometimes in the gym, I still find that to be the quickest way to exhaustion. You can erg [row on an ergometer] for two minutes and be completely dead on that machine.

What did you learn about muscles in drawing the illustrations for the book?

When you draw something, you have to break it down into its components. I think I understood how all the different muscles are individual and yet working in concert—especially with the shoulder. The shoulder is very complex—there are, like, 17 muscles working on the scapula. I threw away so many terrible sketches.

Your father was a professional artist—he drew some of the cover images for the Choose Your Own Adventure book series. Does he still draw?

He does, but not for work anymore. He’s pretty much retired, but he does it every day, like a practice, just like exercise. He does it because he has to do it, for his health, for his well-being. It’s just who he is—it’s his identity to be an artist and a martial artist.

In your opinion, what’s the most underappreciated muscle?

I loved learning about the arrector pili, the muscles that give you goosebumps. They’re the little muscle fibers that are around your hair follicles. It’s cool because they’re all the muscles of fear, awe, and emotional regulation, but also your state-of-being regulation. There’s something very existential about them that I love because you can’t quite control them.

John McMurtrie is the nonfiction editor.