Garrett Koeppicus of Brooklyn, N.Y., knows folding bottle caps with one hand and ripping apples in half with his fingers make excellent party tricks. But to him, they’re fitness training. He wants to be strong enough to hold on to millimeters-thin ledges of rock high off the ground.

To scale gym walls and rock faces, advanced climbers like him need durable digits. The most complicated moves often require holding on to just a nub of stone with the strength of the fingertips.

Emily Harrington, a professional free climber based in Olympic Valley, Calif., has two hangboards installed above the doors in her condo. The mountable pieces of wood or plastic feature varying holds—small edges and pockets often less than an inch deep—that climbers hang on or even do pull-ups from.

“I have three, which is sort of embarrassing, portable hangboards that I travel with,” she says. “They can be hung from just about anywhere that’s weight bearing—a tree, a staircase.” That way she can get what she calls finger contact when she can’t climb.

Ms. Harrington, who is currently in Ecuador climbing and skiing volcanoes, says she affixed a hangboard to her hut situated at 14,000 feet, and hangs from it to train when “we’re not on the mountain,” she says. Locals “get pretty entertained.”

One of the most important components of rock climbing is the ability to grip small nubs and hang from micro-thin edges. Climber and coach Luke Livesey shares some tips for developing fingers of steel. Photo/Video: Natalia V. Osipova/The Wall Street Journal

The training tool had a cameo in “Free Solo,” about Alex Honnold’s effort to scale El Capitan in Yosemite National Park without a rope. The film had the best-ever opening weekend for a documentary earlier this fall. In one scene, shortly after his record-breaking summit, we see Mr. Honnold doing pull-ups from a hangboard attached inside the van where he then lived.

A new audience is embracing rock climbing as the film and others such as “The Dawn Wall” help spread interest in the sport. Climbing also makes its Olympic debut at the 2020 Tokyo Games. Last year saw a record number of 43 climbing gyms open in the U.S., double the number opened in 2016, according to Climbing Business Journal, a website dedicated to the indoor-climbing industry.

Fingers have no muscles, only tendons. The 34 muscles that move the fingers and thumb are found in the hand and forearm. L. Scott Levin, president of the American Society for the Surgery of Hand, says there aren’t many practical reasons to train these muscles unless you are a neurosurgeon, watchmaker or climber.

In 1988, German climber Wolfgang Güllich developed a training tool called the campus board to help him build the specific finger strength he would need to ascend a famously difficult climb in Germany. The suspended wooden board has a variety of different-size rungs that you climb without the aid of your feet. The strongest climbers can “hop” rung to rung with their fingertips.

Emily Harrington practices at home. Photo: Jamie Kingham for The Wall Street Journal

Newer tools include the Block, a mini-fingerboard with an assortment of edges, pockets and pinches you can use to lift weights or attach to a cable weight-training machine. The Rockblob is a hollow, orange-size ball suspended from a rope to practice standard holds. They all have names, such as the half crimp, where the fingers are bent only at the second knuckle.

Wackier gadgets include crimp training resistance bands—essentially mini rubber bands that loop around each finger to stretch and strengthen them. After the workout, acupressure finger massage rings aim to help with recovery.

Emily Varisco, head climbing coach at The Cliffs Climbing + Fitness in Long Island City, N.Y., gives tutorials that can help newbies build strength. Beginner exercises include balling up putty and pushing it out flat with both bent and straight fingers, and opening and closing fingers in a bucket of rice.

“I’ve thought about using peanut butter for resistance, but that might be too messy,” she jokes. Resourceful climbers embrace daily habits, like carrying their heavy grocery bags home with just one finger.

Climbers can see big improvements from these kinds of helpers. “If climber A can hold on to a hold using 40% of their maximum finger strength versus climber B who is at 90% of their maximum, climber A will have more left in the tank on the route,” says Zack DiCristino, the Vail, Colo.-based lead physical therapist for USA Climbing.

Finger-strength workouts started taking off about seven years ago, says Dave Wahl, a coach at Movement Climbing and Fitness in Boulder, Colo. He credits Eva López, a Spanish climber and coach whose Ph.D. thesis at the University of Castilla-La Mancha was on finger strength and endurance training.

Ms. López launched a training program and corresponding hangboard called the Transgression in 2011 based on her research. The concept is like powerlifting for your fingers, with hangs rarely surpassing 30 seconds and the entire workout lasting 10 to 20 minutes. For the first four weeks, climbers cling to a medium-size edge with added weights on their body for 10 seconds, or until they can’t take anymore. For the second four weeks, they hang from the smallest edge possible with no added weight for up to 10 seconds.

Tyler Nelson, a climbing coach and founder of Camp4 Human Performance in Salt Lake City, is considered a finger-strength guru in the climbing world. His office (and Instagram feed) are littered with charts and graphs that track increases in finger strength using various experimental training methods.

“You don’t know the force going through your fingers without measuring it,” he says. “By tracking the force-per-pull during a finger-training session, you know when to manipulate training intensity, volume and frequency.”

Alex Honnold’s climb of El Capitan in Yosemite National Park without a rope is the subject of “Free Solo.” Photo: Jimmy Chin/National Geographic

Dr. Nelson cautions that equipment like hangboards can be dangerous for new climbers. “Tendons and joints in the fingers of the most seasoned climbers are fragile,” he says. “It takes years of climbing to prepare your fingers for that type of strain,” he says.

Just as blisters are badges of honor for distance runners, callused fingers become a rite of passage for any climber. Brooklyn-based climber Luke Livesey uses a double-edged razorblade to trim gnarled calluses and finishes the job with pro-grade sandpaper. He says some friends prefer battery operated mini-belt sanders.

Serious finger-grippers also know better than to rely on touch-ID fingerprint sensors, says Will Anglin, co-founder of Denver-based gear company Tension Climbing. “We tried to use a fingerprint scanner to clock employees in and out,” he says. “But since we’re all climbers, our fingers were too callused for them to register.”

Write to Jen Murphy at workout@wsj.com