Last March, Jon Arnow descended 4,400 vertical feet on crutches, from the South Rim of the Grand Canyon to the Colorado River, and then, the same day, hobbled back out. He started at dawn with four companions, following the others down the winding trail in a silent, focused lope. Strangers sometimes mistake Arnow's disability for nothing worse than a broken bone, but he is partially paralyzed from the waist down. On his right side, his nerves still stimulate, in his words, an "itty-bitty" hamstring, a quadriceps the size of another man's forearm, some gluteus, and a hip flexor just strong enough to swing his leg forward. His left leg has none of this. It is completely inert, but with a rigid brace, he can balance on that foot while he advances his crutches.

It is almost 10 miles on the Bright Angel Trail from the rim to the Colorado River, hidden among the canyon folds. The group made an unhurried descent and arrived on the riverbank before noon. They snapped a few photos and then began to climb back out. Arnow tripped. He took a few more strides and fell again. He sometimes worries about inconveniencing his able-bodied partners, and now, almost a vertical mile below the trailhead, he read desperation in their faces. One was an old friend, the others were recent acquaintances. "I've got this completely under control," he said. His muscles had gotten used to the descent, and he had to adjust. He climbed to his feet again and, later, insisted on detouring to an overlook above the river. "It was a mile and a half out, and everyone's looking at me like, 'Dude, don't do it,' " he recalls. "And I'm going, 'We're leaving nothing on the table.' "

Arnow is soft-spoken and attentive. He can appear either robust or diminutive. In 2013, New Mobility Magazine ran a photo of him from the waist up, wearing a tank top in the sun, flexing his right arm, biceps and deltoid surging, a contrast to his slender legs. His dark hair is beginning to gray, and he has a strong, open face without hard angles. By the time the group approached the rim of the canyon late that afternoon, Arnow was staring at his feet, measuring the final miles in short, tired increments. They stopped to rest, and one of his friends switched on a video camera, narrating for a later replay: "Less than a mile to go and this thing is kicking our butts."

"If I had one," Arnow joked.

Today, disabled adventure sports – "adaptive recreation" – are more popular than ever before. You might chalk it up to innovation; in the last few decades, injured athletes have refined the tools and skills they need to get outside. You might point to the long wars in Iraq and Afghanistan, which have disabled thousands of Americans and channeled public and private dollars toward their sports. Whatever the cause, the limits have shifted for those with physical disabilities. Organizations around the West have found ways to adapt almost any adventure sport; a disabled person can raft, surf, water ski and paraglide – if necessary, in a rig hitched directly to a wheelchair – and that list scarcely scratches the surface. Last year in Colorado, Vail hosted one of the country's first competitions for disabled rock climbers, and an Iraq War veteran who'd lost both arms in combat completed the state's prestigious Leadville 100 Mountain Bike Race. In February, adventurer Sean O'Neill became the first paraplegic to ice climb Telluride's 365-foot Bridal Veil Falls.

But for many disabled athletes, particularly those with spinal cord injuries, getting outside remains a tough and unglamorous endeavor. Arnow was the first person to crutch-hike the Bright Angel Trail – in the end, the round-trip took him 12 hours – but he is quick to note that such records may have little meaning. Partial injuries such as Arnow's, in which the spinal cord is not completely severed, are idiosyncratic, with unpredictable symptoms. "There aren't that many of me around," he says. "I had to come up with a lot of stuff on my own."

Arnow grew up in Syracuse, N.Y. At the age of 26, when he was enrolled in the Yale School of Medicine and not yet paralyzed, he was assigned a rotation in family practice in Kayenta, Ariz., a Navajo town a few miles from the buttes of Monument Valley. Until that summer, he'd slept only a few nights outside, but he joined a Utah raft trip, and hiked in the high desert and the Rockies. "Snow in the summer, and I've never seen mountains, wildflowers," he recalls. "It was an epiphany. I went back to Yale an absolutely transformed person." Intent on returning to the West, he applied for a residency at the University of Colorado hospital in Denver and, while there, made his first backcountry ski turns and learned to climb. After graduation, he partnered with a young alpinist named Charlie Fowler, who was making a name for himself as one of the most daring and dedicated climbers on the continent. Together they scaled ice routes in Canada, and put up a new line on an 18,000-foot peak in the Peruvian Andes. Arnow established himself as an ear, nose and throat surgeon in Reno, Nev. For the next 15 years, he crisscrossed the Sierra Nevada range, climbing and skiing. Each winter, he spent a week in British Columbia, hunting powder turns by helicopter.

Arnow took great pleasure in moving through remote terrain swiftly on skis. He avoided resorts, but in February 2002, he joined a friend at Alpine Meadows, a ski area in California. They rode a chair to the top of the mountain and hiked to the head of a chute called the Keyhole. The air was cool and clear. Lake Tahoe glinted in the distance. Arnow clicked into his skis and dropped in. He took two turns and yelled up to his friend, "I'm good!" He turned again and lost control.