Modern racehorses don’t race nearly as long or as often as they once did. In the middle of the last century, for example, horses would compete in dozens of races over a span of four or five years. Today, if a champion racehorse shows signs of injury, the financially sound decision is to get them out of competition and into a stud barn as quickly as possible. It’s one of many reasons people list when explaining the sport’s declining popularity.

For the athletes who ride the horses, the opposite is often the case. Much like in golf, the career of a professional jockey can stretch for three decades or, for a select few, even longer. Unlike golfers, however, jockeys must endure the incredible strain (and the life-threatening danger) of sitting on top of thousand-pound animals running in packs as fast as automobiles. In order to make weight for races, most jockeys keep up intense fitness regimens and strict diets year round; their small, muscular frames have almost zero body fat. As a result, they can take a licking. “I bounce better than most fifty-two-year-olds,” Gary Stevens, the oldest jockey competing in the hundred and forty-first Kentucky Derby, on Saturday, says.

After thirty-six years of professional racing, Stevens is among the most celebrated jockeys in Saturday’s race. He’s ignored a lot of doctors along the way. When Stevens was six years old, he was diagnosed with Legg-Calvé-Perthes disease, a childhood bone disorder, which required him to wear a full-length leg brace. Doctors told Stevens that he would likely never compete in athletics, or even walk again without pain. “I learned at a young age that we can overcome a lot, and our body has amazing healing power,” Stevens said. After eighteen months, he shed the leg brace. In high school, he joined the wrestling team, eventually earning college-scholarship offers. By the age of sixteen, he was riding thoroughbred horses conditioned by his father, a trainer. He dropped out of school in 1980 to ride full time.

Five years later, doctors were telling Stevens that his career was over. One morning in 1985, Stevens was helping a trainer get a young horse used to breaking out of the starting gate, a dangerous job that veteran jockeys usually avoid. Stevens had been warned that that the horse, a filly named Irish Crystal, had given other riders trouble. But he was twenty years old, and considered himself invincible. “Things didn’t go right that morning,” Stevens recalled. The horse tried to turn the wrong way on the track and collided with the inner rail. Stevens was catapulted into the rail, tearing cartilage from his right shoulder and his right knee, giving him a concussion and rupturing his eardrum. He was found bleeding from the nose and ears. He fell into a coma for sixteen hours.

When Stevens regained consciousness, a doctor asked him if he knew what had happened: “Yeah, my horse made a left-hand turn out of the gate.” The doctor looked at Stevens’s wife and asked, “Does your husband have a speech impediment?” Stevens’s words were coming out jumbled. He had lost equilibrium and some of his speech abilities. In addition, he would require surgery on his his shoulder and his knee. “When can I start riding again?” Stevens asked the doctors. Maybe never, they replied.

A year later, Stevens was the top jockey on the West Coast, and had captured the riding title at Santa Anita with a hundred and three wins. On the first Saturday in May of 1986, Stevens found himself riding in the Kentucky Derby in front of over a hundred and twenty thousand people. He was on Wheatly Hall, a horse that had raced only four times and had never won a graded stakes race. Few people gave the young up-and-coming jockey and the inexperienced horse much of a chance, and they were right. They lost to another long-shot horse, Ferdinand, and a jockey, Bill Shoemaker, who most people figured was well past his prime: Shoemaker was fifty-four years old when he rode Ferdinand to victory, making him the oldest jockey to ever win the Kentucky Derby, a record that stands to this day.

Stevens idolized Shoemaker, who encouraged Stevens to keep riding for as long as he could enjoy it. “I remember telling Bill Shoemaker when I was twenty-six years old that I was going to retire when I was thirty, and he just laughed at me,” Stevens told the Baltimore Sun. “He said, ‘Mark my words. You won’t.’ ”

During the next three decades, Stevens won over five thousand races, including three Kentucky Derbies; in 1997, he just missed the Triple Crown. He was considered among the top jockeys in America, and was certainly one of the most famous; he had a major role in the 2003 film “Seabiscuit,” as the jockey George Woolf. But Stevens’s knee never fully recovered from his accident in 1985, and he had been riding in pain the entire time. He didn’t know it, but his right ACL and MCL had been detached in the accident, and his doctor chose not to reattach them (“You’re not a running back in the N.F.L.,” Stevens said his doctor explained, years later). The only thing stabilizing his knee was his femur, which had grown into his tibia. He treated the pain with periodic injections of lubricants that provided temporary relief, and he had over a dozen surgeries. The pain was enough to sideline the jockey for ten months in 1999; by the end of 2005, it had become unbearable.

That year, Stevens had piloted a racehorse named Rock Hard Ten to an undefeated four-year-old season. In November, Rock Hard Ten’s trainer announced the horse’s retirement after veterinarians discovered that his left front hoof had no cartilage left—the joint was just bone on bone. Stevens called Rock Hard Ten the best horse he had ever ridden, and the decision to retire the horse gave Stevens pause. His own knee was in no better shape than the horse’s hoof. “I said, ‘That’s it. He’s retiring, I’m retiring,’ ” Stevens told the Associated Press at the time. “There is a piece of my heart that would love to continue riding, but my body can’t take it anymore.”

Stevens stayed close to the racetrack in his retirement. He worked as an analyst for a number of TV networks, as a jockey’s agent, and even trained a few horses. In 2011, he signed a five-season deal to star alongside Dustin Hoffman and Nick Nolte in the HBO drama “Luck.” The show was cancelled during the first season after public outcry over the number of horses that were injured on set, including three that died.

Stevens had hoped acting would provide him with the adrenaline rush he missed from racing, and for a while it did. After “Luck” was cancelled, Stevens grew anxious. “That had a major effect on me, emotionally,” Stevens said. He had grown to identify with his character, Ronnie Jenkins, a hard-drinking jockey who was struggling to get back into racing after injuring himself in a serious accident. “He had a lot of demons, a dark side to him,” Stevens told the Los Angeles Times after the cancellation. “I sort of became that guy.” After stewing about the show’s demise for several months, Stevens decided to take a page from Ronnie Jenkins and get back on the racetrack. “I felt there was unfinished business,” Stevens said. In January of 2013, he announced his comeback.