In hindsight, Decker’s injuries were predictable. If a female athlete pushes too hard and is not eating enough through her teenage years, her body is at risk of not developing the fat needed to produce sufficient hormones to trigger menstrual periods and build strong bones. In 1992, the American College of Sports Medicine addressed the problem, defining the so-called female athlete triad: eating disorder (from internalizing the idea that skinnier equals faster), amenorrhea (lack of menstrual period) and osteoporosis (deterioration of bone). Later the N.C.A.A. started encouraging coaches to consider any athlete who showed signs of the triad as injured, until she was cleared to compete by a medical professional.

The focus on the long-term health effects of intense athletic training is especially important now. Nearly 500,000 American girls ran track last year. Cain, of course, is an outlier. She has broken records that existed for more than 20 years, shattering several of them by more than six seconds, an eternity in track. But she is also part of an expanding pack of very fast American girls. In 2001, only two high-school girls ran the 1,600 meters in under 4:50, and only one ran faster than 4:45. Last year, 46 girls ran faster than 4:50; eight broke 4:45.

This increased speed is an inevitable result of girls’ growing up not just after the passage of Title IX, in 1972, when schools first started rushing to put together girls’ sports programs to comply with the law, but several decades later, after those programs matured. Coaches no longer tend to baby female runners, assuming that they can’t handle as much training as boys. Gone, too, is the outdated idea that the best way to make a girl run faster is to make her skinnier, so that she carries fewer pounds around the track. The dominant philosophy now is that girls, like all other runners, should train to become very strong by lifting heavy weights. Running mechanics are fairly simple: Speed comes from a foot hitting the ground, loading with energy, like a spring, then exploding off with propulsive power. For years, sprinters have trained for strength. Recently this focus has spread to middle-distance runners and even marathoners. All Oregon Project runners do squats and dead lifts, some up to twice their body weight.

From the moment Salazar started coaching Cain, he set out to develop her talent slowly over the course of many years, building her up and holding her back as necessary, aiming for her to peak when most female track runners peak, around age 25. All athletic training depends upon a careful balance of physical stress and rest and is governed by the progressive-overload principle. It holds that if an athlete pushes herself slightly out of her comfort zone — ramping up the distance she’s running, or her pace in sprints, or the amount of weight she’s lifting — then once she rests and recovers from that workout, she’ll be stronger or faster than before. But this adaptation, or supercompensation, as it’s called, lasts for only a short time. The key is to apply training stress again, during that window, to spur more adaptation and increase fitness.

While Cain was still in high school, Salazar deployed John Henwood, a former Olympic runner from New Zealand who lives in New York, to monitor her workouts. His job included encouraging Cain to push hard and embrace discomfort, but also to make sure she didn’t go too far “over the red line,” as Salazar puts it, by adding too much speed or mileage too quickly. Cain responded well to Salazar’s program — to say the least. In 2013, the first year under his guidance, she seemed to smash a record every weekend. She broke the American female junior 1,500-meter record. She broke the American junior 800-meter record, becoming the first girl to do it in less than two minutes. She ran a 4:32.78 indoor mile, breaking the American girls’ high-school record, which was set 41 years earlier. That same year she broke her own record by four seconds. Cain describes that season with considerable understatement as “a lot of fun.” After races, she would give interviews to the doting track media, often holding Puddles, her stuffed duck.

Salazar’s cautious strategy for Cain grew out of his determination not to repeat the mistake that he made in his own prime: burning up remarkable talent early in life with a self-destructively ferocious drive and a more-is-better training philosophy. In 1982, at age 23, Salazar set American records in the 5,000 and 10,000 meters. He also won the Boston Marathon and his third New York Marathon in a row. But, he wrote in his memoir, he was living “a life of extreme athletic excess, as far gone, in my way, as a drug addict or alcoholic.”