THREE YEARS AGO, Jeff Mitchell, a 42-year-old business consultant from Jackson, Tennessee, doubled his maximum bench press from 135 to 280 pounds, cut two seconds off his 100-yard-dash time, lost 40 pounds, and shed six inches from his waistline—all in just over a year. His muscles bulged, his skin looked smoother, and he hadn't felt so good since playing college basketball.

"You on steroids or something?" Mitchell recalls a friend asking him.

In fact, there was nothing illicit behind Mitchell's transformation, which some believe may hinge on an exercise-induced upswing in the body's production of human growth hormone (HGH). The catalyst? A short but super-high-intensity workout called Sprint 8 (see "Explode into Shape"). The program had Mitchell running sprints down his street four times a week, leaving him heaving for air and nearly passing out, while his neighbors looked on with bewildered amusement.

The Sprint 8 program was quietly introduced in 2000 in Ready, Set, Go! Synergy Fitness, by Phil Campbell, a strikingly muscular 53-year-old masters runner and college speed coach from Jackson. Back in the mid-nineties, Campbell was a hospital administrator with a doughy gut that wouldn't tighten up no matter how many miles he ran. Then one year, to prepare for his family's annual Thanksgiving flag-football game, he started adding sprints to his daily 45-minute runs. By game day, two months later, he'd shed ten pounds and "felt like 17 again." Campbell kept sprinting through the winter, and as the weight fell off and his muscles firmed up, he reduced his running time while adding more sprints—finally dialing in a 20-minute routine that included eight 30-second intervals.

Other people around Campbell's hometown took notice, adopted his sprint program, and promptly saw similar results. Burt Gillmann, a 39-year-old building project manager, dropped 35 pounds off his six-foot frame. Masters bike racer Tom Gee, 54, claims the sprints helped him clock his best 40-kilometer time trial in 30 years.

Seeing these results prompted Campbell to do some armchair thinking about the science behind his success. Short bursts of intense activity have long been a staple of workouts. (Remember your high school coach making everyone do wind sprints?) But Campbell was interested in explaining the tangible benefits of his routine, especially the dramatic fat loss and notable increase in lean muscle mass. His research of the scientific literature led him to studies that documented a link between intense activity and a natural increase in HGH.

Linking HGH to weight loss and increased speed and strength made for a promising connection, since thousands of aging Americans now inject synthetic HGH—at costs ranging from $500 to $1,000 per month—for a buffet of purported health benefits, including improved sleep and libido. What if Campbell had stumbled upon an all-natural way to achieve the same thing?

Campbell is convinced he has, pointing to a 2002 study in Britain's Journal of Sport Sciences showing that 30 seconds of all-out cycling increased HGH levels by 530 percent over nonexercisers' base levels. Thus, says Campbell, "the harder you work, the more HGH you produce." This, along with a 2003 study in the Journal of Clinical Endocrinology and Metabolism concluding that "the beneficial effects of exercise can mimic the effects of HGH treatment," prompted renewed interest in high-intensity training and equipment (see "Time Machine").

Many scientists, however, aren't sold on the connection. "There's no doubt that high-intensity training is potent, but the theories linking improvements in fitness to HGH are still very speculative," says Dr. Martin Gibala, a muscle physiologist at Canada's McMaster University, in Hamilton, Ontario.

Dr. Mike Joyner, an exercise researcher at the Mayo Clinic, in Rochester, Minnesota, explains that while exercise- induced HGH production is a fact and HGH is known to boost strength and shrink fat cells, its full role is unclear. "There are probably 10 or 12 things going on during high-intensity exercise, and all we can say for sure right now is that HGH is one of them."

Whatever the case, as long as Sprint 8 delivers results, true believers like Jeff Mitchell will keep blitzing neighborhood streets. There's no downside to trying it, unless you don't like the hard work.

"It's tough to get my patients to do Sprint 8 because it pushes them to uncomfortable levels of exertion," says Dr. Chet Gentry, a family practitioner in Sparta, Tennessee, who says his own LDL cholesterol level dropped 60 points on Campbell's program. "But those that stick to it will see very good results."