Over the past 15 years, there has been an explosion in fitness fads promoting all sorts of dubious concepts. Perhaps the worst of all is the idea that shorter, high-intensity interval training (HIIT) can substitute for traditional long-duration aerobic base work in endurance athletes who are looking to maximize their performance.

To be clear, though many of this idea’s proponents have emerged from CrossFit, I’m not singling out that community. CrossFit can be an effective and transformational approach to working out for some athletes. My beef is with a few of its misguided adherents, some of whom claim that their HIIT programs offer new breakthroughs in training that allow athletes of all stripes to achieve the same results with less training time. Others even go so far as to say that putting in the long hours of endurance work is bad for you. To make matters even worse, much of the popular press—Outside occasionally included—seems to promote these programs.

After more than 40 years as an endurance athlete and coach, it pains me to see people latch onto these fads. I want to clear a few things up.

The first thing to get straight is that high-intensity-focused fitness programs such as CrossFit and P90X bring nothing new to the theory or methodology of training for endurance sports. Coaches have long known about the benefits of HIIT. But the proponents of these shortcuts frequently make claims that simply can’t be backed up. Though research is often cited, it’s important to remember that these studies shouldn’t be taken out of context or extrapolated to infer performance gains.

Often, the HIIT fitness studies that get promoted are short term, use untrained or moderately trained subjects, and often use VO2max for the measure of improvement in the subjects. Several fundamental flaws exist here. First, VO2max is known as a first-wave response to exercise, meaning the adaptations responsible for its initial improvement occur very quickly when one begins an exercise program. Poorly trained individuals will naturally see a big gain in VO2max in short-term studies, although the improvements taper off dramatically after a few months of regular endurance exercise. If a steady diet of HIIT is consumed for too long, an athlete can even get less aerobically fit and see a decline in VO2max.

More important, performance in endurance sports is not well predicted by VO2max. I have coached world-class cross-country skiers with rather mundane VO2max numbers who have stood on World Cup and world championship podiums. I’ve also coached cross-country skiers with extraordinarily high VO2max numbers who, though racing at the Olympic and World Cup level, are not especially competitive. Bear in mind that cross-country skiing is the sport with the highest demands on the aerobic system. Endurance correlates much better with maximal sustained work rate. Sports scientists have several names for this: lactate threshold, anaerobic threshold, critical power/speed. The takeaway: You don’t necessarily need to improve your VO2max to improve your endurance performance. Finally, these studies present the average improvement for the study group. Some subjects may have have shown a decrease in VO2max, but if the average shows an improvement, the study is held up as proof of the efficacy of the training protocols being studied.

The real world of endurance training is much more complex. It is messier than a limited lab study, but with enough data points, it can lead in the right direction. Through a trial-and-error process spanning many decades and literally millions of test subjects (that is, athletes), coaches and competitors have tried myriad training methodologies, rejecting the failures and modifying and refining the successes until we have a very good idea of how to get better.

Now is a good time to reiterate that I’m referring to athletes looking to improve their endurance—someone training for, say, a marathon, gravel grinder, or ski-mo race, not someone simply trying to get some exercise for the health benefits.

If HIIT were the only thing required to become a successful endurance athlete, you could be sure all the pros would be on board. After all, these athletes are looking for every advantage they can get, and their paychecks and careers hinge on their performance every time they step up to the starting line. But instead of doubling down on HIIT, the elite ranks of runners, cyclists, cross-country skiers, rowers, and swimmers do just the opposite. In 2010, a meta-analysis by Norwegian researchers examined the actual distribution of training intensity used by elite athletes across the full spectrum of endurance sports. The conclusion: The best in the world complete about 80 percent of their training volume at low intensity, 7 to 8 percent at moderate intensity, and about 12 to 13 percent at high intensity.

The 80/20 approach, as it’s called, can seem counterintuitive. Most people think they need to train hard all the time if they want to get faster. In reality, however, training slower will make you faster. The reasons come down to physiology. The sustainable duration of high-intensity work mainly depends on the aerobic capacity of your slow-twitch (ST) muscles. The more aerobically adapted your ST fibers are, the greater the intensity you can maintain for a longer duration.

That’s because the fast-twitch (FT) muscles that provide the power and speed of high-intensity work produce byproducts, some of which, when accumulated, will slow and eventually stop those very same FT fibers. You know this as fatigue and sense it as slowing down. But the ST fibers take up and utilize some of those byproducts as fuel, lessening their accumulation and hence the onset of fatigue. The greater the capacity of the ST fibers to remove these byproducts, the higher the intensity you can sustain for longer durations. Sprinkle in some higher-intensity sessions (key word: some) and you can lower the rate of production of these fatigue-associated metabolites from the FT fibers.

The hitch is that training the aerobic qualities of these ST fibers is best accomplished by a high volume of long-duration, low- to moderate-intensity exercise. For many, the intensity should be so low that it doesn’t feel like a workout at all.

Successful training for endurance sports is highly nuanced. Athletes do require some HIIT in their programs, but they need a tiny fraction of what is being proposed by many in the fitness industry. The endurance athlete will use HIIT as a supplement to—not a replacement for—the aerobic base work that makes up the foundation of their fitness.

It may not be very sexy training, but it is what works. It has been proven on millions of starting lines, and it forms the foundation of every world-class endurance athlete’s training program. Professional endurance athletes don’t follow fads. They train by well-understood principles developed over more than 100 years.

Folks should stop looking for a shortcut to fitness.

Scott Johnston is a former NCAA Division 1 swimmer and World Cup cross-country ski racer. In 1981, he made the first alpine-style ascent of Ama Dablam. He has coached World Cup cross-country skiers, elite ultrarunners, U.S. Navy SEALs, and record-breaking alpinist Steve House. Johnston currently teaches and coaches at UphillAthlete.com.