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There’s a striking parallel between two of the most famous case studies ever published about elite endurance athletes.

In 2005, University of Texas physiologist Edward Coyle published a longitudinal study that tracked Lance Armstrong’s lab data over a seven-year period. The title of the article conveyed the most remarkable finding: “.” By the time he was 28, Armstrong reportedly needed 8 percent less energy to generate a given power output on his bike than when he was 21.

The following year, University of Exeter physiologist Andy Jones 11 years of lab data from Paula Radcliffe—and he too pegged a steady improvement in efficiency as one of the crucial adaptations that enabled Radcliffe to set the still-standing marathon world record of 2:15:25. Between 1992 and 2003, Jones reported, Radcliffe’s VO2max remained relatively unchanged, but her running economy—a measure of how much energy is required to run at a given pace—improved by 15 percent.

On the surface, the takeaway from these studies is straightforward. To perform at your best as a cyclist or runner, it’s not enough to make your engine more powerful; you also have to make it more efficient. But how? For decades, scientists have struggled to pin down what determines exercise efficiency and how to change it, with little consensus. In fact, it’s far from clear that it’s even possible to significantly increase cycling economy, which (along with all the other rumors swirling around back then) made Armstrong’s results .

This lingering debate is what makes a new study on running and cycling efficiency from researchers at the University of Colorado Boulder, published in the , particularly intriguing. The authors have plenty of scientific and athletic cred: Rodger Kram, the head of the university’s Locomotion Lab, led the external testing of Nike’s Vaporfly 4% shoes last year; visiting student Wannes Swinnen was an elite triathlete in his native Belgium; Shalaya Kipp, a graduate student in Kram’s lab, is an Olympic steeplechaser.

The study itself is fairly straightforward. The research team assembled three groups of well-trained endurance athletes: ten runners, nine cyclists, and nine triathletes. All were highly experienced, training for about nine hours a week on average—virtually all running for the runners, all cycling for the cyclists, and 5.6 hours of cycling and 3.9 hours of running for the triathletes. The researchers measured their efficiency while running at 8:00 mile pace and cycling at 200 watts. (There are a number of different ways of measuring efficiency and different terms like “running economy” and “cycling economy” that have specific mathematical meanings. For the purposes of this article, I’ll use the general term “efficiency”; if you improve your efficiency, you need less energy to sustain a given pace.)

There are two headline results in the new study. The unsurprising one is that the runners were more efficient than the cyclists at running (with the triathletes somewhere in the middle). On average, the cyclists had to burn 21 percent more energy than the runners to maintain the required pace. That’s partly because the cyclists were a bit heavier on average, but even if you correct for weight, they were still 10 percent less efficient. The more surprising headline, in contrast, is that the cyclists �ɱ�����’t more efficient than the runners or triathletes at cycling. Statistically, all three groups were pretty much the same in the cycling test.

Those are the facts. What it all means, on the other hand, is a bit of a Rorschach test. Over the years, numerous theories have been proposed to explain how or why exercise efficiency changes. Jones’ paper on Paula Radcliffe, for example, notes that a training-induced transformation of fast-twitch muscle fibers into more efficient slow-twitch fibers could improve efficiency. Radcliffe’s flexibility also got worse over the years, a factor that to better efficiency. And she did weight training to improve her strength and power, which may have translated into better efficiency through improved neuromuscular signaling.

What’s interesting here, though, is the difference between running and cycling. It’s pretty well-established that experienced runners are more efficient than newbies. The fact that well-trained cyclists don’t get more efficient at running suggests that this effect isn’t simply a consequence of better fitness or of some general internal adaptation like more slow-twitch muscle fibers. It’s something specific to the act of running.

The temptation, of course, is to assume that experienced runners are more skilled at the act of running. They have less vertical bounce, or they overstride less, or move their arms more smoothly, or whatever. There may be some truth to this. But there are also more subtle possibilities. When I pressed Kram and Swinnen for their preferred explanations, they pointed out that efficient runners use their stretchy tendons and ligaments to store elastic energy to be “recycled” from stride to stride. The push and pull between tendon and muscle is so finely tuned that your muscles stay roughly the same length throughout the stride instead of shortening and lengthening with each contraction. Optimizing this aspect of running is invisible to the naked eye and beyond conscious control, but it may be one of the crucial skills that improve with experience.

Cycling, on the other hand—and bear in mind that I’m a lifelong runner writing this—is relatively simple. Your legs are constrained to move along a specific path. There’s very little contribution from stored elastic energy and very little movement in the rest of your body to be optimized. over the years have investigated whether experienced cyclists are more efficient than beginners or whether efficiency improves with training, and the results have been mixed. This new study, which found no difference between cyclists and noncycling runners, is by no means the final word on the topic—but it adds to the impression that any efficiency improvements you get from becoming a more skilled cyclist are small at best.

(A caveat: The cycling in the new study was performed on stationary bikes at a predetermined cadence. It’s possible that the benefits of experience and bike-specific training would be more noticeable on the open road at a freely chosen cadence.)

From a practical perspective, the journal article concludes with this somewhat provocative sentence: “Cyclists who typically spend many hours riding their bike could seemingly replace some time-consuming cycling training with shorter sessions of running training without experiencing negative effects on their [cycling efficiency].” My intuitive reaction to this is that it’s a bit silly—after all, the direct training benefits of cyclists are far broader than just efficiency. But when I try to think of specific counterarguments—“cyclists who replace some of their training with running would miss out on X, Y, and Z”—the case doesn’t seem quite as obvious.

Sure, bike-handling skills and situational awareness are important. And there’s a bit of evidence that improvements in other physiological parameters, like VO2max and lactate threshold, are specific to the mode of exercise you’re most familiar with. But maybe there really is some merit to the idea of cyclists putting on running shoes for cross-training, particularly those whose full-time jobs prevent them from spending six hours a day in the saddle. Conversely, it seems like runners who supplement their training with cycling are missing out on some potential efficiency gains (though that “loss” may be more than compensated for by the ability to rack up more aerobic training without incurring injuries).

So many caveats and on-the-other-hands. Is there anything we can say for sure? How about: Yes, you can get more efficient at running. You probably can’t get more efficient at cycling—at least not by an amount that’s worth spending a lot of time fretting about. Which means that the “better efficiency” narrative is still plausible as an explanation for how Paula Radcliffe developed into a world-beating athlete. For Lance? Not so much. But then, Oprah could have told us that.

My new book, , with a foreword by Malcolm Gladwell, is now available. For more, join me on and , and sign up for the Sweat Science .