For decades, the conventional view among scientists has been that investing time and energy in one pursuit should detract from investment—and success—in another. This concept, known as a trade-off, is simple enough: The more energy that a bird invests in building a nest, the less energy it has left over to make eggs. Likewise, the more money you spend on a fancy car, the less you’ll have to invest in a nice home.
As an evolutionary biologist, I normally study performance trade-offs in reptiles and amphibians—for example, how swimming performance of tadpoles influences jumping ability when they become frogs. But having recently competed in several triathlons, I began to wonder whether these trade-offs in frogs might be replicated in humans who face off in consecutive swimming, cycling, and running events.
Although the idea of performance trade-offs seems intuitive, few studies of human athletic performance have identified such a pattern. A study of Olympic decathletes found that individuals who jump higher than their competitors tend to also sprint faster and throw farther. A study of professional soccer players found no evidence for trade-offs between speed and stamina, nor between power and agility.
Triathlons, however, haven’t previously been subjected to this kind of analysis, and they’re an ideal sport to study trade-offs in humans. In addition to different body shapes tending to have advantages or disadvantages for each portion, each triathlon event also requires different training methods: Swimmers develop a “feel” for the water by completing lap after lap, cyclists work to build power and fluid pedaling strokes, and runners strive to enhance stride length and leg turnover. Triathletes often combine multiple disciplines in single training sessions. In “brick” workouts, for example, a short run following an intense bike ride helps to build endurance and running fluidity.
To assess the magnitude of trade-offs in the sport, a colleague and I pulled race results from the crown jewel of the multidisciplinary event: the Ironman triathlon, which includes a 2.4-mile swim, a 112-mile bike, and a 26.2-mile run. We put together 86 total races from 32 worldwide venues between 2013 and 2016. This gave us a data set of 149,291 results, including 27,269 women and 122,022 men. We analyzed these data to look for trade-offs, such as whether those who swam well suffered more on the bike, or if those who rode fast shuffled through the marathon.
The answer, it turns out, is a mixed bag. In all but the “elite” athletes (men finishing in under 10 hours, women under 11 hours), we found clear evidence for trade-offs in performance between some (but not all) pairs of events. For slower athletes, cycling and running performance had a negative correlation, which became stronger and stronger as race times got slower and slower. Translation: Those who cooked themselves while biking consistently set themselves up for a long, brutal slog in the run, and the slower the athlete, the more devastating the consequences. And while our prediction was that the broad shoulders of the best swimmers would be costly in terms of aerodynamics on the bike, our analysis shows that those who swam well generally performed well on the bike—weaker evidence for performance trade-offs.
In contrast to the slower athletes, both the male and female elite athletes excelled in all three disciplines without showing any evidence for trade-offs in performance. Whereas the correlations between athletic performances were largely negative among the slower athletes, correlations in elite athletes remained positive. To put it another way, the more modest triathlon finishers look a lot like that expensive car parked in front of a dilapidated duplex, the elites defy convention, parking their Land Rovers in front of mansions.
Previous sports science studies couldn’t find evidence for these trade-offs because they focused on elite athletes. In a pool where there’s relatively little variation in athletic ability—a group of Olympians, for example—the trade-offs in performance are masked. In our data set, professionals, elite amateurs, and athletes with a simple goal to finish the race are competing side by side, allowing the whole picture to emerge.
Our study revealed a major take-home message for any aspiring Ironman finisher who isn’t an elite athlete: Get as fit as possible, overall, without trying to master all three events. Although you still need to be physically capable of completing the entire swim, bike, and run, gaining a handful of minutes in the water only to give them back on the pavement is not worth the dedication it would require of the average weekend warrior. Our study showed that if you allocate extra time anywhere in your training, you should do it on the bike to reduce the toll of the run. (This result is in line with popular triathlon training lore: Legs that have been spinning circles for 112 miles will rebel when asked to stride upright through the duration of a marathon.) And although elite triathletes can race as if trade-offs don’t exist, the rest of us should remember that just like buying too much car, pushing too fast and too early will cost you elsewhere later.