Cycling and Running Take the Lead
Every triathlete knows the mantra: swim, bike, run. But when it comes to predicting your overall IRONMAN® performance, which of those three disciplines really matters most? A new large-scale study, published in 2025 in Sports Medicine – Open, offers a clear answer — and it might just change the way age-group athletes think about training.
The research team analyzed data from a staggering 687,696 IRONMAN® finishers spanning 20 years (2002–2022). The dataset covered every age group from 18–24 all the way to 75+, with both male (553,608) and female (134,088) athletes represented. By crunching the numbers, they set out to understand which discipline — swimming, cycling, or running — is the best predictor of total race time. The findings point to a consistent truth: cycling and running dominate the equation, while swimming lags far behind.
This article is based on the study by Knechtle et al. 2025 and reflects my own understanding of its findings. The summary is written in my words and offers my interpretation, not an official reproduction of the original publication. While I strive for accuracy, please consult the original paper for full details.
Knechtle, B., Stöggl, T. & Rüst, C.A., 2025. Cycling and running are more predictive of overall race finish time than swimming in IRONMAN® age group triathletes. Sports Medicine – Open, 11(1), 45. Available at: https://doi.org/10.1186/s40798-025-00835-8
The Numbers Don’t Lie: Bike and Run Rule the Day
When the researchers calculated the correlation between each discipline’s split time and overall finish time, the results were strikingly consistent. For both men and women, cycling and running showed very strong correlations with overall race performance:
- Women: Cycling r = 0.88, Running r = 0.89
- Men: Cycling r = 0.89, Running r = 0.90
Understanding Pearson’s r
Pearson’s correlation coefficient (r) expresses the strength and direction of a linear relationship between two variables. The value ranges from –1 to +1: the closer it is to +1, the stronger the positive link; the closer to –1, the stronger the negative link. An r near zero indicates little or no linear relationship. In practice, higher absolute values of r suggest that changes in one variable are more reliably mirrored by changes in the other.
Swimming, by contrast, had a noticeably weaker relationship with total time. While the paper doesn’t suggest swimming is unimportant — after all, you can’t win if you don’t get out of the water — it highlights a reality most experienced triathletes already sense: over the long haul of an IRONMAN®, the bike and run legs are where the race is truly decided.
From a physiological perspective, that makes sense. Cycling and running occupy far larger chunks of the total race duration, meaning that improvements in these areas have an outsized effect on final results. Gain three minutes in the water and you’ve improved your day. Gain ten minutes on the bike or run, and you’ve transformed it.
The Age Effect: Swimming’s Predictive Power Fades Fast
One of the more intriguing aspects of the study is how age changes the equation. Across all three disciplines, correlations with overall time decreased as athletes got older — but the drop-off was steepest for swimming. In younger age groups, swim time still had some predictive value. By the later decades, its influence was minimal.
The implication? For older age-group athletes, swimming performance becomes a less reliable indicator of the overall IRONMAN® performance. In part, this could reflect a growing emphasis on energy management and pacing in the longer disciplines as we age. It may also signal that, past a certain point, differences in swim speed become less pronounced across the field, while differences in bike and run performance remain significant.
For coaches and athletes, this suggests a more targeted approach: maintain swim efficiency, but invest the bulk of training energy into the bike and run, especially for those in the masters and veteran categories.
Gender Gaps: Narrow in the Water, Wide on Land
The study also sheds light on a long-standing curiosity in endurance sport — the gender IRONMAN® performance gap. In swimming, men and women posted times that were surprisingly close. The spread between average male and female swim splits was notably smaller than in either cycling or running.
This finding aligns with other research in open-water swimming, where women often perform closer to men than in land-based endurance events. Possible explanations include differences in buoyancy, body composition, and stroke mechanics, which may play to women’s strengths in the water.
On the bike and run, however, the gap widens considerably. Men continue to record faster average times, suggesting that physiological differences — such as higher average hemoglobin levels, muscle mass, and maximal aerobic capacity — retain a stronger influence in weight-bearing or power-dominant disciplines. For female athletes, this highlights a clear opportunity: targeted training in cycling and running could yield the biggest overall performance gains.
What This Means for Age-Group Triathletes
While the findings won’t surprise seasoned competitors, they offer a data-driven foundation for smarter training decisions:
- Double Down on the Bike and Run
The numbers are unequivocal: these two disciplines are the most powerful predictors of IRONMAN® performance. Prioritize your training hours accordingly. - Keep Swimming in Perspective
A strong swim sets you up for a good day, but it won’t make or break most age-group races — especially as you get older. Focus on efficiency and endurance rather than chasing marginal speed gains. - Consider Your Age and Gender Profile
Older athletes may see diminishing returns from swim-focused training, while female athletes looking to close the performance gap should lean into bike and run development.
Optimize your IRONMAN® performance with smart pacing strategies using the paceWiz calculator.
The Bottom Line
Triathlon will always be about mastering three sports. But if your goal is to maximize your overall IRONMAN® performance, this new research makes the priorities clear: cycling and running are where the race is won — or lost. Swimming still matters, but as a smaller slice of the total time, it offers fewer opportunities for game-changing improvements.
For the data-minded athlete, the takeaway is simple: if you want the biggest payoff for your training investment, put your energy where the minutes are.