One of the oldest gems of wisdom among runners is that more mileage is better for your performance.
But, perhaps equally old wisdom holds that more mileage also increases your risk of getting hurt—especially if you pile on a lot of mileage all at once.
But are either if these statements true?
Now that millions of runners log their training digitally, researchers are starting to leverage this new data resource to answer these critical questions.
- Will adding mileage always make you faster, and always raise your risk of injury?
- If you do choose to add mileage, how much can you safely add at once?
With new research coming out, we are starting to get a clearer answer to questions like these.
Mileage and running performance
Researchers are often interested in the major determining factors of performance in runners. These determinants of performance are particularly interesting when you can control for other factors that might influence performance as well.
For example, while it’s possible to directly examine the correlation between weekly training volume and race performance, it’s more interesting to look at that relationship after adjusting for factors like body weight, which might otherwise affect the correlation—heavier runners are likely to run lower mileage, and are also likely to have slower race times, so it’s easy to see how failing to adjust for such an influential factor could cloud our conclusions.
One paper that took just such an approach to investigating the relationship between mileage and performance was published in 2017 in the Journal of Sports Science & Medicine by a team of researchers in Spain [1].
In this study, the research team developed predictive equations for half marathon performance that controlled for training volume, years of running experience, body mass index, and body fat percentage.
Perhaps not surprisingly, the researchers found that, even after controlling for all of these other variables, greater mileage led to faster race times.
Half marathon performance dropped by about 1.75 minutes for every ten-mile increase in training volume.
Similar work has been done in 100-kilometer ultramarathoners, with similar results: greater training volume was correlated with faster race times, though training speed was an important variable as well [2].
Adding mileage without getting hurt
It’s clear from the research that mileage matters when it comes to race performance, but if you aren’t running a lot right now, what’s the safest way to bring your mileage up without getting hurt?
Several studies have explored this question, but a new scientific paper took a systematic look at all of the research to date on the injury risks associated with acute increases in weekly mileage.
The paper, published by a team of scientists in Denmark and Luxembourg, underscores the difficulty of trying to separate the effects of absolute training volume versus changes in training volume [3].
For example, if you typically run 20 miles per week, then increase your training volume immediately to 25 miles per week and subsequently get hurt, was it the five mile increase, or would you have gotten hurt anyways if you’d maintained 20 miles per week?
Disentangling these effects requires a lot of data with detailed information on daily training volumes. The authors of this study systematically searched the scientific literature for studies that were careful enough to examine changes in mileage (as opposed to mileage by itself). They identified 65 potentially eligible studies, but surprisingly, only four were detailed enough to look at changes in training volume as it relates to injury in runners.
Likely because of the dearth of high-quality studies, even after pooling the results of these four studies (which included a total of 1,563 runners), the results were fairly weak.
The results suggest that an acute increase in weekly mileage of 30% leads to about a 1.6-fold increased rate of injury, compared to mileage increases of less than 10% per week.
However, no differences appeared when the researchers considered changes in weekly mileage between 10 and 24%.
Even within the high-quality studies examined in the systematic review discussed earlier, there are some big limitations.
- Most of the runners studied were running relatively low volume overall: an increase in 30% when you are doing ten miles per week is very different than increasing your mileage by 30% when you are doing 50 miles per week.
- Also, even with over 1,500 runners, the authors acknowledged that more research on bigger groups of runners needs to be done to illuminate the injury risks associated with changes in weekly training across different types of runners—old, young, male, female, experienced, novice, and so on [4].
What we know about mileage, performance, and injury
Despite the flaws of research done so far, we do know a few things:
- First off, mileage is an important factor for performance, whether you’re training for run-of-the-mill road races or 100-kilometer ultramarathons. However, it’s not the only factor, and perhaps not even the most important.
- We didn’t examine the research in this article, but the speeds you run in training have a greater effect on your risk of injury than your mileage. The faster you try to run, the more explosive you need to be, which increases strain on your muscles and tendons.
- If you are increasing your mileage, it’s definitely riskier to increase your mileage by 30% or greater in a single week, even if your absolute volume is very low. While the oft-quoted 10% rule isn’t right for everyone, 30% does seem to be a bright red line that’s well worth not crossing when planning out your training.
- Finally, it’s important to emphasize that these numbers are calculated based on actual mileage, not what you initially planned on doing. Quite often, runners will overshoot or undershoot their weekly mileage plans by quite a bit. So, make sure you accurately track your true training volume, whether on paper, using a GPS watch, or an online training log.
