Suppose you need to run 15 miles (24 km) in the next week. You can distribute this volume however you want, both within and across days. Your goal is to cover the requisite distance in a way that minimizes your risk of injury. Does it matter how you schedule out your week?
This is one concrete way of thinking about the question of cumulative damage in running: given that you’re going to cover some combination of distances and speeds, does the ordering affect how much tissue damage you’ll accumulate?
We’ll look at three different theories on how tissue damage accumulated during training. These aren’t exactly competing theories—more than one can be correct—but considering each framework separately helps sharpen up how you think about programming your training.
I think all three have some truth to them, though I’m still skeptical of the strongest version of each of the three (and I’ll explain why).
A quick review on the biomechanics of overuse injury
If you’ve read my recent articles on biomechanical training load and tissue load, tissue damage, and injury, you already know the basics behind how running injuries occur. With every step you take, your load-bearing tissues (bones, tendons, muscles, joint surfaces) experience one “loading cycle”: the increase and then decrease of mechanical force at the tissue level.
The amount of tissue damage you sustain per step is a function of both the tissue-level force and the structural integrity of the tissue (e.g. tendon stiffness or bone density). Studies on cadaver tissue and animal models suggest that there’s an exponential relationship between tissue-level forces and tissue damage per step.
The biomechanical math suggests that faster speeds can do vastly more damage per mile or per kilometer of running—perhaps up to a 4–6x increase in damage per mile when running fast versus running slow.
The assumption is that after accumulating a certain amount of tissue damage, an injury occurs. At a high level, this is not a bad assumption: you can easily replicate overuse injuries in cadaver tissue or animal models with repeated-loading protocols.
However, here’s one big issue with this biomechanical approach to overuse injury: it pays no attention to the order in which you experience these tissue loads, or the time between different “bouts” of loading—10 miles today and a rest day tomorrow is treated the same as 5 miles today and 5 miles tomorrow.
If you think tissue repair matters, or if you think that tissue structural properties change over the course of an individual training session, then “order effects” would matter quite a lot if you hold overall tissue load (loading cycles × load per cycle) constant.
There are three possible ways of framing these order effects, in terms of how a given program of weekly (or monthly, etc) training might modulate this tissue loading / tissue damage / overuse injury pathway.
Theory 1: Warming up improves the structural integrity of your tissue
This is something of an old-school hypothesis: injuries (or at least some injuries) are caused by “insufficient warm-up.” How might you operationalize this claim into a biomechanically sound hypothesis?
Well, one way to do it would be to treat tissue properties as a function of how long you’ve been running. So, running one mile easy + one mile hard would cause less damage than running one mile hard + one mile easy.
Just from an anecdotal perspective it’s not hard to see why you might think there’s some merit to this idea. If you get up first thing in the morning and try to run hard with no warm-up, you’ll find that your muscles, joints, and tendons all feel stiff. Once you get some blood flow going, it’s much easier to run fast.
Now, this might all be neurological, not biomechanical, but clinical evidence in some injuries (like tendinopathy or plantar fasciitis) points in this direction. Later on during the recovery process, connective tissue is often stiff or sore in the first few minutes of a run, then “loosens up.” Plausibly, as more blood flows to the tissue and as more lymph fluid is passively pumped out of your extremities, the tendon (or fascia, muscle, etc) might become more resistant to damage.[1]
There’s some experimental evidence to suggest this “warm-up theory” has some merit: a 2022 study on 40 healthy young adults found significant increases in tendon blood flow (of ~50%) and increases in tendon stiffness (of ~4–6%) after doing a 10-minute easy run (at 9:55/mi / 6:10/km) or a ten-minute, ~250-contact plyometric routine.[2] Interestingly, eccentric heel drops and static stretching had no effect on blood flow or tendon stiffness.[3]
Other evidence is mixed: a 2012 review paper on tendon biomechanics cites some studies finding increases in tendon stiffness after hopping and jumping exercises, and others that find no effect. Later work, like this 2019 study on hopping exercise, likewise found no change in tendon stiffness (maybe they weren’t jumping high enough?).
What’s the evidence that the warm-up theory is correct?
If you believe the “warm-up matters” theory, you would expect to see the first few miles of a run causing more damage than the later miles. That’d suggest it would be “safer” to do fewer, longer runs, as opposed to many short runs (though the exact findings might depend on exactly how long it takes to get warmed up, and whether you think Theory #2 is at play as well).
To the extent that we have evidence on this front, it suggests that the warm-up theory’s predictions are not correct, at least for recreational runners. The strongest evidence comes from a prospective analysis of almost 7,400 runners logging their training on Garmin Connect, which found that running less than twice per week was associated with a higher risk of injury after 1000 km (630 miles) of running—clearly this “mileage normalization” is important, since if you look at injury risk after, say, six months, the data will clearly favor people who barely run at all, since they cover less mileage.[4]
Even so, I find the warm-up theory more compelling for situations involving fast running. At the very least, it seems reasonable to avoid kicking down the door at a hard pace from the start. Also, in practice, if you tell yourself “the first mile is an easy warm-up” and run five miles at a hard pace, you’re really doing one mile easy + four miles hard, instead of just five miles hard. So, you’ve decreased your effective biomechanical training load!
Theory 2: Tissue damage per mile increases over the course of long runs
This framing is essentially the opposite of Theory 1: It suggests that the dominant driver of tissue damage is not warm-up, but fatigue. Concretely, if you believe that tissue damage increases as you get tired (at the end of a long run, or at the end of a workout) you should ascribe to this theory.
It’s important to tease apart the two different mechanisms that could be at play here. The first is changes in the mechanical properties of tissue: tendons get less stiff, muscles become more damage-prone, etc. The second is changes in running gait itself: your running form “breaks down” and you run in a way that creates larger forces per step.
When it comes to changes in mechanical properties, there is some evidence that tendons get less stiff after a prolonged run. That’s what this 2018 study found: an 8.8% drop in Achilles tendon stiffness following a 90 minute run. Interestingly, total energy stored in the Achilles stayed the same, which suggests that the tendon must have stretched out more—which, per the state of the art theories of tendon damage, would cause more damage (because tendon strain is increased). A similar mechanism is plausible in muscles: as muscle damage accumulates over a long run, you could plausibly see how the muscle itself would become less elastic and more vulnerable to damage.
For bone, I don’t see an obvious mechanistic path in the same way: bone’s physical properties come from the organization of bone cells at the microscopic level, and inducing enough change there to see macroscopic changes in material properties over a single run seems implausible to me. Maybe there is an argument to be made around water content of osteoclasts or something, but on the whole I don’t see much reason (or research) indicating bone becomes more vulnerable to damage over the course of a single run.
When it comes to the effects of fatigue, my views might surprise you: I don’t think fatigue-related changes in gait are a significant driver of injury risk. I do think gait changes as a function of fatigue, but the research indicates these changes are highly individualized, and not necessarily harmful from an injury perspective. For example, if fatigue is limiting your maximum muscular force production abilities, you might expect to see lower peak biomechanical loads, since you have to “spread out” your force production over a longer period of time. To give just one example, here’s a thesis showing that peak tibial bone strain decreased after a fatigue-inducing treadmill protocol.
I have a lot more to say about injury risk from fatigue, so I’ll save it for another article—for now though, let’s at least loosely go with my argument that, for the “long runs are more damaging” theory, the main driver is changes in mechanical properties of tissue, not changes in gait.
What’s the evidence that the long-run theory is correct?
There was a nice new paper out of Aarhus University last year on over 5,000 runners (mostly novices and amateurs) that provides some interesting evidence on this front. The upshot was that weekly mileage increases were not predictive of injury, but sharp increases in the longest single run (compared with the longest run in the last 30 days) was predictive of injury. In other words, it’s more dangerous to increase your running volume by drastically increasing your long run distance, as opposed to drastically increasing your weekly mileage evenly across the board.
This study’s results also seem to track the intuitive result you get when thinking about my original thought experiment: Doing 15 miles all in one shot feels like it would leave you more sore, and more injury-prone, versus spreading out those same 15 miles into three different runs.
Theory 3: It doesn’t matter, it’s all about volume and intensity
The final framing is essentially the “anti-view”: focusing on the distribution of running within the week is a distraction; the overwhelming driver of injury risk is total volume and intensity. That’s essentially the framing you get if you take the cumulative damage stuff seriously; any small effects of warm-up or long-run related tissue property changes get washed out by changes in mileage or speed. In other words, it doesn’t matter how you distribute those 15 miles, what matters is that it’s 15 miles and not 10 or 20.
We’ve already seen some evidence to suggest that the strongest version of this view is not right, but you still shouldn’t neglect this sort of “training load maximalism” view—it has very strong explanatory power, and it’s the only theory of tissue damage accumulation that doesn’t rely on hand-waving.
I’m including this “anti-theory” just because I sometimes see people who fixate on very technical aspects of their workouts, like various weighted heart-rate metrics, without zooming out and looking at the big picture: how much mileage, and how fast. As my sketch of an “equivalent-damage framework” shows, it’s quite plausible that small changes in workout speeds can drastically change total tissue damage, which could dwarf any marginal changes in tissue properties.
Practical upshots for training
As is often the case with the science of running injuries, we don’t have a clear winner, at least in the sense of a single dominant framework we can use to get a handle on injury risk. Still, there are some useful lessons:
For tendons (and maybe also for muscles), a sufficient warm-up is worth doing before quality sessions. “Sufficient” here means at least 10 minutes of running, plus some dynamic drills and strides.
Tendons also plausibly sustain more damage at the end of longer runs, so be especially careful when building up the distance of your long run. A “10% rule” for long run distance makes more sense than a “10% rule” for weekly mileage. The window to consider here is the last four weeks: if you did 16 miles a month and a half ago, but haven’t gone longer than 10 miles since then, don’t immediately jump up to 18 miles for your next long run.
For bone, theories about injury risk being amplified by a lack of warm-ups, or by excessively long runs, are less plausible based on the evidence thus far. For these injuries, total mileage and total volume of intensity (e.g. “total mileage at threshold pace”) are the important things to pay attention to.
I would also add that fatigue per se should be less of a concern as a cause of injury, though I don’t expect my cursory coverage here to fully convince you—we’ll revisit the question of fatigue-related increases in injury risk another day.
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Footnotes
[1] I don’t think “loosening up” is the right metaphor here, since what causes tissue damage is high levels of strain—i.e. a greater change in length. What you actually want is stiff tendons and dense bones that don’t change in length very much when they experience high levels of mechanical stress. A better metaphor might be changes in “brittleness” versus “toughness” — how much damage a material can absorb before breaking.
[2] The exact routine was 5 sets of: one depth jump from a 30 cm box; 10 squat jumps; 5 high jumps (not exactly sure how these differ from squat jumps); 15 single-leg jumps, only on the leg being tested; 10 lateral jumps; 10 forward and backward jumps; two standing long jumps. 1:15 rest among all.
[3] From one of the regression models in the paper it looks like stretching actually caused a small but statistically significant decrease in stiffness, but the methods section is a little confusing so I would not take that finding to the bank.
[4] The exact method used was a variant of survival analysis; the runners did not actually need to successfully cover 1000 km to be “eligible” to contribute information to the study. 1000 km is just an arbitrary, pre-specified point to compare the cumulative risk of injury across different groups of runners.
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"If you believe the “warm-up matters” theory, you would expect to see the first few miles of a run causing more damage than the later miles. That’d suggest it would be “safer” to do fewer, longer runs, as opposed to many short runs"
That part lost me as it ignores the recovery after a run.
A short run x 3 might cause more overall damage than 1 long run, but the 1 long run might push you over your limit and cause an injury.