The Science Of Downhill Running
This article originally appeared on Trail Runner
In coaching, I have a persistent mantra playing in the back of my head when determining long-term training approaches:
"The fastest athlete usually wins."
Next, my brain says, "Des-pa-cito." Stuck in my head since summer 2017.
Before the physiologists start writing letters to the editor, I'll admit that the mantra is a gross oversimplification of a complex process (The thing about speed, that is. Despacito is perfect.) While running economy values on uphills, downhills, and flats are correlated for the same athlete at a given point in time (see this 2021 study in Frontiers of Physiology), the variable stresses of trail races add a ton of uncertainty into how that will translate into outcomes. Speed matters, but speed only expresses itself with adequate biomechanical practice, fatigue resistance, metabolic efficiency, and fueling, among many other things. And I think the number one barrier for most athletes in fully expressing their speed in trail running is what might be considered the "easiest" part of the sport… downhill running.
Or more specifically, what I'm fascinated by in coaching is how each athlete's physiology responds to repeated bouts of downhill running, reducing and managing exercise-induced muscle damage. You probably know the feeling–a few fast or steep downhills, and all of a sudden your legs are moldy jello. No matter how fast an athlete is, moldy jello legs are not going to get it done. Des-pa-ci-NO!
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The Science Of Downhill Running Performance For Trail Running
Before getting into a really cool review article, let's break down the importance of downhill running with a thought experiment in a hypothetical study (we'll get into some real studies later that have similar protocols). Imagine this fake study had athletes run uphill at 10% grade for 15 minutes at 90% of their VO2 max. Then, the athletes run downhill at 15% grade for 15 minutes at 80% of their VO2 max. Finally, they repeat the uphill protocol one more time, with another 15 minutes at 10% grade at 90% of VO2 max. My predicted results based on the literature and training theory:
The time on the first uphill bout would be largely dependent on their speed/running economy. Even an athlete coming straight from track racing could excel with little specific training (rationale here).
The time on the second uphill bout would have a much greater offset for some athletes, particularly those that are undertrained for downhill running.
The magnitude of the offset would be predictive of how much an athlete's speed correlates with trail racing outcomes, even in ultras.
I love hypothetical studies because I can just pull results out of my butt. It's where the sun doesn't shine, but made-up science does. Thankfully, you can track this type of model in the real world at trail races on any weekend. The fastest athlete as measured by 10k speed or a similar metric usually does great, unless they absolutely implode. And I think those implosions are largely dependent on how those athletes handle the mechanical strain of downhill running (plus fueling in longer races).
So why do I still want athletes to focus heavily on speed and running economy development? Because we know that it's trainable over many years, improving by leaps in bounds even in advanced athletes (see this 2006 study on Paula Radcliffe). Meanwhile, the ability to use the speed over a long distance (whether we call that fatigue resistance or some other metric, which likely has a large downhill running component) can only improve so much, since it'll eventually run into the speed ceiling. Which finally, mercifully, leads us to the thesis of this article:
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Athletes can prepare for downhill running (and can thus improve a key element of their fatigue resistance) very quickly.
A 2020 review article in Sports Medicine from the great research team of Bastien Bontemps, Fabrice Vercruyssen, Mathieu Gruet, and Julien Louis examined the effects and adaptations from downhill running. It features a whopping 256 citations, a monumental achievement incorporating decades of evolving science, and enough footnotes to bring a lawyer to completion. The article is a wonderful template to understand why downhills are so important for race outcomes, but also why downhill training might not require a massive, year-over-year focus.
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Downhill Running Fatigue
The review starts by breaking down the components that make downhill running a unique strain. Eccentric muscle contractions are often a product of "negative work," when the force applied to the muscle exceeds the strength of the muscle. That can happen in downhill running when the muscle lengthens under load–think of the race photo where you land on an extended leg and and the knee bends as your body absorbs the impact, lengthening the quad muscle in the process. The result is exercise-induced muscle damage.
As stated by a 2016 review in the European Journal of Applied Physiology: "Downhill running induces severe lower limb tissue damage, indirectly evidenced by massive increases in plasma creatine kinase/myoglobin concentration or inflammatory markers." Steep downhills without enough preparation can lead to blood and urine with the properties of a toxic waste dump.
Even with tons of preparation, athletes are left managing that breakdown process. At the Western States 100 drug testing tent last year, where the top 10 men and women pee into a cup, I was mortified to see some samples that looked like Coca Cola, but with pulp. That's muscle breakdown in real time.
But it's not just the mechanical breakdown caused by the impact forces and transmission. There is also peripheral fatigue that could stem from overstretched muscle fibers, and muscle fiber damage may contribute to excitation-contraction coupling failure. Muscle contractions require rapid electrical signaling, and if those signaling mechanisms fail, it can result in extreme fatigue, and possibly even cramping (though it's debated). There is also an element of central fatigue related to the spinal cord and nervous system, leading to reduced muscle activation. A 2020 study went a step further in adding another neuromuscular wrinkle, theorizing that delayed-onset soreness like that experienced after downhill running could be the result of "acute compression axonopathy of the nerve endings in the muscle spindle." I have no idea what exactly that means, and you can't make me learn.
Put together, downhill running can cause a musculoskeletal and neuromuscular crapstorm. I do know exactly what crapstorms are, because I once ate a plate of Skyline Chili.
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Running Performance Impacts
How does that crapstorm translate to running performance? A 2007 study in the Journal of Sports Sciences got delightfully close to the hypothetical protocol I suggested. The authors had 10 men do level running before, immediately after, and a few days after a 30-minute downhill running bout at 15% grade. They found a 7% increase in oxygen demand during the level running after the downhill running bout, and up to a 21% reduction in maximum strength of the knee extensors, with reductions persisting for days.
Those findings are backed up by a 2021 study in the European Journal of Applied Physiology, which used a similar protocol to find an 18% increase in oxygen demand after the downhill running bout, and a similar ~22% reduction in maximum voluntary contraction. Interestingly, the oxygen demand changes didn't significantly correlate with biomarkers of muscle damage, stride length, or stride frequency. Thus, it's not as simple as limiting muscle damage or changing form, likely tied to the complexity of the fatigue caused by downhills in the first place. So it's a complex crapstorm, like cryptocurrency.
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So what is responsible for variation among different athletes?
The primary culprit is likely training status and how it interacts with genetic differences. In studies on trained athletes, the 2020 review found that there was a less substantial reduction in maximum voluntary contraction rates (-16.4% for trained athletes, -23.5% for untrained athletes), along with reductions in muscle damage. However, there was a big limitation: the trained athletes were rarely specifically trained for downhill running. It's possible that even in studies in untrained athletes, variable amounts of exposure to eccentric contractions could explain some differences, underscoring how hard it is to isolate inter-individual variability when so many mechanisms are involved.
Maybe gender plays a role. Interestingly, while men and women had similar negative responses to downhill running, women demonstrated a quicker return to baseline levels. A 2001 study theorized that estrogen may even have a protective effect against exercise-induced muscle damage.
There are plenty more possible explanations for variance among individuals. My explanation of choice is muscle fiber typology, with faster-twitch athletes being more prone to muscle damage and neuromuscular system fatigue. But the jury is out on that.
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Now we get to the million-dollar question (worth approximately 7 bucks with inflation): how can athletes improve their own responses to downhill running over time?
The strongest evidence for a major training response comes from studies on the "repeated bout effect." Just one downhill running session can reduce muscle damage on a subsequent session. One 1985 study found that the downhill sessions could be separated by 3 weeks and still see those improved metrics! However, the benefits vanished after 9 weeks. That study opened the floodgates for many other similar studies over the years, with similar findings too (see Table 1 of the review to have your mind freaking blown by their fantastic research).
And the benefits of downhill running may go beyond preparing you for more downhill running. A cool 2020 study in Scientific Reports found that 14 days after a single 30-minute downhill running session, athletes reported less fatigue from a flat-ground running session, possibly pointing toward some benefit to downhill running in fatigue reduction generally. A 2022 study in the European Journal of Applied Physiology found that just four weeks of downhill running training could promote neuromuscular adaptations. Other studies show mixed results for running economy (2018 study showing no change) and strength (a different 2018 study showing improvement), depending on the study protocol and output measurements.
The hard part is translating the scientific understanding into a training system. What's the optimal way to harness the repeated bout effect? There are a few studies on downhill training protocols that seem to find slight improvements, but no set answer.
The authors completed a massive scientific undertaking, and their conclusion is direct: "These results strengthen the rationale for including specific [downhill running] sessions in the athlete's training regime." Think back to that hypothetical study, where the uphill running bout after the downhill was much slower than the bout before the downhill. What training sessions can bring those closer together, thus improving trail performance and endurance?
The specific design of those sessions is up for debate, and coaches vary. My co-coach/wife Megan and I focus on five elements, based on the physiological mechanisms and applied studies.
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One: Practice good form
A 2019 study in Frontiers of Physiology found that performance across individuals in a 7 km, steep trail race varied most on downhills, with 25% performance differences on downs compared to just 10% on flats. Athletes have wildly different downhill running abilities! That offset likely has to do with form and practice. The studies are mixed on the impact of form on muscle damage, with some (but not all) showing small changes with increased step rate or altered footfall, and it likely depends on the individual athlete.
These 5 form tips are where to start, before even thinking about the specifics of downhill training sessions. The goal is to harness the power of light, quick strides, finding flow with intentional relaxation, like water streaming down the trail rather than a pinball rocketing from step to step.
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Two: Throughout the year, aim to run some downhills with purpose in your long runs
Those biomechanical adaptations from proper form take reinforcement, so it's helpful to practice when you can throughout the year. And the muscle damage metrics in the studies show some evidence of being cumulative, with more downhill running leading to more damage (and more potential for adaptation). Thus, long runs are a great chance to add up those stresses without having to absolutely hammer downhills and risk injury.
We like our athletes to run downhills in many long runs "with focused, purposeful flow." For most athletes, that means 50k/50 mile effort. For very advanced/pro athletes, it might mean 50 mile/100k effort. The rationale is two-fold. First, downhills involve lower heart rate even when running a bit faster, so it increases the aerobic strain and will help athletes harness free speed. Second, and most relevant for this article, the resilience/repeated-bout-effect benefits of a downhill after doing 3000 feet of vert are likely greater than a downhill done closer to the start of a run. These background adaptations from efficient downhill running will leave an athlete ready to pivot to more specific trail goals.
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Three: At least once every three weeks during the trail season, train more specifically for downhills
While the repeated-bout-effect studies show that the adaptations are sticky over a few weeks, it's likely that becoming maximally proficient requires more focused training than found in typical long runs. Our favorite way to program downhill running for athletes is in the context of tempo runs–something like 30 minutes moderately hard over terrain with steep ups and downs. In addition, on some of those weekly long runs, athletes can do a few downhills harder in an unstructured way.
For athletes that live in flatter locations, check out this article for some tips, which today's article builds off. For athletes in Florida, move.
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Four: In specific training for a very steep race, at least 4 weeks before your race and ideally 8 to 12, include a weekly training day with faster downhills
Those first three tips will get you where you need to go for most races. But what about very steep races, like Hardrock or UTMB, or races where downhills play a huge role, like Western States? Based on the mechanisms for muscle damage and nervous system-related fatigue, we like athletes to push some steep downhills in training much harder than they ever will on race day. The theory is that athletes need to be as close to impervious to the impact-related issues as possible, since even small concerns will add up a ton over the course of one of these races.
In practice, that usually means focusing on strong/fast downs throughout some long runs leading up to race day, or in training races. But we also like steep uphill intervals, with the downs at a smooth and fast effort. Our go-to workout is The Hill Beast: 10/8/6/4/2 minute steep hills moderately hard with run down smooth recovery.
Five: Strength train
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Eccentric muscle contractions are not confined to downhill running. In many ways, they can mimic the demands of strength training. That's why we developed two routines: 3-Minute Mountain Legs and 8-Minute Speed Legs. While strength work is no substitute for specific work, it can prepare you to get the most out of it.
Takeaway
And perhaps the most important tip of all: decide NOW that you are a downhill beast. So much of strong downhill running starts as a choice, and the first step is to control the narrative about your strengths and weaknesses. By deciding you are a downhill monster, you'll be more confident with proper form, allowing you to adapt to the background stimuli in long runs. You'll gain confidence with positive reinforcement, and your physiology will undergo fundamental alterations that allow you to go even faster. The feedback cycle can get so damn fun, with downhill running getting better and better.
But the origin point is just a simple piece of self-talk. YOU ARE A DOWNHILL BOSS.
Now go train (and adapt) like it.
And as always, remember one thing: "Des-pa-cito."
David Roche partners with runners of all abilities through his coaching service, Some Work, All Play. With Megan Roche, M.D., he hosts the Some Work, All Play podcast on running (and other things), and they wrote a book called The Happy Runner.
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