The Training Theory Of Uphill Treadmill Doubles

Updated December 1, 2025 10:13AM

A few weeks ago, I asked a pro athlete to do their first “treadhill” double—20 to 30 minutes on the treadmill at 15% grade in the p.m. after an a.m. run. They asked if I had written an article on the reasoning and what to expect. I realized I had only talked about the power of treadhill doubles in passing, as throwaway lines in other articles.

So I responded: “No articles on that. But lots of athletes on the team have done treadhills before breakthrough races.”

Of course, training is like a locker room or group text message—there are no secrets. Aside from removing the bottoms of your training shoes molecule by molecule, anything that seems like a secret is probably snake oil.

But over many hundreds of articles, I have written about treadhill doubles way less often than expected based on how much I believe in them. The offset is for two reasons.

One, there are no specific study protocols that really apply here except in a tangential way. So we are venturing deeply into Anecdote Land, a magical world where the monsters are, like, kinda big, or kinda small, depending on who you ask. I’ll describe the theoretical underpinnings of treadhill doubles, along with some observations, with the understanding that it’s all uncertain.

Two, I don’t want to write articles that go out to thousands of people that might not apply to everyone. Treadhill doubles are pure bonus training elements, a narrowly applicable training theory. But exploring the nooks and crannies of training theory can be fun sometimes, even if the lessons lead to other conclusions for your individual training. So let’s do this!

What are treadhill doubles?

The basics are super simple—20 to 30 minutes at 15% grade. Most athletes should start with a hike around 2.5 to 3.5 miles per hour, with some starting closer to 4 miles per hour. Very advanced/pro athletes may even start with a run around 4.5 to 5 miles per hour. The unifying factor is a slight increase in pace over the course of the session, with freedom to exceed aerobic threshold on good days.

For beginner or intermediate runners, that may be a hike focusing on good form, with some jogging mixed in at slightly faster paces if they feel good. For intermediate to advanced runners, it may be a hike progressing to a run, with faster run pulses mixed in. For top pros, it may be a run with faster finish, possibly with fast hiking added at the end if they are training for a mountain ultra.

Treadhill doubles are light stimuli that supplement existing running training, rather than replace runs entirely (though they can also act as a type of cross training on non-running days). The rationale is that running economy is still the primary driver of performance, and economy development comes from consistent training most of all.

These types of workouts have preceded many championships for trail and mountain-running athletes on our team, but that observation is overwhelmed by potential confounding variables. For example, the type of athlete adding treadhill doubles is likely very motivated and fit to begin with, since they are ready for bonus stress. In addition, they can tolerate extreme amounts of boredom and questioning life choices. Choosing to do a treadmill run is choosing to go the extra mile for fitness development … or it could be choosing an existential crisis that ends in burnout.

Why do treadhill doubles?

Now, we are diving into the deep end of training theory. There are sharks here, but instead of eating you, they comment on how your P-value is insignificant.

Let’s break the treadhill into its component parts before bringing it all back together.

Benefits of a double: hormonal stimulus, aerobic stimulus

Doubles—two training sessions in a single day—are nearly ubiquitous across the training of elite athletes in many sports. They are one of the best examples of how training theory usually drives the science, rather than the other way around. We see all of these stellar athletes do these types of efforts, so there must be some physiological reason, right?

Right! Doubles may provide a hormonal stimulus that enhances adaptation, they may increase adaptation markers and protein expression associated with better performance, they may even have some fascinating relationship to epigenetic signaling. The big conclusion is that two aerobic system pulses in one day could have outsized benefits on overall aerobic development. One plus one may equal two-point-one, and those extra point-ones add up over time.

Or maybe not. The science is not settled, particularly in how it applies to individual athletes living complex lives.

Takeaway: Treadhills provide a strong aerobic stimulus, but they don’t need to be too long. Past 30 to 40 minutes, there is a possibility that some of the adaptation benefits of doubles are reduced by increased breakdown. There is a razor-thin margin between overtraining and optimal training with these more complex additions to a plan.

Endurance benefits: specific muscular and biomechanical stress of a long climb, added aerobic strain

Treadhills may be better than a normal run or bike due to the specific musculoskeletal and biomechanical adaptations to sustained climbs. Studies show that around 15% grade is right at the ceiling of when it may be more efficient to run or it may be more efficient to hike in short events, depending on background and fatigue levels. Spending additional time at that grade can make the body more efficient with all sorts of climbing–shallower grades where you might run and steeper grades where you have to hike.

With hiking specifically, treadhills can cause rapid and stunning changes. In 2019, an athlete we coach did his first treadhill session to practice hiking, resulting in an average speed of 4 miles per hour and an average heart rate of 160. After just 3 sessions, his heart rate was in the 130s at the same output. Hiking is a large neuromuscular strain, so the adaptations can happen fast. I do not recommend going over 15%, since the output is not the limiter when hiking, it’s the neuromuscular/biomechanical adaptation to the unique style of output.

With running, similar rules apply—it’s a different form than many runners are used to (see this 2017 review article in Sports Medicine), and practicing that form can make sustained climbing much easier. It’s a mix between a run and a strength workout at first. Later on, hill running (and higher outputs) may become easy, since a part of why climbing is impossible is that it’s tough to practice in a specific way without twists and turns that reduce output, followed by a downhill that increases impact. When running, you can reduce the grade to 8-12% if wanted, though the rules likely apply differently at shallower grades.

Plus, treadhill doubles often venture from purely easy to easy/moderate or moderate. While that makes it easy to overtrain, it also implicates some emerging physiological theories explaining elite performance. Two moderate workouts in one day (rather than just going easy on a double) are used by Coach Renato Canova (2019 article), the Ingebrigtsen brothers (2019 article in the International Journal of Sports Science and Coaching), and countless other coaches. Treadhills can compound interest on fitness by the same principles, likely related to how aerobic development is influenced by adaptation markers and cellular-level protein expression.

Minimized injury risk: substantially reduced impact forces

What really sets treadhill doubles apart is that you can do them with little pounding on the body. A 2005 study in the Journal of Biomechanics showed that uphill running at 9% reduced impact forces significantly, and at higher grades the impact is almost absent relative to flat-ground or downhill running. Plus, a 2019 study in the European Journal of Applied Physiology found that uphill, downhill and level running economy were strongly correlated, indicating that improving uphill running should feed back into all gradients in a well-rounded training plan. All the benefits of running without the elevated injury risk; all the benefits of cross training without the non-specific muscular stress.

When to do treadhill doubles

I like to apply the same rules as regular doubles. The best time to start is on a typical workout day. That optimizes potential hormonal and adaptation benefits, while stacking stress in a concentrated window. On those days, you can push a bit harder at the end, doing simple versions of some of the double workouts popularized in track and road running pros, but with reduced risk.

You could also add them on a normal aerobic day. Keep these efforts easier to avoid excess stress on a day you need to be recovering. They could replace a running day altogether as well, with the option to increase duration.

I wouldn’t suggest doing more than two a week unless you have infinite capacity for contemplating the infinite desolation of the Universe. But pure hiking days may help if you are doing a very steep or long adventure.

For the pro athlete that started the article and is training for a big ultra, a typical week looks like this:

Should you do treadhill doubles?

Maybe. The first single off my upcoming album is going to be an uncertain love song called “Maybe, baby.”

I think that the greatest benefit of treadhills may be developing into a stellar hiker, so if you are not confident in your hiking or have a hike-heavy event upcoming, it’s a great addition. You can even use the same principles as a weekly cross-training session to replace a run or bike.

If you are an older or injury prone athlete, treadhills can replace some mileage with reduced injury risk.

If you want to improve climbing or overall speed, treadhills may provide a new stimulus that helps your body move past a period of stagnation.

And maybe that’s the biggest conclusion of all. It’s uncertain whether treadhills are helpful due to specific adaptations or due to the general principle of giving the body a different stress on top of a well-developed running base.

While there is little that we know for sure about treadhill benefits, they certainly are different and difficult. And when it comes to the murky, unknowable world of adaptation, different and difficult has to count for something.