The Exciting Complexity of Threshold Training for Trail Running

A perfectly designed training program might aim to have ideal intensity distributions. But what happens to a perfect program when you add hills? The answer to that question can lead to fitness breakthroughs... or collapses.

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Training intensity distribution is all the rage. If you walk into a crowded auditorium of exercise physiologists and announce your feelings on it, you might start a fight or an orgy. People are ready to throw fists and/or underwear.

One thing that’s certain when looking at the evolution of training theory: different things work for different athletes, and anyone who says that there is a one-size-fits-all answer is about to ask for your credit card number. But at this specific moment in theory evolution, the training intensity that is getting the most attention is threshold. And I think trail running can flip the entire application of threshold training upside down.

RELATED: When (and Why) to Train Below Aerobic Threshold

Training Intensity Zones

First, a quick primer so we can all get our bearings for the bigger discussion. The recent literature generally classifies training intensity into three “zones,” and the amount of training in each zone determines training intensity distribution (see this 2015 review article in Frontiers of Physiology).


  • Zone 1: under the first ventilatory threshold (or first lactate threshold depending on the source), less than ~2 mmol lactate depending on the athlete. Think easy up to steady in some cases.
  • Zone 2: between the first and second ventilatory thresholds, capped by either traditional lactate threshold or critical speed, between ~2 mmol lactate and ~4 mmol lactate, with variance based on the athlete. Think moderate up to moderately hard.
  • Zone 3: above the second ventilatory threshold, when lactate accumulates more rapidly and efforts become less sustainable. Think moderately hard up to hard. For tons of fun with the physiology and application of these zones and more, buy Dr. Philip Skiba’s Scientific Training for Endurance Athletes.


All serious training approaches involve most of the training time in Zone 1 (sometimes up to 90% in sports like cross-country skiing). That Zone 1 base improves capillarization around working muscles, mitochondrial function, metabolic efficiency, and mechanical strength/muscle fiber efficiency–all of which are indispensable at all effort levels. But while Zone 1 time is of paramount importance, that’s unlikely to start an orgy anytime soon. The distribution of Zone 2 and Zone 3 is when things get sweaty. 

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Threshold Training

Two weeks ago, I wrote about the Norwegian model for endurance training. Since that article was published, Jakob Ingebrigsten set the world record for the indoor 1500 meters. Last week, I wrote about Nils van der Poel’s wildly cool training prior to setting a world record in the speed-skating 10K at the Olympics. Both of those approaches are characterized by a shockingly heavy dose of Zone 2 threshold at certain parts of the training cycle. In the literature, that training intensity distribution would actually be classified as “Threshold”–it’s like when a band names their CD after the hit single. Capital “T” Threshold training is relatively uncommon, but doing some work in Zone 2 is a staple of most training styles.

The two main training approaches are “Pyramidal” and “Polarized.” Pyramidal training involves more time in Zone 2 than Zone 3 (most easy, some medium, less hard), and Polarized training involves more time in Zone 3 than Zone 2 (most easy, some hard, less medium). I agree with a February 2022 article from the Medicine & Science in Sports & Exercise journal, that most elite training is Pyramidal, at least when classified by time in each zone. But for our purposes, most athletes should be spending some time in each zone, and there’s a strong argument that Zone 2 should occupy a substantial amount of training time.

The theory behind Zone 2 training revolves around the aforementioned lactate (see this 2013 review in the American Journal of Sports Science and Medicine), which is produced as our bodies use glucose to fuel ATP production during glycolysis. Lactate is a fuel source for cells, and it’s accompanied by a hydrogen ion that changes muscle pH and contributes to fatigue. Zone 2 training involves more steady lactate production and clearance, and improving this lactate shuttling mechanism can improve all performance, even at very high or somewhat low intensities. 

RELATED: Easy/Moderate Fartlek Workouts For Building Endurance

Threshold Training Complication for Trail Runners

Now, we’re getting to the sexy thesis. To all the English majors out there, I’ll give you $5 to include that line in your next term paper. 

Sexy thesis: athletes that train over steep hills may already be doing Zone 2-heavy training, thus the way they approach each training week may need to be fundamentally different.

Before getting to the details, it’s time for some disclaimers. Play that funky disclaimer music, lawyer boy. First, this is just my approach to training theory based on seeing athletes of all levels progress over time. There are other approaches that view uphills very differently. Second, very advanced athletes may be able to run steep uphills without worrying about Zone 2. Like Mark Allen excelling with heart-rate capped MAF training, those athletes are so strong that even low efforts correspond with high outputs, and they can break the most well-intentioned formulas. Third, every part of these discussions would need pages of detail to fully encompass all the complexities, and I’m getting a case of the stress farts just thinking about all of the problems and terminology I didn’t address or brushed over. It also varies based on an athlete’s background and training ground. If someone lives in Florida, they are not getting any Zone 2 from the terrain, so they may need added structured workouts for intensity. The same goes for athletes who aren’t running steep trails in winter.

Let’s go back to the article on Norwegian training for some examples. Athletes using that system are training for the track and road, keeping their easy days very easy and doing two double-threshold workouts per week. An example of a double-threshold day from Dr. Marius Bakken’s article:


  • AM workout: 5 x 2K at 2.5 mmol lactate (lower end of zone 2) with 1 minute easy recovery
  • PM workout: 25 x 400m at 3.5 mmol lactate (upper end of zone 2) with 30 seconds easy recovery


That equates to around 1 hour of zone 2 work, spread across two sessions of varying intensity. The total training load overlaps with some of the research on the optimum adaptation stimulus for lactate threshold in very trained runners, and splitting it into two sessions allows for more efficient and faster running for these advanced male athletes.

Now let’s think of our old friend Nils van der Poel, the world-class speed skater who published a training guide that is better than anything I have ever written. I’d be jealous if I weren’t so awed, like when Brad Pitt took off his shirt in Fight Club. 

Nils would sometimes string together 5 threshold workouts on the bike in a single week, with sessions like this:


  • 4 x 30 minutes at threshold with 5 minutes recovery
  • 5 x 20 minutes at threshold with 4 minutes recovery
  • 9 x 10 minutes at threshold with 3 minutes recovery


That’s what I call a Rick James training plan, because he’s a super freak, he’s super freaky, yeah. Biking is non-weight bearing, so he can layer in more threshold work prior to the mechanical load resulting in substantial performance deterioration. It was the absolute maximum he (and likely any other human) could do without collapsing into a puddle of liquefied soul juice.

Road and track runners all do sessions that involve a heavy quantity of threshold training, from Coach Jack Daniels’ cruise intervals to Coach Renato Canova’s kilometer repeats at half marathon effort. These runners are so fast that often the threshold intervals are also extremely fast, leading many people to think they are classic burn-yourself-up intervals. But they’re not. In fact, Dr. Bakken said that when he went to Kenya in the early 2000s with his lactate reader, the pros had a great understanding of Zone 2 without actually monitoring the levels in training. 

I’d bet that many trail runners have a similar ability to dial in Zone 2 training. But it’s not in structured workouts. It’s running up long hills.

RELATED: How To Use Hill Intervals For Stronger Climbing

Uphill Running and Zone 2

Here’s the crux of the problem: it’s very difficult for most athletes to run up really big hills without leaving Zone 1, unless you slow down to a hike or a shuffle. Let’s use my training as an illustration, since I wear a semi-accurate heart-rate monitor for some runs, so can provide a ballpark estimate of intensity distribution (for perspective, my LT heart rate using the Friel Method is ~173-177 beats per minute, with variance depending on the season).


  • On rolling dirt here in Boulder last week, I was at 146 average heart rate at 6:49 pace, firmly in the middle of Zone 1. I would need to run a good bit harder to get into the lower end of Zone 2, and it would have to feel fast. In other words, to do a threshold workout on flat or rolling terrain, I would need to do structured intervals or tempo/moderate running, and it would not feel relaxed.
  • On a long run with Western States 100 3rd place finisher Drew Holmen in January, I averaged 162 heart rate for a 1-hour climb when we really eased into it and let the steepness dictate effort (I have gone 6 minutes faster in a hard effort). For me, that climb spends much of the time in Zone 2, ranging from the lower end on shallow grades to the top end on the steepest pitches. It never felt fast in the same way that a flat workout at the same heart rate would feel fast.


To run at 160 to 170 heart rate on flats, I’d need to accelerate substantially. The aerobic and musculoskeletal strains would be high. 

To run at 160 to 170 heart rate on steep uphills, I just need to continue running. The aerobic strain would be high, but the musculoskeletal strain lower.

And to run up steep hills at 146 heart rate, I would need an oxygen mask or Russian figure skating supplements. I could slow down a ton for the Zone 1 aerobic stimulus, but I’d be working different biomechanical patterns than used for faster running. That’s great in moderation, but athletes that do that all the time may find that they end up a bit slower overall. 

Thanks for reading my diary entry. But that example overlaps with what we see constantly in coaching. Many trail athletes will climb in Zone 2 even when they feel like they are not pushing. And that may persist after years of Zone 1 aerobic development. If we had lactate meters to measure every minute of every session, some trail athletes may be doing Norwegian-style training intensity distributions without it being the goal. A prime example may be Jim Walmsley, whose training in the Grand Canyon before Western States famously involved lots of running up steep grades. 

My guess is that Zone 2 climbing explains how some trail and ultra runners can be world class performers with less structured training, something you would never see in road and track running. For years, critics would say that it shows that trail running is not that competitive. What I think it shows is that the athletes are doing a Threshold or Pyramidal training distribution (sometimes intentionally, sometimes accidentally) but without specific workouts. The terrain and perceived exertion dictate the workout.

That’s why I think the best training location before a race like Western States is Walker Ranch here in Boulder, an 8-mile loop with two sustained climbs. For a pro athlete, the long climb is around 20 minutes with a very steep start and gradual finish, and the shorter one is around 10 minutes with a steady grade. If that athlete runs the ups efficiently in a 24 mile easy/moderate run when they feel good, the workout might look a lot like something out of Nils van der Poel’s training log of horrors: 


  • 3 x (20/10 minutes in Zone 2) with 10 minutes between intervals and 20 minutes between sets (with the downhill or flat running acting as the recovery)


That’s 90 minutes in Zone 2! Wildest of all: the athlete might report back that it was not a hard workout. It was just a normal long run, but it was close to the maximum physiological stimulus that any athlete could handle.

Zone 2 is not a monolith, however, with the lower end and the upper end being very different stimuli. Most of what I’m talking about here would be the lower end of Zone 2, which might not be an appropriate stimulus in many cases. To encourage more time at the upper end of Zone 2, we’ll sometimes incorporate structured tempos or controlled long runs workouts that have the added bonus of making the run play longer for adaptation purposes. 

RELATED: Why You Should Be Skeptical About Wrist-Based Heart Rate

Takeaways for Trail Runners

That theory comes with a few implications for trail runners. First, make sure easy running stays truly easy when you are recovery-limited or need to develop an aerobic foundation. Zone 1 aerobic development is still the most important variable in long-term growth. If an athlete inadvertently spends hours each week in Zone 2 during hilly runs, it’s a one-way ticket to the physiological ditch of overtraining, injury, aerobic inefficiently, or burnout. While Zone 1 time matters, though, it may help to try to run more and more uphills as you progress. There is so much time to be gained for every athlete, from beginner to advanced, by running a few more steps than might be your baseline.  

Second, account for time spent in Zone 2 running uphill, and don’t layer in Zone 2 climbing time with too many traditional workouts. Athletes we coach usually only do one interval workout each week (rather than the traditional two or three) when they are spending time on trails with uphills. On flats, the “grey area” of moderate training involves lots of impact and mechanical stress. On hills, that same intensity range has reduced load and less risk, making it a training opportunity. The same goes for uphill treadmill running. We’re often asked on podcasts: how can some of your athletes win big races without more workouts? The answer is: they’re doing workouts, they’re just harder to discern when looking at Strava.

Third, make sure speed development is still at the forefront of training. Zone 2 climbing can be a dangerous game, specifically because it is not fast. On the cellular level, it may be similar to the aerobic stimulus for lactate shuttling like a workout from Norway. But on the systems level, even advanced athletes are going pretty slowly in the big scheme of their abilities. The neuromuscular, biomechanical, and musculoskeletal systems must be continually developed unless an athlete has so much speed talent that they don’t even need to worry about it. It doesn’t require many strides, short hill intervals, and flat workouts to translate the aerobic ability from climbing into speed, but ignore speed and an athlete may be faced with what my co-coach Megan and I call “The Climbing Paradox”–run excess amounts of vert, and get slower at everything, including climbing.



I wish that we could go around and measure the lactate levels of 1000 dedicated trail runners of all levels for a few months during the summer. From seeing tons of athletes progress over time, I bet there’d be an offset between what the average athlete training in a hilly area thinks they are doing, and what they are actually doing.

Uphill running (and hiking) may involve a substantial quantity of time at different levels within Zone 2, which changes the training calculus. Account for that Zone 2 work on climbs–seek uphills out and embrace trail runs, just don’t overdo the vert or sneaky moderate intensity, constantly monitoring how you feel and develop over time.

Climbing is a gift that can turn trail runners into threshold monsters, sometimes without even realizing it. But it can also be a curse, turning them into sloggers with a massive aerobic engine that can’t overcome nervous/endocrine system fatigue and muscular/mechanical inefficiency.

Run or hike up hills. Continually develop speed. Eat lots of food to fuel the work. Or to summarize it in three words:

Climb. Stride. Pizza. 


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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