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I can’t ski. Never done it, never will. When someone asks me to ski, I think PIZZA PIZZA PIZZA and bring that to a hard stop.
But I love ski training.
Cross-country skiing is a fantastic analogue from which we can draw conclusions about how physiology works at the pointy end of human performance potential. The reason? It’s just about the hardest thing a human can do.
Cross-country skiing uses both the upper body and lower body, possibly contributing to VO2 max observations that are greater than other endurance sports. It’s low impact, unlike running, so total output is less limited by mechanical load and breakdown. And it involves holding extremely high power levels for races varying from under an hour to several hours or more, with big sprints interspersed. As summarized by this 2017 review in the International Journal of Sports Physiology and Performance on the physiological capacity of cross-country skiers, they are the ultimate aerobic monsters.
Sports as a Real-World Performance Lab
In designing a real-world study on training theory, we have the output metric we are looking for: Who are the most monstrous of the aerobic monsters?
We have participants: thousands of athletes willing to push themselves to the limits.
The timeline: decades of development of training theory and science, with some of the top physiologists and coaches in the world. This point is especially key for our purposes, since cross-country skiing and the various disciplines of the sport are subject to numerous scientific articles.
Now for the big reveal: with all of that testing and time, what monster winds up on the top?
The answer, in skiing at least: it’s the monster that slows down most of their training and embraces that it doesn’t have to be a monster all the time to be a monster when it counts.
Before delving into the sexy science details, some disclaimers. First, I am not an expert on skiing, so I will try to characterize the studies directly before diving into potential implications for running. Different ski disciplines likely have different demands, and this is not a complete literature review. Second, these studies describe elite performers, who may be chosen (at least partially) to be genetic outliers when it comes to adaptation and stress management (see our “Sexy Science Corner” podcast episode on this topic here). Third, it’s always tough to know how much to trust self-reported training data. It’s similar to how in a Tinder dating profile, you may need to subtract three inches.
That’s gross. You know what’s not gross? SCIENCE!
A 2021 study in the Frontiers in Sports and Active Living journal examined the training characteristics of 12 world-class male long-distance skiers training for events that were several hours long. The annual volume of endurance training was 795 hours, or around the length of that final Lord of the Rings movie. A whopping 88.7% of those hours were spent doing low-intensity training classified as below 82% of max heart rate. For a hypothetical athlete with a max HR of 185, that would be around 152 or less, but in reality it may be even lower since these elite athletes are basing those measurements off blood lactate levels, and they’re very good at keeping those levels low at relatively high outputs. The remainder of the volume was split between medium intensity and high intensity at ~6% and 5%.
Basic principle: a ton of training, mostly very low intensity. We’re not just talking sessions that allow athletes to be recovered for the next day. These athletes could likely go harder and still nail subsequent training sessions.
This intensity distribution means that athletes are likely below those recovery-focused thresholds, going slower because it’s optimal for adaptation, not because they have to in order to avoid breaking. It’s easy for easy’s sake.
How might that change for athletes as they progress into the sport? A seminal 2006 study in the Scandinavian Journal of Medicine & Science in Sports was instrumental in classifying the intensity ranges used in subsequent studies. That study looked at the training of 11 male junior skiers, and found 71% of training was at the very low blood lactate levels indicating low intensity training. While that percentage is a bit lower, I think it’s relevant that the study was looking at junior skiers. Who winds up excelling in the pro ranks? The answer to that question (at least in most of the studies reviewed for this article) is settled.
A 2021 study in the PLoS One journal examined 31 skiers’ training from the age 16 season to their age 22 season, and found increased endurance volume through more low intensity endurance days. Another 2021 study in the Journal of Human Sport and Exercise compared elite male Russian and Norwegian cross-country skiers. For the Russians, 670 of 740 annual endurance training hours were very low intensity, over 90%! For the Norwegians, it was 715 of 801 hours, or 89%.
A 2017 case study looked at the training of the greatest female cross-country skier of all time, and she did 92.3% of her endurance training at low intensity. The number was even higher during altitude training camps, and she only increased higher intensity work in very short windows before races (sometimes just a couple weeks). A 2016 study examined the training of 6 world-class and 6 national-class female skiers and found that the world-class skiers did 26% more low-intensity training. Another case study analyzed the long-term training of a Nordic combined gold medalist to find that their training volume and performance both increased when low-intensity training became a higher proportion of overall workload.
The athletes aren’t just doing low-intensity work. The high-intensity sessions involve a lot of time at threshold (especially at the massive volumes they are doing over the course of a year). They do strength and sprinting too. But a huge percentage of their training time is spent noodling around at low heart rates.
Those training principles don’t just apply to skiing. The Norwegian Ingebrigtsen brothers have had stellar international results applying similar principles, with a higher proportion of intensity around threshold, as outlined in this 2019 study in the International Journal of Sports Science and Coaching. Recent triathlon gold medalist Kristian Blummenfelt seemed to apply many similar ideas, with a substantial amount of lower intensity work mixed with threshold intervals (see this podcast with his coach).
In a 2021 review in Sports Medicine, the authors refer to the “Golden Training Divide,” where athletes excelling at distances 1500 meters and above usually have a heavy aerobic emphasis in training. An amazing 2019 study in the Journal of Strength and Conditioning Research looked at 85 elite athletes over their first seven years of serious training to draw conclusions about the type of runs associated with top performance. The volume of easy runs had the highest correlation with performance, from 0.72 at 3 years to 0.68 at seven years.
Increasing low-intensity training volume has clear benefits. A non-sport-specific 2010 study in the Scandinavian Journal of Medicine & Science in Sports outlined the basics. There is an increase in mitochondrial content and capillaries around muscle fibers, improvement in metabolic function at both high and low intensities, and a more economical use of oxygen to power performance at all intensities as well. The problem is that many of these adaptations take years to translate into max performance, with longer-term growth often being driven from the bottom-up rather than the top-down, but most interventional studies having a time constraint of weeks or months.
As the author of the 2010 study said: “While the immediate effect of low-intensity high-volume training on intense exercise performance can be difficult to assess, it would appear that the insertion of these low-intensity training sessions has a positive impact on performance, despite being performed at an intensity that is markedly less than that which is specifically performed at during intense exercise competition.” That’s why real-world observational studies are so helpful–show me what the best in the world actually do, and then we can figure out the mechanism later.
A cool interventional study approached this question from a unique angle. The 2013 study in the European Journal of Applied Physiology had two groups of sedentary women perform three training sessions a week, with one group doing high-intensity intervals and the other doing lower-intensity intervals. The improvement for both groups over 12 weeks was similar, but 60 percent of the progress in the high-intensity group happened in the first three weeks, compared to just 20 percent for the lower-intensity group. Chart that out long-term, and it’s likely the low-intensity group would continue progressing, particularly with periodic harder efforts mixed in.
What it Means for Running Training
Most running coaches who work with athletes for long-term success agree with the bottom-up approach, from Arthur Lydiard in the 1960s to Renato Canova today. An article I wrote earlier this year goes into some of the theory that applies a bit more uniquely to easy running. The variation likely lies in the loading patterns of running.
Running differs from skiing in that it requires much different biomechanical stresses and impact forces. Runners doing 800+ hours a year will likely break into a thousand little pieces. And even if they don’t, running extremely slowly for a very long time would likely create inefficient movement patterns that apply less specifically to faster running.
Think of another sport with similar training dynamics as cross-country skiing: cycling. In cycling, the motion to put out 400 watts for an hour is basically the same as the motion to put out 50 watts, and neither come with much injury risk. Meanwhile, running up steep grades versus flats, or at 6 minutes per mile pace versus 15 minutes per mile, can seem like almost different sports. There are three big takeaways for running training.
One: Aerobic base is key
The skiers show that the aerobic system can continue adapting long-term with high volumes. Emphasizing slight aerobic volume increases over time is a way to improve performance at almost all endurance sports. Running introduces complications due to the high rate of injuries, so consider adding a mile here or there, along with easy doubles or cross training, to level-up long-term performance. Supersized long runs may have diminishing returns due to injuries and endocrine/nervous system overstress. Next week’s article will be on how very chill doubles can fit into this long-term framework.
Two: Easy running can be very, very easy
On most of their training, the skiers are going at relaxed efforts. It takes patience and love, but it’s OK to slow down a significant amount on easy days. Shuffles are your friend when you are tired, and there is no valor in fast easy days unless that’s a specific training goal.
Three: Neuromuscular and biomechanical demands for running fast likely need more reinforcement
Make sure your speed remains high nearly year-round to avoid becoming a terrific slogger. This primer on base training gets into the big problem with just doing easier running, like in strictly heart-rate-capped approaches. Even given the skiers’ high proportion of low-intensity training, they are still doing a ton of intensity given that the annual training volume denominator is through the roof.
An athlete that can do higher volumes with health and happiness has two potential avenues to go down. The first avenue involves consistent strides and speed reinforcement (or the speed talent that makes faster running natural in most contexts), and it should lead to long-term growth.
The second avenue involves higher volumes without an emphasis on running economy development, where higher outputs become nearly impossible to access due to neuromuscular and biomechanical changes. Those athletes may put in a ton of work, and they may progress a bit as they layer in aerobic volume on top of past training. But over time, they’ll likely find themselves getting slower and slower at the same heart rates as they become limited by the mechanical work.
Like the skiers, go easy to get fast. Just make sure you maintain the adaptations to actually go fast, and keep health at the forefront.
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.