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If you gave me some gels for fuel and a Ludacris playlist for motivation, I could probably dive into the exercise physiology journals and find you 100 studies with this general design:
Group A introduces intensity into their training (sometimes with a Group B that sustains training levels). Athletes in Group A get faster, regrow lost hair, and have erections lasting 3 hours, 59 minutes, and 59 seconds. In these types of short-term studies, speed work can look like a magical elixir that cures all of the problems you have and most of the problems you don’t.
Meanwhile, you hear coaches like me preaching easy running as if it’s saving your athletic soul from the fiery pits of lactate hell. How do you square that circle? Usually, I’ll say that it’s a problem of time horizon, with the adaptations from low-level aerobic work accruing over months and years. Easy running builds capillaries around working muscles, improves the cellular ability to shuttle lactate during intense efforts, and enhances the expression of Type I slow twitch muscle fibers, among tons of other beneficial adaptations that power faster running.
That’s why you’ll see numerous different approaches to speed work, from the Norwegian emphasis on threshold to more polarized, high-intensity models. But you’ll almost never see an athlete in the modern era have great success without plenty of easy running. For example, a 2022 study in Sports Medicine – Open found that the best athletes in the world train 450 to 700 hours per year, with 80+% being easy running (and often more like 90%). A 2019 study in the Journal of Strength and Conditioning Research found that over the seven-year study horizon, volume of easy running had the highest correlation with performance. Heck, even in cross-country skiing and speed skating, high levels of easy volume are indispensable to world-class performance.
“Cool,” you might think. “Those studies are on elite athletes over multiple years. I want to be faster next month.”
A wonderfully fun new study in the Medicine and Science in Sports and Exercise journal addressed the time horizon problem with an ingenious study design that pitted volume against intensity in a startlingly short 2-week training intervention.
The study was led by Olli-Pekka Nuuttila and a research team in Finland, and instead of looking at volume and intensity in different studies, they brought them together. Is it intensity or volume specifically driving fitness changes, or is it related to training alterations more generally? And how might that microcycle change play out over longer training mesocycles?
The study recruited 30 participants, with the intensity group and volume group each consisting of 9 men and 6 women. The average 3K time for both groups was about 12:30, or 6:42 minutes per mile pace, so the participants were strong athletes. Both groups underwent the same 2-week preparatory period consisting of easy training plus one interval session of 6 x 3 minutes at the “maximal sustainable effort” with 2 minutes recovery. That was followed by a recovery week with a 50% volume reduction.
Next came the study intervention. The intensity group completed 10 sessions of 6 x 3 minutes during the two weeks. The volume group increased easy running by 70%. Both groups followed that with another recovery week. Time trial performance for 3,000 meters was measured before the training intervention, after the two-week training intervention, and after the recovery week, along with heart rate variability (which acts as a proxy for nervous system stress), perception of fatigue, and blood/urine samples.
The intensity group had similar training volume before and after the intervention, around 5.5 hours per week, but the faster running led to an increased total volume from ~45 km to ~50 km a week. The weekly intensity breakdown looked like this, where Zone 1 is easy, Zone 2 is moderate/threshold, and Zone 3 is hard/VO2:
- Zone 1: 61.9% of training volume
- Zone 2: 18.4% of training volume
- Zone 3: 19.7% of training volume
While that’s more intense than traditionally recommended, it probably looks pretty darn similar to what a lot of athletes do when they aren’t following a structured plan. It’s hard, but it’s not CRAZY hard.
The volume group increased from 5.4 hours per week (45 km) to 9 hours per week (77 km), with 99.6% of that training in Zone 1. To picture that intensity distribution, imagine if Margaritaville became a training plan.
As always in these articles, I want you to put your hypothesis hat on to guess what happens. Speed causes rapid beneficial adaptations, so that group should excel. However, the volume group really added A LOT of weekly distance. Perhaps speed will be wastin’ away in Margaritaville? I personally would guess that the intensity group would improve by 1-2% more than the volume group.
Let’s report the 3,000 meter time changes in seconds, since the two groups had similar starting points:
- From pre-intervention to post-intervention: 13 seconds faster for intensity, 11 seconds faster for volume
- From pre-intervention to post-recovery week: 19 seconds faster for intensity, 17 seconds faster for volume
The differences between the groups were not statistically significant. Looking deeper at the data, one athlete got 2% slower in the intensity group, so perhaps that athlete had a sickness or undisclosed injury, which perhaps hurt the overall group totals slightly. But the volume group had a few athletes that did not seem like short-term responders, so maybe it balances out.
A 2018 study in the same journal found that lower intensity training was more likely to be associated with positive responses. However, that study was subject to a 2019 response paper that was critical of the methods, and that response was subject to yet another response paper from the study authors. For our purposes, the takeaway is that the predictability of individual response to training intensity is complex and debated.
As you’d expect, muscle soreness was higher in the intensity group. Here are some fascinating findings to pay attention to though: perceptual reports of readiness to train increased in the volume group, while decreasing in the intensity group. And while fatigue was similar, stress in the intensity group was higher. HRV increased in the volume group by 1.8% on average, while decreasing by 1% in the intensity group.
I freaking love this study.
Before professing my undying affection, a few disclaimers. First, there’s a chance that the short-term intervention may actually benefit the high-volume group. All of the athletes did three hard 3K time trials over a few weeks, and that might be enough intensity to interact with the easy training. I think that an athlete that only runs easy would likely slow down once they lose the neuromuscular and biomechanical adaptations to actually run fast.
Second, the intensity protocol was clearly excessive by design. I’d be curious what would happen with two to three sessions per week, rather than five, which may have introduced excess fatigue.
Third, a two-week time horizon likely measures lots of noise, rather than just fitness signal, depending on numerous individual factors. Anything from an illness to life stress to training background to small injuries could overpower the training response in individual athletes in ways that would likely be smoothed out over longer studies. This study is a brick in a really big wall of exercise physiology, and it’s only meaningful in that broader context.
Plus, these types of massive increases in both groups may be unsustainable, and they definitely have diminishing returns at some point. A 1992 study in the British Journal of Sports Medicine took similar interventions to the extreme, having athletes increase from 86 km per week to 176 km per week (!) in the volume group, and the intensity group increased speed work from 9 km total per week to 22 km per week. Both groups demonstrated signs of overtraining.
But I think the study does show that easy running isn’t just a long-acting adaptation stimulus, particularly with a small amount of intensity worked in. Easy training can work fast and may even act similar to an intensity intervention. Given what we know about “base building” and the importance of easy running for long-term growth, the higher volume would likely precede further breakthroughs as well. If those athletes introduced balanced intensity on top of that base, they could probably improve by leaps and bounds.
While the volume group was just getting started, the intensity group was already showing signs of burnout. Most relevant to me was the HRV response. There is strong evidence that depressed HRV corresponds with reduced adaptation rate, so not only were the athletes reporting less readiness to train, their physiology was likely less primed to respond to future stimuli. If this study played out over eight weeks, I’d bet those athletes would be burnt to a fine crisp, and the volume group would be steadily improving (especially if they added some strides and light intensity).
The big takeaway is that volume increases do not happen at the expense of speed. Adequate aerobic volume may actually be the key to speed, and not just when we zoom out over years. Easy running will likely be accompanied by a lower injury rate too, plus it will allow athletes to do harder speed sessions later.
Increasing training levels needs to be done in a methodical and iterative fashion, though, unlike in the study. A 2020 review article in Sports Medicine looked at the concept of “functional overreaching,” when training loads cause short-term performance decrements, followed by supercompensation improvements. While functional overreaching can lead to improvements, it is playing with physiological fire. So be careful when introducing new stimuli. And in general, easy volume is safer to add than intensity, and increased easy volume will prepare the body to adapt to higher intensity later.
Build an aerobic base, and continually reinforce that aerobic base. Use strides to maintain the neuromuscular and biomechanical adaptations that allow you to run fast. Introduce strategic workouts in moderation to turn that aerobic fitness into top performance. Repeat that cycle over and over for years, and running economy can improve by leaps and bounds even as the underlying metrics of aerobic capacity barely change.
I’ve written over and over again that “speed wins.” But I think there’s a strong argument that the aerobic ability from easy running actually is speed. Often, athletes will run super fast, and we’ll look to their most intense workouts for a justification. Maybe we should actually be looking at their easy runs, stacked back over years.
That easy running is the pressure that builds the physiological diamond. The intense workouts just polish it off.
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 you can find more of their work (AND PLAY) on their Patreon page.