Issue #9: Outdoor vs Treadmill Running

✍️ Author’s Note

Welcome to this edition of the Threshold Lab newsletter! I’m Stephen Pelkofer, an experienced HYROX athlete with a men’s pro personal best of 59:41 and men’s pro doubles best of 51:39. If you want to learn about my story and how I got here, check out this Instagram post here.

The goal of this newsletter each week is to pick a training topic related to running or HYROX, do deep research into it, and provide actionable protocols that the reader can take away and apply to their training immediately. Let me read the research, listen to the experts on podcasts and youtube, and then give you the tools to make it work for you.

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👨‍💻 Introduction

Topic: Outdoor vs Treadmill Running

We’ve all had days when the weather or schedule forces us onto the treadmill, leaving us wondering: “Do these miles count as much as running outside?” This week, we dive deep into a 2019 systematic review that compared treadmill vs. overground running in dozens of studies. The findings might surprise you – revealing when treadmill running mimics outdoor conditions and when it doesn’t, and what that means for your training.

A Systematic Review and Meta-Analysis of Crossover Studies Comparing Physiological, Perceptual and Performance Measures Between Treadmill and Overground Running

⚡ Fast Finds for the busy reader

• Treadmill running can closely align with outdoor running, but if you race outside, you shouldn’t only rely on treadmill miles.

• Learning to train by Rate of Perceived Effort (RPE) instead of pace is a valuable tool.

• Treadmill performance varies a lot depending on the brand, belt type, and environment.

🔬 Deep Dive

The comprehensive review analyzed 34 crossover studies to see how physiology, perception, and performance stack up on the treadmill versus outside. They looked at runners’ oxygen use, heart rates, lactate levels, effort ratings, and actual run performances in both settings. Here are the key takeaways:

• Aerobic demand: At equivalent paces, oxygen uptake (VO₂) was virtually the same on the treadmill as outdoors, especially when the treadmill was set to a 1% incline, which many studies used to simulate outdoor air resistance. In other words, if you run a steady pace on a slight incline treadmill, you’re burning about the same energy as you would outside on flat ground.
  

• Lactate and muscle strain: Interestingly, runners showed lower blood lactate levels on the treadmill compared to outside. For the same steady pace, treadmill running produced less lactate accumulation than overground running. This suggests treadmill running might be less taxing, possibly because the moving belt assists leg turnover slightly or there’s no wind resistance. Your muscles don’t need to generate quite as much force to keep you moving forward, which translates to less lactate for a given pace.
  

• Perceived effort and heart rate: It seems how hard treadmill running feels can depend on the speed. At easy paces, the treadmill often felt a bit easier – runners reported lower Rating of Perceived Exertion (RPE) and had slightly lower heart rates compared to the same easy run outside. The controlled environment and consistent belt speed can make an easy treadmill jog feel very relaxed. However, at faster speeds (think threshold run or fast intervals), the trend actually flipped: RPE and heart rate were higher on the treadmill than outside at the same speed. I think in general a lot of runners I have talked to find treadmill interval sessions mentally challenging to get through, so this makes sense.

• Performance outcomes: endurance time trials (like a timed run to exhaustion or a set distance) were poorer on the treadmill on average. Runners tended to stop sooner or run slightly slower on treadmills, corresponding to a moderate performance decrease. Sprinting on a treadmill was more variable – some people actually hit higher speeds on a treadmill, others slower, but overall there was no clear difference in sprint capability. The review’s authors suspect psychological and environmental factors play a large part in this. For example: on a treadmill, you lack the cooling breeze; heat builds up and thermoregulation is worse, which can slow you down.

• Not all treadmills are equal: An important insight for coaches and athletes is the variability between treadmill models. The review noted that differences in treadmill design (ex: a very stiff deck vs. a bouncy one, calibration accuracy, etc.) can lead to different physiological responses. One cited comparison found a ~7% higher oxygen cost and 5% higher heart rate on one motorized treadmill versus another at the same speed. A rock-hard treadmill belt with less give can make you work noticeably harder than a softer machine. This high variability and, quite frankly, lack of good study design, contributed to the mixed results across all of the studies in the review.

So, is treadmill running a good stand-in for road work? For the most part, yes. Treadmill running is not a waste of time and you will see improvements in running and overall fitness if you run on a treadmill a lot. Be aware of the subtle differences: you might experience lower lactate buildup on the treadmill, changes in perceived effort at different paces, and a tendency to heat up or hold back during hard efforts. With those nuances in mind, let’s translate this knowledge into training tips.

Time for the lab notes ⬇️

🧪 Lab Notes

In every issue of this newsletter, the “Lab Notes” are going to be the protocols that you can apply to your training and routine right away. The goal of this section is to translate the science into actionable steps for the reader, whether you’re a recreational runner/HYROX competitor, or someone pushing the limits of their peak potential.

• Leverage the Treadmill as a Training Tool: Used wisely, treadmill running can complement your outdoor training. It offers precise control over pace and incline, and it’s a safe haven from bad weather. Treadmill running is also slightly lower-impact (softer surface and some belt assist), which can be useful for easy days or injury recovery. Just remember to still include outdoor runs if you race outside. You want to maintain that road-specific muscle conditioning and pacing feel.
  

• Practice Running by Feel, Not Just Pace: Given the variability in treadmill calibration and conditions, rely on RPE (Rate of Perceived Exertion) or heart rate to gauge intensity more than the displayed speed. A 7:30/mile pace on one treadmill might feel like 7:00 or 8:00 on another due to machine differences. Instead of locking into a specific number, tune into your effort level. For instance, if your threshold effort is normally an RPE ~7/10, set the treadmill to whatever speed gets you to that feeling (with a 1% incline to simulate outdoors). This approach accounts for days when the gym is hotter (raising your HR) or when the treadmill’s belt drag is different. Training by effort ensures you hit the intended stimulus, regardless of the treadmill model or room temperature.
  

• Adjust and Adapt: Finally, monitor how you personally respond to treadmill running. Some athletes thrive on the treadmill’s consistency, while others find it mentally tough. Over time, training in both environments will make you a more versatile athlete. The treadmill is not cheating – it’s a different modality that, when used thoughtfully, can enhance your fitness and prepare you for race day just as well as miles on the road (to a certain extent).

That’s it for this edition of the Threshold Lab. If you enjoyed this and found it useful, please share it with a friend!

What I Read and Researched for this Issue

• [1] Miller, J. R., Van Hooren, B., Bishop, C., Buckley, J. D., Willy, R. W., & Fuller, J. T. (2019). A Systematic Review and Meta-Analysis of Crossover Studies Comparing Physiological, Perceptual and Performance Measures Between Treadmill and Overground Running. Sports Medicine, 49(5), 763-782. https://doi.org/10.1007/s40279-019-01087-9