Hello and welcome back to the Endurance Lab newsletter.

In last week’s newsletter, I broke down the training it took for Josh Kerr’s mile world record. Turns out, his coach, Danny Mackey subscribes to the newsletter and saw it. While that caused a bit of anxiety, he said, “You did pretty darn good for piecing it together, actually”, and then he agreed to come on The Endurance Lab Podcast.

I took that as a big compliment. 

Our team of athletes that myself and Miss Stephanie Brokaw coach at the Endurance Lab have been killing it with the summer training, and we are eager to add more dedicated and hungry runners join our team. Book a free consultation call with me and see if you’d be a good fit! https://www.endurancelaboratory.com.

This week’s newsletter will tell you EXACTLY what happens to oxygen consumption during repeats, and some of the ways you can modify your repeats to maximize time at VO₂ max, including why you don’t actually need to run at VO₂max to hit your VO₂max.

Very simply, your VO2 is a measure of the amount of oxygen you can consume at any point in time relative to your weight. In lay terms, it shows how well your heart and lungs work together to deliver blood and oxygen to make energy during exercise. If it’s low, you can’t walk up steps. If it’s high you can run a marathon really fast.

Generally, the value ranges from 14 ml/kg/min in those with congestive heart failure [1], all the way up to 80 or 90 ml/kg/min in elite cross-country skiers.

Table 1. Physiological metrics in those with heart failure

Side bar: I spoke with Dr. Filip Larsen last week on whether this was a function of using upper body mass or due to the cardiac features of cross-country skiing. The rationale behind this question is that you use more muscle, so more tissue is consuming oxygen which contributes to the overall oxygen consumption to inflate the measure. He said he thinks it really has to do with the physiological adaptations. In other words, you can’t put cross country skis on someone like me to artificially inflate my VO2 during a test. Similarly, cross country skiers still have insanely high VO2s while running according to Dr. Larsen. So there you have it. There are implications to this, but another time…

To measure VO₂max, we perform a graded exercise test. A classic is the Bruce protocol [2]. It’s gnarly. I’ve gotten to stage 7 and almost died: 6 mph with a whopping 22% grade. For context, in 3 minutes, you would cover 0.33 miles and about 350 ft of gain… 

 To actually measure it, you wear a fancy mask that measures expired air. It measures amount of air, Nitrogen, Oxygen and CO2. Nitrogen doesn’t change, so we use that as a normalizing factor with external air, then we use environmental conditions to calculate the change score in CO2 and as well as the flow rate to get the amount of air. Do all that and voila. You get VO2. Back in the day, they used to use these bad boys: Douglas bags.

You’d literally fill these up with air and literally measure the CO2 in them. Jack Daniels swore by this and never trusted the electronic stuff. Love it.

The moment you’ve all been waiting for! So, to be honest, I can read as much as you want on oxygen consumptions, lactates, max testing etc., but until I do it, feel it, and measure it all, it doesn’t really click.

In short, I ~tried~ to do 5 x 5 minutes at what I thought was my threshold (5:24 pace – 11.1 mph). I say thought because it definitely was not on this day. Whether that’s from fasting, trying to run with the mask or a genuine drop in fitness it’s hard to say, but the mere fact was SUPER informative.

So a few things, during the first rep, the mask came lose, so had to stop and readjust, second rep was fine, third rep was fine, but I took my mask off after it to measure lactate (I was curious how it corroborated with my RER and effort), then I took 2’ after the 4^th rep to put the mask back on and finish the 5^th rep before measuring a lactate.

Point 1: VO2 for the first rep takes about 2-3’ from rest to hit steady state. Now compare this to the second rep. VO2 hits steady state within 60 seconds. This is why the first rep always feels the worst. You are largely using anaerobic processes to compensate for the slower mechanics of blood flow and “oxygen kinetics” – how fast it increases (yes, that is my m dash, not AI’s).

This brings up point 2: How fast you reach steady state matters and changes dynamically. For one, the rest will determine how far oxygen consumption will fall. When you are warm, you get the much more rapid increase in oxygen consumption, because 1) you are starting from a higher baseline (see rep 2 below), and you have local vasodilation from metabolite accumulation and shear stress, so there’s not as much resistance to flow peripherally. Basically your blood vessels are more reactive and have less resting tone. People who are more fit have faster oxygen kinetics. These people perform better, and can better adjust to changes in intensity during a rep or race.

Point 3: Oxygen consumption increases rep over rep due to fatigue and the slow component of oxygen uptake. This is why you don’t necessarily need to run at VO2max to hit VO2max. If you look, you can see a VERY slight positive slope on reps 2 and 3 even after reaching steady state.

Basically, the VO2 is drifting upwards. We call this the slow component of oxygen uptake. This is most visible in the 5^th rep.

Despite the fact that I decreased the speed to 10.9 mph, and took 2’ rest as opposed to 1’ rest to put the mask on, you can see a massive increase in O2 consumption all the way up to 70 ml/kg/min. Now, as you can see by my blood lactate, I wasn’t actually at my lactate threshold. I was much faster than it (lactate should be at 4.0 mmol not 9.0 mmol…) This is why you see the increase in O2. Now interestingly, you can SEE THIS in my stride. It get’s ugly, I start flailing, and I’m less smooth. Basically, I fatigue.

Point 4: Lactate and VO2 work together, but VO2 drifts will lag behind lactate spikes. VO2 and lactate are obviously correlated, but you can see here: My VO2 was still stable despite my lactate going from 5.2 mmol during rep 3 all the way to 9.2 mmol during rep 4. It wasn’t until rep 5 that my VO2 started to corroborate what was happening with my lactate, and this was largely because the fatigue started to mess with my motor patterns and impair efficiency and economy (I started to use too much unnecessary muscle mass, mainly upper body.)

Point 5: Your rest will determine your VO2 as much as the intensity you pick. When you use short rest, you can increase the total time you spend at any given VO2 because you never return to baseline. As such, you can use a 25-30s rest to basically keep VO2 elevated to reduce increase the total time you spend at any given VO2. If you take 2-3’ rest, and only do a 2-3’ rep (typically a 1:1 work to rest ratio is recommended), then you only get 1.5 minutes at VO2max. It just depends what you want.

Point 6: Once you go past your lactic threshold by too much, there’s no coming back. I was surprised by this. Even though my VO2s didn’t spike necessarily until the very end, my lactate hit 9.0 mmol, then I took 2’ rest AND slowed down to 5:30 pace, and it still stayed elevated around 9. Think about that. I doubled my rest. I slowed down substantially, and still was doomed. This is a word to the wise: trust your pace and don’t go out too hard. You’ll ruin the workout before it starts.

Point 7: Trying to do 5 minute repeats at your actual velocity at VO2max is impossible unless you take massive rest. I know Jack Daniels says you can, but he also says VO2max pace is 3k pace, and I have yet to see that. Granted, my treadmill doesn’t hit that velocity, but I have tested many people recently, and by and large velocity at VO2max is MUCH closer to mile pace than 3k pace. Now, I was running at 88% of my VO2max, and I could BARELY do 5 minutes reps. My lactate was 9 mmol. I was rigging. My actual velocity at VO2max? 4:28/mile pace. That’s awfully closer to my mile time (4:24).

I recently saw an elite athlete’s VO2 data (13:00 flat 5k), and his velocity at VO2max was 3:52… guess what that was? About his precise mile time.

Moreover, here’s a study where they measured time to exhaustion at velocity at VO2max. Guess what? It was 150 seconds to 300 seconds or 3-5 minutes… And, it was highly repeatable [3]…

Now, here’s what I also want you to hear: I don’t know if this matters.

At the end of the day, whether you run at 71 ml/kg/min or 75 ml/kg/min, you’re still going to adapt, drive up VO2/maintain it.

There are tradeoffs to both. Run at 71, and you’ll be running 3k pace, still awfully close to VO2max and you’ll likely hit max towards the end of the workout because of VO2 drift. Tradeoff: you get very little time at actual max.

Conversely, run 75ml/kg/min, and actually run at VO2max, but now you are just doing mile work. How much of that can you do? Probably about 800m at a time at maximum, maybe 4 reps total and with full recovery. If you run a 4:20 mile, that’s only 1 minute of time at VO2max per rep, or 4 minutes total. Meaningful? I don’t know. You tell me! 

I get that this is complicated and there are a lot of nuances, but here are the key takeaways:

1)    You can run above lactate threshold to hit VO2max without running at your actual velocity at VO2max

2)    Intervals increase or maintain VO2max

3)    Lactate spikes before VO2

4)    VO2 is stable at an intensity until fatigue makes your form sloppy

5)    Shorter rest increases time spent at any VO2

6)  VO2max velocity is probably closer to mile pace than 3k pace.

Go listen to the podcast, go follow me on Instagram, and share this newsletter with your friends!

Cheers,

1.         Roibal Pravio, J., et al., Determinants of maximal oxygen uptake in patients with heart failure. ESC Heart Fail, 2021. 8(3): p. 2002-2008.

2.         Bruce, R.A., et al., EXERCISING TESTING IN ADULT NORMAL SUBJECTS AND CARDIAC PATIENTS. Pediatrics, 1963. 32: p. Suppl 742-56.

3.         Pallarés, J.G., et al., Time to Exhaustion at Traditional Physiological Indicators in Runners: Between-Subject and Between-Day Variability. Int J Sports Physiol Perform, 2026. 21(4): p. 590-596.

Book a free coaching consultation call if you are interested in joining the team, here: https://www.endurancelaboratory.com