Hello and welcome back to the Endurance Lab newsletter.

In last week's newsletter, I talked about the Joyner model and the physiology of the marathon. Be sure to check it out if you haven't already.

I also had Emma Hoffman on the podcast. She is a freshman at the University of Colorado Boulder, and she talked about what she did in high school and how she went from a 20-minute 5K to 16-flat in the span of a few years. She was great to talk with, and I'm sure she's going to do big things in the coming months.

In this week's newsletter, I'm going to cover a topic I discussed in one of my recent Instagram posts: my experience with double-threshold training. I've already discussed what double threshold is and how to do it, but this time I collected data across my sessions and repeatedly performed the same workouts to see what would happen. I'll break all of that down here and compare it with some older studies that used more traditional approaches to discuss the potential pros and cons.

All right. If you don't know what double threshold or the Norwegian method is, go check out one of my older newsletters, where I discuss it in detail. For the sake of this newsletter, I'm going to assume you know a little bit about it.

Before I started double-threshold training, I made sure I was already in pretty good shape. I ran a marathon in March in roughly 2:37, and then immediately began a mile-training block. I was able to get my mile down to 4:24 and measured my VO₂max at 76 mL/kg/min. I also measured my lactate threshold and estimated it using 85% of my VO₂max, which is a commonly reported relationship in the literature.

Lo and behold, the two were similar but not perfect. Lactate threshold often occurs around 85% of VO₂max in trained athletes, which is what we call the fractional utilization of VO₂max.

What was interesting was that when I did 5 × 5 minutes at 5:24 pace, granted, while wearing a VO₂ mask, my lactate approached 9.4 mmol/L by the fourth rep. My VO₂ was about 65 mL/kg/min, which suggests that this was probably a little faster than my true threshold under those conditions. Again, this can vary depending on how threshold is measured and under what conditions. I did another newsletter on lactate threshold and critical power, which you can read for a longer discussion.

The main point is that although the pace corresponded closely with the VO₂ measured at threshold, I wasn't able to perform it like a controlled threshold workout, especially metabolically. My heart rate and oxygen consumption fell within a range that would largely be considered a threshold effort, but my lactate suggested something totally different.

There's a small point here: everyone is different, and guidelines are guidelines. If you've come off a heavy speed block, especially like I had, you may find that your lactate tolerance is quite high. What feels like threshold might actually be far more intense.

Over six weeks, I performed four basic sessions. In the morning, I either did 6 × 5 minutes or 10 × 3 minutes with one minute of rest. I started around 5:30 pace and progressively cut down to about 5:24 per mile. I did these on the exact same treadmill, at the same time of day, on the same day of the week, and in the same shoes. Yada yada yada. You get the point.

I kept everything consistent and alternated between 6 × 5 minutes and 10 × 3 minutes. My work-to-rest ratio was 5:1 during the five-minute reps and 3:1 during the three-minute reps. Threshold work can typically permit a work-to-rest ratio of about 5:1. I recognize that 3:1 does not necessarily do that, but that is okay. You do not need to perform the most intense session every day. The shorter work-to-rest ratio helped me recover while still allowing the training intensity to undulate, so I was not hammering week after week and was fresh enough to continue progressing by the third or fourth week.

I measured my lactate regularly toward the end of these sessions. The goal was to accumulate more work at threshold without crushing the workouts.

My Friday sessions were a bit more intense and specific. I completed several 20- to 25-minute tempos. During a few sessions, I also did hill sprints and some 400s to make sure I maintained enough intensity to continue driving adaptations. I have a goal of PR’ing in the 5K this fall, and I know I cannot do that without some dedicated work. This is also a feature of double-threshold training, or the Norwegian method. You still need specific sessions, sometimes called X-factor workouts, to ensure that you are actually running at the appropriate pace. I believe that to build running economy, you have to run the paces at which you want to race, and these Friday sessions helped me do that.

The first finding was that I had a 45% lower lactate at the exact same pace. Before training, at 5:24-per-mile pace, my lactate reached 9.4 mmol/L by the fourth five-minute repetition with one minute off. After six weeks, at the same 5:24 pace, my lactate was only 5.2 mmol/L, and that measurement came at the end of six reps rather than four. This is a reduction of roughly 4.2 mmol/L, or about 45%.

Figure 1. Blood lactate during five-minute repetitions performed at the same 5:24-per-mile treadmill pace before and after six weeks of double-threshold training.

During the 10 × 3-minute reps, my pace improved from 5:28 to 5:24 per mile, a four-second-per-mile change, or from approximately 11.0 to 11.1 mph. At the same time, my lactate fell from 4.6 to 3.2 mmol/L. That is 1.4 mmol/L lower, or a reduction of about 30%, despite the faster pace.

Figure 2. Pace and blood lactate during the 10 × 3-minute sessions before and after six weeks. Four seconds per mile equates to approximately 52 seconds across a half marathon.

My pace improved from 5:28 to 5:24 per mile, but my average heart rate fell from 152 to 149 beats per minute. Heart rate relative to speed fell from 13.82 to 13.42 beats per mph, which represents roughly a 2.9% improvement. This corresponds well with my lower perceived effort, which I also tracked.

The main point is that I did not look only at blood lactate. I also looked at heart rate and perceived effort, which suggested a lower internal cost at the same or higher workload. A lower heart rate does not directly prove that my VO₂ or running economy improved, but when combined with the lactate and perceived-effort data, it supports the possibility that I became more efficient. I'm going to measure this directly on the treadmill to get more data.

Figure 3. Average heart rate and heart rate relative to running speed during the five-minute sessions. The post-training session was faster, yet average heart rate was three beats per minute lower.

One thing I want to point out is that lactate rises nonlinearly as workload exceeds lactate threshold. A 45% reduction in lactate is not necessarily a 45% improvement in performance because there is a threshold effect. If the lactate curve shifts slightly to the right, you may see a modest improvement in pace but a massive change in lactate at a workload that previously sat above threshold. This helps explain why heart rate relative to workload changed by only about 3%, while lactate fell by 45%.

A lot of people said, 'Well, you didn't run another mile to see whether your performance improved.' I do not necessarily expect my mile performance to improve. I wanted to get better at running threshold, hence double threshold.

I mentioned the mile only to confirm that I was not starting from a totally unfit place. I was starting from a trained state in which my VO₂max and lactate threshold were already at a high level, considering my training background.

Technically speaking, I got better at running 6 × 5 minutes with one minute of rest. Six weeks later, double-threshold Bill was better than six-weeks-earlier Bill at running those sessions. I was also better at running a 25-minute tempo, which makes sense because those efforts are far more reliant on the ability to run at threshold than the mile. The mile is still largely aerobic, but many other factors determine its outcome.

Here is the full breakdown. A 5:28-per-mile threshold pace produces an estimated half marathon of approximately 1:11:40, almost exactly matching my previous half-marathon PR. At 5:24 per mile, the estimate falls to roughly 1:10:48. That is an improvement of approximately 52 seconds over a half marathon.

The important distinction is that this training did not automatically improve my half marathon by exactly 52 seconds. It suggests that my threshold pace improved to a level consistent with approximately 1:10:48 if I could sustain that pace across the distance. The half marathon is one of the race distances most closely associated with lactate-threshold fitness.

Obviously, this is not without limitations. I'm one athlete, and this was not a controlled study. I controlled for weather and terrain by using a treadmill, and I also used the same time of day and the same shoes. Lactate can vary with nutrition, glycogen, sleep, hydration, and measurement error. That is why I measured it week after week, and the overall pattern showed consistent improvement.

This cannot establish causation, but the direction and magnitude of the changes, for me at least, are difficult to ignore, especially when combined with the anecdotal evidence we're seeing internationally and nationally. Jakob just won the 5K again, and we know he is a massive proponent of this approach.

Unfortunately, I cannot find a good study examining this exact threshold-training approach in elite endurance athletes who are already highly trained. That probably helps explain why the effect I observed was smaller than the changes reported in historical studies such as the Hickson protocol, in which participants showed enormous increases in VO₂max and power output. Those participants and training conditions were very different from mine.

Some studies also show that when athletes decrease overall training load while maintaining or increasing intensity, a plateau is common. Basically, the fact that I saw any improvement from an already trained state is meaningful to me.

In summary, double threshold did not create a massive shift overnight, but it gradually allowed me to run a little faster and perform more work at a lower physiological cost. From everything I can tell, an improvement in running economy is one possible explanation, although I am going to measure this directly on the treadmill as soon as my Adidas shoes come in.

This is a slow and controlled approach to improving fitness. It is not super sexy, and over six weeks you might say, 'Well, that is a minimal improvement in threshold.' You would be right. But I saw the improvement, stayed healthy, and think that if I keep doing it, which I am going to do, I should continue to see gradual progress.