Hello and welcome back to another installment of the Endurance Lab Newsletter. I hope everyone’s doing well. Frankly, I’m getting pretty tired. The semester started in early September and I’m only one month in. I’m kind of cooked between testing people, writing newsletters, doing podcasts, making content, coaching, having a family, trying to stay involved in church, and many other things.

Hopefully none of that influences the quality of this newsletter. I suppose I’m using this somewhat as an open journal, but alas, on to the content for today.

I was torn between a few things, so here’s a brain dump of the papers I read and what I gathered. I’ll wrap it up with the best paper at the bottom.

There’s a recent paper suggesting you can use the Wingate test to predict muscle fiber type, particularly the proportion of muscle area occupied by fast-twitch fibers. I thought that was cool, but I might save it for another week because it’s kind of complicated.

Another paper looked at the use of near-infrared spectroscopy. It was published by Jake Hudgins[1], who was on the podcast two weeks ago to talk about power output. He uses something called the zero-slope method with a Moxy. There was a letter to the editor in Medicine & Science in Sports & Exercise, the journal where the paper was published. It’s kind of infuriating to read because a lot of the criticisms seem to miss that Jake was looking at changes in the slope within the model, not trying to interpret absolute oxygenation values.

There was also a paper on core temperatures in people running Western States[2]. The researchers got usable in-race temperature data from 23 finishers, and the faster runners tended to reach higher peak temperatures. Nine exceeded 39°C and two briefly exceeded 40°C. My first thought was that the faster runners were probably producing more metabolic heat because they were sustaining a higher output. But they also encountered the hottest parts of the course at different times, so I wouldn’t take this to mean that getting hotter caused them to run faster. Still, it makes the relationship between core temperature and performance more interesting than “hit a certain temperature, and you stop.” Seven runners actually reached their peak temperature in the final 2% of the race.

For some reason, there were a ton of papers on heat exposure in the past month. Maybe it’s because we just got done with summer, or maybe there’s DOD money. I’m not sure.

 I read another paper that somewhat validated the use of newer sensors that infer blood lactate from sweat[3]. The researchers tested whether the sensors could identify the second metabolic threshold and maximal lactate steady state. In 36 active people and endurance athletes, the sweat-inferred estimate of threshold differed from the ventilatory threshold by an average of only 1.9 watts. That sounds incredible, but the average absolute error was 9.1 watts. Compared with the threshold derived from capillary blood lactate, the average absolute error was 10.6 watts. Those numbers tell me more than a correlation alone would.

I still have questions about using one during a workout. Sweat lactate doesn’t necessarily change at the same instant as blood lactate, and the device is estimating blood lactate rather than measuring it directly. I’d want to know how it behaves when someone makes a quick change in pace, and how often it puts an individual athlete on the wrong side of their threshold. A good estimate from an incremental test doesn’t automatically mean the live number is useful for adjusting intervals in real time. But I have to give the researchers credit: they compared the estimates with capillary blood lactate, gas exchange, and maximal lactate steady state. This is more convincing than showing a high correlation and calling it a day.

• High vs. low load resistance training was another[4]. The researchers assigned 201 people who weren’t resistance trained to different groups. The training groups did unilateral elbow flexions to failure three times per week for six weeks, using either a load they could lift 8–12 times or a lighter load they could lift 20–30 times. Both groups increased muscle thickness, but the low-load group gained more: about 0.30 cm versus 0.20 cm. Tendon thickness increased slightly in one region in both training groups, but it also increased in a group that came in for testing and a mid-study one-rep-max test. There was no detectable change in the other tendon region.

My initial thought was that runners might want the heavy-load version to get the tendon adaptation without as much muscle growth. But I need to be careful with that one. This was an elbow-flexor and distal biceps tendon study in people who weren’t resistance trained. It doesn’t tell us what happens to a runner’s Achilles tendon, and high-load training did increase muscle thickness. The interesting part is that muscle and tendon thickness didn’t respond to the two loads in the same way.

 The Main Paper, Number Six

But the one I really want to talk about is an old paper by R.C. Hickson[5]. Yes, the Hickson of the famously hard Hickson interval protocol. His work sits alongside that of John O. Holloszy and others who helped establish much of what we know about endurance training adaptations. This particular paper asks a question I find very practical: When you have to train less, what should you actually cut?

Hickson and colleagues first had people train for 10 weeks, six days per week. Three days involved cycling intervals: six 5-minute bouts at a work rate approaching VO₂max, separated by 2 minutes of rest. On the alternate days, they ran continuously. The runs progressed from 30 minutes in week one to 35 minutes in week two, then 40 minutes thereafter, and they were told to run as fast as possible. This was a hard program. Notice how I said approaching VO2max. You can’t actually hold max for that long, especially on that rest, but that’s a different point.

After 10 weeks, VO₂max had increased on both the bike and treadmill. Three separate studies showed that VO2max increased by 15-20% from this study.

Here’s what I found most compelling, though. Across this paper and Hickson’s earlier studies, the researchers reduced one training variable at a time for another 15 weeks: frequency, duration, or intensity. The final panels of the 1985 paper put the results next to one another. And boy are they a beaut. 

Above is the treadmill data.

Above is the cycling data.

What do you notice? Which parameter seems to be the least important to maintain? 

Frequency. When they cut frequency by 1/3, the VO2 continued to climb. When they cut the duration, VO2 continued to climb. The only thing that resulted in substantial drops when cut was none other than intensity. Intensity is always key to maintaining VO2max. You have to do the thing you want to be good at. If you want to maintain VO2, you need to exercise at VO2. It’s that simple. 

The performance data make this more interesting. In the group that reduced intensity by one-third, performance during tests lasting about five minutes was maintained despite the decline in VO₂max. But their time to exhaustion during a longer cycling test dropped from 184 to 145 minutes, a 21% decrease after 15 weeks. In the group that reduced intensity by two-thirds, five-minute cycling performance had already declined by week five, and long-duration cycling performance dropped from 202 to 141 minutes, a 30% decrease by week 15. They only repeated the long test at the end, so we don’t know exactly when that decline began.

This is also why I wouldn’t say duration doesn’t matter. In the earlier study, cutting sessions from 40 to 26 minutes preserved long-duration performance, but cutting them all the way to 13 minutes came with about a 10% decline. Maintaining VO₂max and maintaining your ability to keep going for a long time are not necessarily the same thing.

To me, the practical takeaway is pretty straightforward. If life gets busy and you need to reduce training, cut frequency first while keeping some sessions hard and maintaining their duration when possible. You can cut duration and still preserve VO₂max, but if you cut it too far, you may give up some long-duration endurance. What you probably don’t want to do is keep showing up six days a week but make every session substantially easier and assume you’re preserving all the same adaptations.

I think this has a ton of implications for triathletes. You have three exercise modes to hit, and there are weeks when you simply can’t maintain your usual frequency in all three. Hickson’s data suggest that, during a temporary reduction in one discipline, keeping a smaller number of purposeful sessions may preserve aerobic fitness better than spreading your available time across a bunch of sessions that no longer provide much of an intensity stimulus.

Of course, these were small studies in people who had just completed 10 weeks of training. They were maintaining newly gained fitness, not preparing trained triathletes for a race. And “maintained VO₂max” doesn’t mean every aspect of performance was maintained. Still, I find the result reassuring: when the schedule falls apart, you may have more room to cut sessions than you think. Just be thoughtful about what you keep.

1.​Hudgins, J.H., M.I. Schoeberlein, and B.W. Wilkins, Reliability and Repeatability of Determining Power Associated with Maximal Metabolic Steady State Using Changes in NIRS-Derived Muscle Oxygenation. Med Sci Sports Exerc, 2026. 58(3): p. 517-526.

2.​Mougin, L., et al., Gastrointestinal temperature of elite and recreational adult runners during a 100-mile ultramarathon under heat exposure. Physiol Rep, 2026. 14(17): p. e71089.

3.​Cosio, P.L., et al., Determination of Second Metabolic Threshold and Maximal Lactate Steady State using Sweat-Inferred Blood Lactate in Active Individuals and Athletes.Medicine & Science in Sports & Exercise, 2026. 58(10): p. 2314-2327.

4.​Hammert, W.B., et al., High- and Low-Load Resistance Training Produce Distinct Skeletal Muscle Growth but Similar Changes in Tendon Morphology. Medicine & Science in Sports & Exercise, 2026. 58(10): p. 2274-2282.

5.​Hickson, R.C., et al., Reduced training intensities and loss of aerobic power, endurance, and cardiac growth. J Appl Physiol (1985), 1985. 58(2): p. 492-9.

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