Hello and welcome back to the Endurance Lab newsletter.

In last week’s newsletter, I talked about double threshold and all the specifics, including types of workouts, lactates you should hit, and even some of the less-talked-about details, such as the x-factor workouts and the use of heart rate in your warm-up or reps to determine how you should progress. Keep in mind that this is all based on Bakken’s articles and not necessarily his book, so if you’re interested in that, you should go check it out. He was also recently featured on the Running Effect podcast with Dom. Dom is a good friend and works hard on his content. Another great resource.

This past week, we had Brannon Kidder on the podcast from Brooks Beast, and it was a total blast. Check it out here if you haven’t done so already.

As always, if you’re interested in being coached, check out our website. We’ve had 1 high school athlete race thus far… In his opener, he ran 14:48 thru 3 miles. That was an 11 second PR and over a minute faster than last year.

Alright, this newsletter was inspired by a conversation I was having with some of the runners on Elon’s cross-country team this past week. They were about to do hill sprints, and I said run these at 5:20/mile pace or about tempo pace.

One of the freshmen spoke up and said, “I thought tempo was slower than threshold, so why are we running these at our threshold pace if they should be tempo?” Then another said, “I thought threshold was 10K pace. Why is threshold faster than tempo?”

The truth is they all overlap.

But it really matters. If you don’t accurately define the terms, it adds to confusion in the field among coaches and athletes. If the effort isn’t what you actually want it to be, that matters. Because now you’re prescribing workouts of the wrong duration or wrong intensity that are going to elicit the wrong adaptation at the wrong time of the year, all because of a lack of clarity.

This is even more important in younger athletes because they’re taking you at your word. You may not realize it, but when you tell them to run a pace, they’re going to run that pace, or at least try to. They’re not going to have the experience to know, especially for themselves and their individual physiology, when to rein it in or pull back.

Also, consider that you’re going have different athletes with different strengths. If you give a miler hill sprints and tell them threshold effort, that’s gonna feel very different than if you tell a 10K runner threshold effort on a hill sprint. The miler is inevitably going to run faster because they have a much higher VLaMax and hydrogen tolerance.

The first I’ll talk about is critical velocity. 

The origins of the critical velocity concept can be traced back to A.V. Hill’s work in the 1920s on the relationship between race duration and average speed.

In 1925, he published a paper looking at the relationships between time occupied in a race in seconds and the average speed in yards per second [1].

Basically, what he did was take top velocities across distances for speed skating, running for men, and running for women. He even had men walking.

As you might expect, there’s a curvilinear relationship between race duration and average speed. As race duration increases, the velocity you can sustain decreases. But importantly, it doesn’t decrease at the same rate forever.

This is discussed thoroughly in Dr. Joyner and Dr. Coyle’s publication entitled Endurance Exercise Performance: The Physiology of Champions, published in 2007 [2].

Av Hill’s work established what we now know as CV. In short, if you take your race performances or perform two or three all-out time trials lasting between about 2 and 15 minutes, you can pretty accurately determine your critical velocity.

If you plot velocity against time, what you’re going to see is a hyperbolic relationship. Velocity falls rapidly as duration increases, but eventually that decline begins to flatten out and approach a horizontal asymptote. That asymptote represents critical velocity. You can see it well listed above in rodents. You can also see how the curve might shift with aging, heart failure or training.

There is a critical intensity beyond which the amount of time you can sustain a given velocity starts to fall very rapidly with even relatively small increases in speed. We’ll come back to this in a moment.

That’s why getting these intensities right actually matters. Believe it or not, this is actually why I like treadmill work for double thresholds. You can control the speed and make sure athletes aren’t constantly creeping above the intensity you’re trying to target.

Want to know another cool thing about CV? We can roughly estimate for elite runners. For example, Dr. Jones calculated the CV for Haile Gebrselassie using an alternative calculation for CV based on a linear distance time model. His CS was 4:32 minutes per mile [5]. Madness.

Okay, so that’s critical velocity.

When we talk about a threshold intensity, this is an intensity slightly below your lactate threshold, which is determined by doing a graded incremental exercise test on a treadmill or track. You increase the speed in known increments by about 0.3 mph every 3 minutes, with very, very short rest, ideally less than a minute and as short as 30 seconds, keeping the rest consistent throughout.

You can then use methods such as DMAX or even a simple eyeball test to identify where the blood lactate-speed relationship begins to change and lactate starts rising more rapidly. That gives you an estimate of your lactate threshold, where we get the term threshold from.

Unlike CV, this test doesn’t involve any maximal efforts, but it does involve a lot of sampling, finger pricks, and a biohazard.

So how do critical velocity and lactate threshold actually differ?

Well, this is where it gets interesting because they’re measuring different things, but they are closely related.

Critical velocity is mathematically determined from the relationship between performance and duration. Lactate threshold is physiologically determined from the blood lactate response to increasing exercise intensity. But both are trying to tell us something about where exercise transitions from something that can be sustained for a relatively long time to something where fatigue starts developing much more rapidly.

In a classic study by Andy Jones, when they had individuals exercise at 10% lower power than their critical power, they could hold it for a very long time. Conversely, when they increased the power 10% above their critical power, they lasted for a fraction of the time; only 3-15 minutes [4]. See below. THe time to exhaustion is vastly different, and so too is the blood pH. In one condition, it sits around 7.05 and the other it drops as low as 6.8 until total exhaustion. Shout out bicarb for helping with that.

And guess what: if you did something similar around someone’s lactate threshold, you’d see a pretty similar phenomenon. 

For example, my lactate threshold is about 11 to 11.2 mph. If you told me to run at 9.9 or 10 mph, I could probably hold that for over 2.5 hours because that’s my marathon pace. Conversely, if you told me to hold a speed about 10% higher than my threshold, which is about 12 mph, I could only hold that for 12 to 15 minutes.

Small changes in speed around these intensities can create very large changes in how long you can sustain them.

I’ve talked with two of the foremost researchers on this topic, David Poole and Andy Jones, separately, and they’ve both confirmed the general concept I’ve written above.

According to Jack Daniels, a tempo is simply a threshold intensity for a steady-state run of 20 to 40 minutes. Keep in mind, you can only hold something around your threshold for about an hour. I got some heat on this from a post a while back, but again, I validated this with several exercise physiologists, and my suspicions are correct.

Sidebar. I meant to ask Andy Jones if he felt like someone could hold their lactate threshold for a marathon at the elite level, or roughly two hours, but I didn’t get enough time. Next time I have him on, I’ll ask.

Nonetheless, I think it’s unlikely, especially depending on what you consider to be their actual threshold. Do you mean sub-threshold, super-threshold, or at-threshold? At the end of the day, they’re going to fluctuate throughout the race.

The reason your tempo speed will generally be slower than your threshold is simply because you don’t have any rest and it’s harder to sustain without a race.

Even though you could hold something around that threshold intensity for a long time, it doesn’t mean you should sit directly on it during tempo workouts because inevitably you’re going to run up a hill or hit a gust of wind, or it might be hot, or whatever it is. Now you’ve gone slightly above the intensity you’re trying to target and fatigue starts accumulating much more rapidly as shown above.

Remember the critical velocity curve. A relatively small change in speed above that critical intensity can have a huge effect on how long you can sustain it. So tempo pace is generally going to be slightly slower than your actual threshold. You’re giving yourself a little bit of space.

You’re protecting the athlete from blowing up on the workout, getting in their head, letting whatever fatigue, cognitive dissonance, or distortion infiltrate and prevent them from finishing the workout. 

Also, from a physiological perspective, it’s not like running those extra few seconds faster always matters in those tempo sessions. Now, there may be a time when it does matter, but that’s typically when you’re trying to progress someone downwards and they’ve got a few tempos under their belt. So it’s better to stay conservative on a tempo, especially earlier in the season, than to rip.

Which answers the Elon students’ original question: why is tempo slower than threshold?

Yes, work is work, and both will continue to push physiology in the right direction, but that doesn’t necessarily mean it’s optimized. If you want to optimize work, then it’s best, although not always ideal, to have these intensities dialed in and progress them down accordingly.

And with that, you have to take the athlete’s current state into consideration: 

Are they in a high-mileage week?

Is it hot and humid?

Are they wearing super shoes?

When was their last workout?

How are they telling you that they feel?

Any or all of these should give the coach and the athlete space to adjust accordingly during the workout.

You can always come back and increase intensity later, but it’s much harder to bounce back if you’ve already gone above and beyond your threshold intensity.

References

1.         Hill, A.V., THE Physiological Basis OF ATHLETIC RECORDS. The Lancet, 1925. 206(5323): p. 481-486.

2.         Joyner, M.J. and E.F. Coyle, Endurance exercise performance: the physiology of champions. J Physiol, 2008. 586(1): p. 35-44.

3.         Copp, S.W., et al., Reproducibility of endurance capacity and VO2peak in male Sprague-Dawley rats. J Appl Physiol (1985), 2009. 106(4): p. 1072-8.

4.         Jones, A.M., et al., Muscle metabolic responses to exercise above and below the "critical power" assessed using 31P-MRS. Am J Physiol Regul Integr Comp Physiol, 2008. 294(2): p. R585-93.

5.         Jones, A.M. and A. Vanhatalo, The 'Critical Power' Concept: Applications to Sports Performance with a Focus on Intermittent High-Intensity Exercise. Sports Med, 2017. 47(Suppl 1): p. 65-78.

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