HIIT is not endurance training for elite athletes

It's been a month since I posted, and despite being quite busy, I'm going to type into the internet void.  High-intensity-intervals-as-endurance-training is all over my usual internet haunts, specifically r/science and r/velo.

As usual, the link goes to a news article rather than the study, though the study is linked at the end of the article--a practice I wish were more common.  From the article:

Short bursts of just a few minutes of exhausting physical activity [boost] the production of new mitochondria...which culminates endurance enhancement much like more time consuming endurance training.  High-intensity exercise triggers the breakdown of calcium channels as a result of an increased production of free radicals. The muscle cells thus have anti-oxidative systems for trapping and nullifying the radicals.

Moving backwards, all cells have methods to nullify unwanted radicals, but let's recall that radicals are not necessarily bad.  Here's a primer that's long and excellent.  So the ryanodine receptor type 1 (RyR1) and sarcoplasmic reticulum (SR) are disrupted by HIT, which causes radical generation that the cell recognizes as oxidative stress.  In turn the cell makes more mitochondria.

Popularly, this is hailed as hard evidence that HIT is just as good as low intensity training.  An astute observer from r/velo notes: "To me this reads like, train like a track sprinter and become the next TT world champ."  Indeed.  Clearly that never happens, so what's going on?

All we really need to know from the study to answer this question is in the abstract, which is publicly available.  It states that the main study was done on moderately active individuals, and that in elite endurance athletes "the measured transcript levels related to mitochondrial biogenesis and endurance showed a general decrease— rather than the expected increase—24 h after the HIIT exercise."  The explanation for this is simple.  Experienced athletes spend a good deal of time activating their muscles, and any easily fracturing RyR1 or leaking SR would be some of the first adaptations that take place in order to facilitate more exercise.  Because let's face it, if your calcium machinery isn't working properly, you're going to have serious problems with muscle contractions.  To confirm this, another quote from the abstract: "the same HIIT exercise does not cause RyR1 fragmentation in muscles of elite endurance athletes."

MLSS = FTP? Plus VO2max and 20' power variability

I've been sick and NOT racing Gloucester, so I've got more time on my hands than usual to think about these things.  I started with this question: is MLSS the true functional threshold of power which, by definition, equals the maximal one hour time trial effort?

What used to be here were two paragraphs of ranting about how lab tests can't inform our training and how what we use as cyclists doesn't relate.  Then, instead of assuming my logic was infallible here in my ivory tower (which I would have done ten years ago), I looked to confirm it.  Every single study I found that tested time to exhaustion at MLSS was 50-70 minutes.  Here is a fascinating study that shows that 90% of your MLSS velocity (not power because air resistance isn't linear blah blah), give or take about 5%.  Let's talk about it before we go on.

Four subjects actually do 38-40kph for a true 1 hour time trial (or as close as I'm willing to forgive based on no criteria).  The AVS column tells the whole story, showing velocity at MLSS and velocity in the 40k TT.  What I'm fascinated in is the variation of the 5km TT time.  I calculated 40kmh at around 8 minutes, so what we know as the long side of a VO2max interval and this we know to be variable among cyclists.  We can conclude from the table above is that while the average of 5km TT speed is about 92% of your MLSS speed, the variability is between 86-95%.  This is a very wide range, and let's make up some numbers.  Let's pretend briefly that power and speed are close enough to linear at these speeds, so 5% of 300w is 15w.  Over a long TT, if this overestimates your FTP, trying to hold the extra 15w is going to blow you up in short order.  Going under won't blow you up, but 15w can make a big difference over 30-60 minutes.

Real life example of short efforts being variable: last year with a 300w FTP I was doing 5' VO2max intervals at 380-400w, let's average it at 390.  My friend was doing 400w on the same 5' hill with an FTP of 370w.  That's 108% of his FTP and 133% of mine.  He now trounces me on any duration longer than 2 minutes, but this illustrates that I've got a larger amount of readily available anaerobic reserve.  This is going to greatly influence any efforts I do in terms of estimating my true MLSS/FTP.  Including 20' tests?  Yes.

I just equated MLSS and FTP, so, quickly: FTP is a reference defined as the power maintainable for one hour.  In many studies where MLSS has been held to exhaustion in various sports including running and cycling, the times I've seen were 50-70 minutes.  So for my purposes, equating the two will be close enough.  It's not precise, it's not scientific, but it's my way of relating commonly used training parameters to the existing body of literature.

Anyway, since I don't know any cyclist who sets their training zones based on speed, well go back to last post's study that some great variability in 20' TT performance when it comes to predicting MLSS.  My educated guess is that this comes from a high level of FRC involvement which will vary by individual.  I'm sad that the individual power data wasn't published because then we'd be able to perform some of our own statistical analysis.

It'd be nice to see is 20' power vs measured MLSS by rider type.  The hypothesis is that the variation in 20' test results is influenced by FRC, and in conjunction with the advanced metrics in WKO4, we'll be able to get a more accurate picture of where an athlete's FTP is in terms of %20' mean maximal power.  You're saying this is a very small point to harp on, and it is.  It's also reasonable, based on the analytic tools to want to be able to tell a rider his or her FTP is only 93% of a 20' test, or 97%, or wherever their phenotype puts the estimate.  95% is a pretty good estimate for people inside the bell curve, but for the people outside of it (speaking for myself and some athletes), it can be frustrating.

Lab tests, 20 and 90 minute time trials

There's frighteningly little literature on how lab tested variables relate to the standard field tests cyclists use.  There's more than I'll ever be able to read when it comes to comparing in-lab tests to other in-lab tests.  VO2max and peak wattage in a ramp test?  Got it.  Comparing MLSS, Dmax, CP, OBLA, or a certain % of VO2max?  Got loads of those.  I've been digging a lot to find some good studies that actually use things that cyclists do on a daily basis.

Enter exhibit 1.  20 and 90 minute time trials from elite and internationally competing cyclists were used to test a variety of previously defined in-lab LT markers.  It's got some very, very interesting data.  First, 20 and 90 minute TTs were not well correlated: r =0.66, p=0.54.  I would rather they had done a 60 minute test because of the relationship we always hear between a 20 minute maximal effort being 105% of your FTP, which is ostensibly the power you can hold for an hour.  But it's actually fine that they didn't use a 60 minute test, because a 90 minute test is going to be pretty close too, and I'll examine why.

We've seen of TTs that are well over an hour, like this year's Giro Stage 14 that was won by Vasil Kiriyenka (spoiler alert: he just took the world ITT title).  Second on the Giro stage, LL Sanchez, who also competed in worlds, was down 2:45 at worlds and only 0:12 down in the Giro.  I know, worlds and grand tour ITTs are different, but because Sanchez and Kiriyenka competed in the Giro, Vuelta, and worlds this year, so we can feel comfortable in comparing their performances.  As a side note, many other top finishers in the Giro TT were well known breakaway riders and long TTers, such as Sylvain Chavanel and dare I say Alberto Contador on the rare chances he can get away, like Stage 17 of the 2012 Vuelta.

Why'd I go over that?  Oh yeah, 20 minute power doesn't correlate well with 60 minute power, and most likely doesn't correlate well with 90 minute power.  In the graphic above, the x-axis is self explanatory, but the y-axis test went like this: 3 minutes at 50% VO2max (previously determined), then increasing the workload by 5% VO2max until voluntary exhaustion.  Highest wattage reached during this test was the Wpeak.  And how well did this correlate?  Compared to most other variables tested, very well.

All the variables tested here are lab things, nothing cyclists can generally do on their own.  They all require equipment that measures VO2max or blood lactate (though there are some semi-affordable ones of the latter) and occasionally a little graphical or mathematical inspection to find inflection points and tangential lines.

What I find particularly fascinating is the low correlation of VO2max and the TT powers.  This kind of thing shows that over longer periods of time (20+ minutes according to this study) the maximal oxygen transport that a body is capable of producing is limited by other physiological factors.  For instance, over a 2:10 marathon, an average of 80% of the fuel used is carbohydrates (source).  So we can conclude that a 60 or 90 minute time trial, though creating great aerobic stress, are still primarily glycolytic in nature.  A rider's muscle fiber makeup, VO2max, and other factors like fatigue will influence performance, and just how aerobic a 60 or 90 minute TT really is.  That's a fascinating paper as well as a classic, so I'll get into it more sometime later.

Now what can we conclude about a 20 minute time trial?  It uses a very, very large proportion of glycolysis as the primary fuel source.  I was always skeptical of the standard 20 minute test being preceded by a 5 minute maximal effort, but now it's a bit more obvious why it's there.  Burning through your first hard effort when you're fresh is a good way to get your 20 minute time trial to behave more like something longer.  For riders with high functional reserves and VO2max power, this is important.  This can set training zones at more reasonable targets so an athlete is able to spend more time in the target zones and reap greater training adaptations, especially since power from 20 minutes may fall off rapidly as it approaches the 60 minute mark, but again, this depends on the individual.

Postscript:
I spend a lot of time thinking whether we really should care about our MLSS values, VO2max, or such things to set training zones.  How long can we really spend at MLSS?  Can you really spend an hour at the FTP as set by a 20 minute time trial?  (In most cases no.)  What I've been seeing is that it doesn't matter that much, every rider comes with a different set of strengths and weaknesses at all power levels.

Although here's a study that offers a clue: these TdF riders (figures 2A and 3A) did way over 4.0mmol/L blood lactate in a 30 minute time trial, and several were well under.  Unfortunately this study was from 2006 and may very well be EPO tainted, hence I won't hold it in high regard.

Lactate intolerance

Growing up and doing martial arts I was always told I got sore because of lactic acid.  Acid? That stuff burns!  Makes total sense.  Many years later while learning metabolism from the one and only Hans Kornberg (discoverer of anapleurosis and writer of this lovely memoir), I started having some of those oh, this is how things work moments.  Since then I've been reading articles, both scholarly and semi-journalistic, on blood lactate accumulation and exercise.  This is the first of what I'm sure will be many posts.  This one is titled "Lactate intolerance" because it seems to be the villain molecule in amateur physiologists, like many cyclists and cycling coaches are.    But it's not, and is quite useful for many reasons.  Let's get into it.

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For a brief period, I studied the enzyme that catalyzes this reaction:

http://www.aaltoscientific.com/purifiedhumanproteins/images/lactate-dehydrogenase.jpg

Pyruvate is the end of glycolysis.  After pyruvate, the next step is rate determining (read: slow-ish, for enzymatic catalysis).  Pyruvate->acetyl-CoA happens more slowly than the reactions before it, so pyruvate can add up.  Other things that add up are NADH and H+.  During highly glycolytic times, NAD+ needs to be regenerated to be used in more glycolytic reactions.  And because the TCA cycle doesn't seem to be particularly fast compared to glycolysis, it can be slow regenerating NAD+.

Like many metabolic reactions at rest, lactate and pyruvate are kept in near equilibrium concentrations.  During exercise when glycolytic muscle fibers become engaged, pyruvate starts to pile up, and lactate dehydrogenase (LDH) regenerates NAD+ and mops up a spare H+ as well (it catalyzes the above reaction both ways, depending on the concentration of lactate and pyruvate).

Think of it like a bucket that has a divider in it.  As pyruvate piles up, it'll spill over into the lactate half.  Now they're both filling up.  Glycolysis is still happening because you're still exercising.  Now if lactate piles up too fast, so will pyruvate, which might fill up the bucket (bad metaphor, overflowing cells would be a serious problem) and back up all of glycolysis!  So now what do we do?  Throw out lactate.  Into the blood stream you go!

What happens to lactate in the blood?  Does it pile up?  It can, but it also gets taken up by other cells and converted back to pyruvate for aerobic metabolism.  One of the big lactate users during exercise is heart muscle, which is very, very aerobic.  When you eat heart, it's very dark meat because there's so much mitochondrial density, and the iron makes it dark.  Incidentally, aerobic training increases the density of enzymes that import lactate and pyruvate from the blood (MCT, monocarboxylate transporter).  And contrary to what you might read in some older literature, lactate itself is not oxidized, but is converted back to pyruvate.  Not that they were bad researchers, they just didn't have the tools that we have now.

Lactate appearance in the blood has several names, and they serve different purposes.  What most people say is "lactate threshold", researchers have defined as: the highest VO2 attained during incremental exercise before an elevation of blood lactate is observed; a blood lactate concentration 1mmol above baseline; blood lactate concentration 2.5mmol above baseline; &c.  Also there's onset of blood lactate accumulation (OBLA), the VO2 attained during incremental exercise corresponding to a blood lactate concentration of 4 mmol/L (source).  So if that's where your current understanding of LT is at, that study just cited as a source is about to blow your mind.

Category 2 and 3 cyclists' average steady blood lactate in a 20k TT is in the upper left hand graph.  In each graph the dotted line is one of the previously defined lactate threshold definitions.  Nuts, right?

It's fortunate for us as cyclists that we don't do physiological testing to define our training intensities.  A standard 20 minute test may well have some of us doing a steady 12mmol/L blood lactate.  Although if we did do such blood tests to set training zones, we probably would have figured out sooner that lactate isn't guilty of anything except being the most obvious blood molecule during exercise.

As it stands, the 20 minute test is generally not excellent at setting training zones, and how long an athlete can hold a certain % below or above the reference value indicates what the athlete is naturally good at.  I'm a sprinter/pursuiter, some people I know are good in a long breakaway but not so good in a short TT.  Incidentally, figuring out this stuff is another thing WKO4 does well (it's a feature I always pined for using trainingpeaks) as long as the athlete has good maximal power values all around, although it still takes a good eye to interpret the data and curves.

For the record, here's mine from WKO4.  You can guess what power zone I'll be working on next spring.  And just to make it easy, we can keep calling it lactate threshold.  And before you ask, there are many reasons I'm not an elite.  Like beer and cookies.

MLSS and training in swimmers

A quick thought for the morning, probably to be followed by something more involved later.  I've been trying to find a graph of MLSS (maximal lactate steady state) in different distance athletes, and in my search am running into articles that contain interesting tidbits.

Today's article looks at improving 500m swim times, which are around 2 minutes long, and in cycling this is a highly glycolytic anaerobic capacity interval.  So it's perhaps no surprise that this is figure 2:

Over the course of two months the athletes' times improved and the lactate concentration at MLSS did as well.  Despite this test being at anaerobic capacity, it also shows how much of an aerobic component there is at this exercise level.  Why does it show this?  Let's use Table 1 to guide us.

This looks like a traditional cycling plan to peak for early spring.  Let's note the two directions that MLSS moves in Figure 2.  During the lower intensity training the speed doesn't significantly improve, but the MLSS concentration goes up.  As training involves more time at threshold, the concentration doesn't go up again, but the athlete gets faster and gets to MLSS some seconds earlier.

To explain the December-January increase in MLSS, let's consider the effects of training slightly below and above MLSS, which in cycling we know as tempo and threshold, though due to the imprecision in swimming metrics, this might even be "sweet spot" and "low vo2max".  While these intensities stress the aerobic engine, they also burn a good deal of glucose and glycogen, engaging glycolytic muscles and increasing the rate at which they are capable of burning through their preferred fuel.  The increase in MLSS shows the improvement of the "glycolytic engine".

The January-February improvement in time and not another rise in MLSS could be explained by several things, but my guess is a larger increase of glycolytic enzymes and the building of type IIa/medium twitch fibers from a rest period in the training cycle.  I find myself wanting to see a 500m test performed in November, since low intensity training has different effects on MLSS than we see here.  More on that later, though.

This study encompassed both short and long distance swimmers.  No real distance specialties were mentioned except to group the two into different phenotypes, sprinters and endurance swimmers.

My first thought when I saw this graph: how cool is that?!  There's no explanation for it in the paper since its primary purpose was to investigate an inexpensive way to monitor training progress for swimmers.  I proffer two reasons behind why there should be different rates of improvement: lower intensities (November through January) lead to larger gains in endurance athletes who have greater type I fiber density, while higher intensities lead to larger gains in type II dense athletes.  My second and less likely explanation is that it takes the sprinters more time to see gains from their low intensity training, though you really can't be sure until it's tested.

The other thing I would like to see is Figure 2 separated by sprinters and endurance swimmers.  Hopefully I'll run across something like that soon.

Cadence and metabolism

A few days ago I was reading a physiology textbook and ran across this graph (from my phone, sorry about the quality):

This is the article referenced.

In case it's not obvious at first glance, these graphs say that the particular twitch (i.e. fast or slow) recruited for a muscle contraction depends both on its duration and force.  This makes sense from an economical standpoint: slow twitch fibers rely primarily on fat metabolism, and there's no need to get high force muscle fibers burning through glycogen for doing something as simple as typing on a keyboard.  (Don't know why that comes to mind.)

As in everything I read, I ask: how can I use this in cycling?  Maybe we can maintain more reliance on aerobic metabolism by pedaling at higher cadences.  Or maybe we can induce greater aerobic stress at high cadences.  This will be extremely useful in base training and general engine maintenance through the year.

Next I did a quick calculation, disregarding force evolution for a moment: if a muscle is engaged for 25% or 50% of a pedal stroke at 90rpm, how long does it contract for?  At 25%, 167ms, and at 50% it's 333ms.  At 120rpm we get 125 and 250ms.  It's looking okay right now, especially if the pedal force and total power output are kept low in addition to high cadence.  But the graph doesn't actually show us what motor units are engaged or even if this happens in leg muscles.  (First dorsal interosseus is in the hand.)  Let's dig deeper and see which muscles are engaged for how long in a pedal stroke.

Thanks for the fun colorful stuff, trainingpeaks.

I found a trainingpeaks article which led me to the above graphic, but I checked the references anyway.  This is one of the studies referenced.  Go to figure 6 for the meat and potatoes.  I suppose we can forgive the authors for working with triathletes, whose pedal strokes are very downward heavy due to their positions (different muscle recruitment is why you should always practice on your TT bike, kids).  Here is a second, most excellent article that reviews muscle engagement in relation to pedal and shoe conditions as well as seat height, and actually looks at two active and two former bike racers who can competently engage muscles entirely around the pedal stroke (look at subjects CDEF in figure 3, with cleats and normal saddle height).  Black regions of the graph represent muscle engagement.  In relation to fiber recruitment and aerobic stress, this shows that many muscles are engaged in half or more of the pedal stroke, indicating a good deal of fast twitch recruitment and glycolysis.

Because I make things more complicated than they need to be, I went through all of the above.  And only now do I look for articles studying metabolism in relation to pedal stroke.  This only does 50 vs 100 rpm.  Not a lot of bike racers pedal at 50rpm, so while informative in a proof-of-concept way, isn't exactly what we're looking for.  I dismissed several more articles on their ridiculously low cadences (down to 15rpm) until I came across this gem, which looks at 60, 80, and 100rpm in female recreational cyclists.  As predicted, HR and VO2 were higher at 80rpm than 60rpm, and higher still at 100rpm.

What we're learning is that despite how much it sucks for a rider like me who prefers high torque situations, high cadence pedaling during endurance exercise will strain the aerobic system more than at lower cadences.  This aerobic stress is, after all, why we base train.  Now we can make it more effective.  In terms of practical application, you should pedal at a higher cadence (if you have that option) if want to save your precious glycogen stores for later in a race.  It's something we all kind of knew anyway, but now we know why.  That's important to me, anyway.

As an aside, I ran across this article showing the neuromuscular adaptations cyclists have in pedal stroke efficiency.  It shows that our muscles don't work against each other as much as compared to untrained individuals.  With the same overall power output, we use less oxygen.  Someday I'll get a pair of Vector pedals and really get to go nuts with pedal stroke analysis.

Crits and Cross: The simplest analysis

Following are four standard power outputs from two recent cross races and two crits from earlier this year.  Three are the same from the last post, but I swapped out White Park for Sucker Brook because its power profile involved a lot more sprinting out of corners followed by some power sections.  I'm including these in a separate analysis to show that you can't always rely on this view to properly assess the demands of a particular discipline.

Quad CX

Two things stand out about this.  First, the peaks.  Rarely are they under 500w, and oftentimes they're between 600 to 1000 or more.  If you want a sprint-out-of-corners race, this is the cat's pajamas.  Second is the blank space underneath the lines.  Those black spots that stand out among all the yellow are times of constant power output.  The biggest, most obvious and repeated one is the long uphill fire road.  There are a couple more that correspond to other long stretches of constant power output, but for the most part the graph is either fully on or fully off.

Sucker Brook CX

The graph for Sucker Brook tells the same story as Quad.  I count 22 peaks alone above 825w, and countless above 600.  Similarly there are black spaces at the bottom of the graph for constant power output sections.  Again, very few.

Wells Crit

Now would you look at all that black space under the trace!  Much more constant power output, but still a very spiky graph.  Even the biggest peaks have some black space under them (I go for long sprints).  Also in contrast to the cross races, there are a few times where there are no peaks to really tax the sprinting muscle fibers.

Keith Berger Crit

Now that looks more like a cross race.  The six corner office park crit does require a lot of power spikes to keep up with the group out of corners and move up when gaps appear.  As for differences with cross, refer to the last post for a rundown on how the physiological pedaling strain compares to cross.  And if you really want to see more gaps under the power trace, just blow it up to full size.  They're shorter and more frequent than in a cross race.  The largest black space under the trace is my last lap attempt to get away, which looks like a very rough version of a "race winning interval."

This type of looking at the graph analysis is the kind of thing that leads to the "crits are cross on pavement" adages.  But after these few posts you know some of the finer details on what makes the disciplines similar and different, and can plan your training to reflect the differences.

Crits and Cross: the same but different

"Crits are just cross on pavement."  Or vice versa.  Are they really?

In a manner of speaking this is true.  Both involve races of short duration with high aerobic and anaerobic components.  The following analysis will look at two crits and two cross races, each with different qualities.  I'll include four graphs with power and pedaling metrics for each, which I hope will illustrate the differences in race demands and therefore differences in the training required (aside from the obvious bike handling differences), especially for a perfectly average roadie like myself.  Jeremy Powers has a 400w(?) ftp and can hit 1400w out of every corner in a cross race, but few of us mortals can do the same. All graphs come from WKO4, build 173.

The first cross race (Quad) was a decent combination of grass crit and riding in the woods, complete with an uphill barrier, U-turn in sand, and a couple long power sections to balance out the turny/carving sections.  A pretty well rounded course.  The second cross race (White Park) had only one long and one medium power sections, and otherwise required very short bursts between sharp turns, both on flat ground and off camber.

The first crit (Wells Ave) is a long oval with one corner barely sharp enough to require not pedaling.  It's not technical at all (and is in fact the closest thing to a race in a straight line as you can get), but this day was full of primes, making and chasing breaks, and a (rare) race winning sprint.  The second crit (Keith Berger) is a classic six corner office park deal with 90ish degree corners, some crosswinds, and a 200m last-corner-to-finish-line sprint.

Here are the first four time-in-zones charts.

Quad CX

White Park CX

Wells Crit

Keith Berger Crit

All four graphs show the effective aerobic demands of cross and crits are similar based on time in zones.  However, compare the yellow line (time in zones) in zones 2 and 3 between the top two graphs and bottom two graphs.  They're a bit lower in the cross races, where you're either pedaling as hard as you can or you're negotiating a turn.  Otherwise the yellow line traces are fairly similar.  The bar graph portion (time in normalized power zones) is interesting, though.  Quad and Wells bear more resemblance to each other than they do to the other races of the same discipline.  Perhaps it's the courses.

But now let's look at the last metric on these graphs: average and normalized power for the races.  Both metrics for crits are significantly higher than they are for cross.  You may be asking why, with lower average and normalized power, I'm spending the same time in the same zones between crits and cross.  WKO4 models your effective FTP, and with a lack of long steady efforts, right now mine is modeled at a meager 250w (in June/July it was in the low 320s, but now with little maintenance since August, it's probably back around 300).  What this really says is that I'm sprinting out of each corner and then recovering on straightaways at mid to high tempo wattages.  So like I discussed in my last post, I may need more aerobic intervals in cross season in order to bring up these power numbers.  It may be different for you.

Next, to confirm, let's look at the mean maximal power (MMP) graphs.

Quad CX

White Park CX

Wells Crit

Keith Berger Crit

On the left side of the graphs, you'll notice the first two (cx) fall off pretty quickly.  These are always start, and then it's settling in, trying not to blow up in the first lap.  Mean wattages are higher all around with crits due to the recovery time.  Same goes for normalized too, but I'm not including those graphs because they're a bit redundant and boring to inspect.

The right side of the cross graphs show some different bumps, thanks to the terrain of the courses.  That kind of thing is rare to see in MMP graphs of road races, but it's here at Keith Berger thanks to a well executed but ill timed last lap attack.

Now we'll see some graphs that illustrate the different muscular demands.

Quad CX

White Park CX

Wells Crit

Keith Berger Crit

Now things are coming into sharper relief.  The first and most obvious aberration to the pattern is how little of Wells was spent coasting, but like I said it's basically a straight line so very little coasting through corners.  The cadence distribution in both crits has a median over 95rpm (highly aerobic), while for the cross races the median is around 85, and heavily skewed left.

Also notice the time spent not pedaling.  In Keith Berger, the more technical crit, there's only a few less minutes not pedaling than in either cross race.  Time in those super low cadence bins does add up.  Longer contractions are highly anaerobic, and create large demands on fast twitch fibers, which in turn creates a premium on mobilizing glycogen for these things.  Don't mistake this for saying cross isn't an aerobic sport, though.  Your heart rate is pinned for 40-60 minutes because you're trying to supply as much oxygen as you can to your muscles.  This still puts cross firmly in the endurance category, but it does blur the line between aerobic and muscular endurance.

Again let's confirm what we just discussed with some quadrant analysis.

Quad CX

White Park CX

Wells Crit

Keith Berger Crit

I think that's about as stark as it can get.  The first two graphs (cx) show significant distribution at low CPV, high pedal force, while the second two graphs (crits) show most of the pedaling done in the high CPV, low pedal force quadrant.

This hints at what I discussed in my last post.  In cross your aerobic engine is pinned through the whole race, and fibers heavily relying on anaerobic metabolism need to refuel with blood sugar, but such high muscular demands create more need for glycogen mobilization than a crit does (though some certainly can do the same).  Of course there are no hard and fast rules to any of this, though I hope I've illustrated some of the physiological differences between cyclocross and criteriums.