Ironman series, part one. This one starts where almost everybody starts, on the figure at the top of the dashboard, and part two takes the other half of the problem, where mobility work is worth the time you give it and where putting load through tissue does more.
A maximal aerobic capacity test answers one question, and it answers it well. Load somebody until they cannot hold the pace any longer, and oxygen consumption levels off; that plateau is your VO2 max. It has become the number people reach for to say how fit somebody is.
What it will not tell you is what happens to that person in hour six. The test is built around a physiological state that a long event never enters, so the answer it hands back is aimed somewhere else.
My own figures ran the wrong way through 2025 and the racing still came out fine. Three measurements across one build: April gave 54.7, July 52.4, October 51.6. The race was Cascais, full distance, on 18 October 2025, at 30 years old. It took 11 hours and 8 minutes to cover the 225.97 km, and the day worked out roughly as I had written it down. It annoyed me, more than it deserved to, because I had spent a year watching the wrong dial.
One man, one build, no control, no repeat. That proves nothing, and it needs saying first: the weight here sits on published physiology and my three readings are only what sent me to read it.
This is an educational and strategic perspective, not personal medical advice. The views are the author's own and not statements by Atlas Cove Lda.
What a maximal test is actually measuring
Read the protocol and the limits of the result are visible inside it. The subject turns up rested and fed, the effort lasts around ten minutes, and everything that makes a long event difficult has been kept out of it: no accumulated hours, no heat load, no depleted fuel. Oxygen uptake is sampled breath by breath while the workload steps up, and the figure reported is the highest rate the body reaches, confirmed by its refusal to climb when the work does.
Two things set that plateau. One is central, at the pump: how much blood the heart can move each minute at full effort, which comes down to volume per beat and beats per minute. The other is peripheral, at the tissue: how much oxygen the working muscle strips out of that blood on the way past. Multiply them and you have the ceiling. Both halves answer to a block of hard intervals inside about six weeks, which is a large part of why this particular number is so popular. A figure that shifts that fast feels like proof the training is doing something.
It is proof that the training is doing something. What moved was the roof of the house. A maximal test is a portrait of you under the best available conditions, and that is a design choice rather than a defect in the design. The trouble starts when a number produced under those conditions gets read as a forecast for a day that shares none of them.
The ceiling a long day never climbs to
Over the eleven hours at Cascais my heart rate averaged 143 beats per minute, which is 72 percent of my maximum. Nothing in a ten minute ramp to exhaustion describes a body held at 72 percent for most of a waking day and then asked to keep going. The two efforts share the word aerobic and very little else.
Which is how two people can walk to the same start line with identical laboratory numbers and finish forty minutes apart. No mystery there. They matched on a trait the event was not charging them for. It was charging them for three other things: the oxygen cost of holding a given pace, which is running economy, how much of that economy is left once they are tired, and which fuel is covering the demand by hour five.
Where extra ceiling still pays, and where it stops
None of this makes aerobic capacity unimportant. Below a certain level it is the constraint that binds, and lifting it improves nearly anything lasting more than a couple of minutes. Somebody sedentary, or fitting two light sessions around a demanding week, is standing exactly there, and telling them to go and raise their aerobic capacity is sound advice. I would say the same.
Higher up the picture changes. Where it changes I would put at roughly the low fifties, measured in millilitres of oxygen per kilogram per minute, among people who already train seriously. Be careful with it: that figure comes from other people's practice, and I cannot hand you a citation for it. Its edges are soft. Beyond it, adding headroom does progressively less for the speed you can sustain over several hours, because a different constraint has taken over. Height stops being the question. Occupancy does, and so does how little you lose along the way.
So, the opinion, with no cushion round it: past that point, a training block spent chasing the number is a block spent on the wrong thing. It stands until somebody shows me long-course results that line up better with laboratory maxima than they line up with how much each athlete faded over the closing hours.
Without a start line the translation barely changes. Given a decent base, a bigger ceiling is seldom what decides whether you manage a long walk on a coast path, or an hour of physical work that still leaves you something for the evening. Both depend on sustainable output and on how you cope once load has been piling up for weeks, and neither is trained by the sessions that raise a maximum. What you keep once you go home answers to the same pair.
Durability, a name the field only settled on in 2021
Durability is the term, defined as the extent to which an athlete's physiological profile holds up rather than degrading as an effort goes on, and offered as a fourth determinant of endurance performance next to efficiency, the sustainable threshold and maximal capacity itself (Maunder, Seiler, et al., 2021).
Be clear about what that paper is. It is a framework proposal, gathering observations that already existed and arguing they describe a distinct quality nobody had named. Proposing a fourth determinant is some distance from demonstrating one, and the authors make no such claim. What the paper does say out loud is the thing no laboratory protocol can get around. Measure somebody rested, by any protocol including the maximal one, and what you hold is a picture of them before the event started, made in conditions swept clear of fatigue on purpose. Durability is the question of what survives into hour six, once glycogen is low, core temperature has climbed and the movement has repeated itself tens of thousands of times.
Measured fresh, gentle decay and total collapse are indistinguishable.
My own race in pieces. The swim took 1 hour 17 at an average of 143 beats. The bike took 5 hours 40 at 142. The run took 3 hours 55 at 144. Three disciplines, eleven hours, and the average moved two beats across the whole of it.
Presenting that as proof would be overclaiming, for at least three reasons. Heart rate can hold perfectly steady while pace falls apart underneath it. A cautious pacing plan flatters anybody. And there is no control here, no second version of me who spent the year training some other way. The splits do show one thing: whatever I had built kept its shape from the first stroke to the finish line, and that shape is what I was being charged for. The number I had watched fall all summer belonged to a ceiling nothing in that day came near.
The three adaptations that carry a long effort
Three things carry a long effort, and none of them is built by the stimulus that raises a maximal test. Start inside the cell. Training endurance increases how much mitochondrial material a muscle carries and how active the oxidative enzymes in that material are. Holloszy showed it when a running programme left trained muscle with close to twice its previous oxidative capacity (Holloszy, 1967). Read that with its date and its subjects in view: rats, a small sample, 1967, and tissue biochemistry rather than anything you could run on a living athlete. Foundational work, nearly sixty years old, and rodent muscle is not human muscle.
Human work has added a layer since. Inside each mitochondrion the folded membranes called cristae, where the oxidative machinery is mounted, sit at a higher density in muscle trained for endurance, and that density moves with whole-body oxygen uptake. Pack the folds tighter and a given quantity of mitochondria offers considerably more surface to work with (Nielsen et al., 2017). Human tissue, which is the improvement, though it is a small group of already-trained subjects seen at one moment in time, so there is no before and after in it.
Next, delivery, this time at the tissue rather than at the pump. Easy volume, sustained over time, thickens the capillary network wrapped round each muscle fibre. The classic human study put the rise in the thigh at roughly a fifth after eight weeks (Andersen and Henriksson, 1977). Small study, young subjects, muscle biopsies, and again 1977. The mechanism is geometry. Squeeze more vessels around a fibre and oxygen has less ground to cover on the way in, metabolites less on the way out, which pays off most at submaximal intensities where diffusion has time to matter. At peak power the ceiling is set by how much muscle the nervous system can call on, so local oxygen supply is no longer the thing holding you back. Which is why this adaptation barely registers on a maximal test and matters enormously across a long steady one.
The three also depend on each other. Extra mitochondria with no capillary bed feeding them sit there supply-limited. All three arrive over months rather than weeks, and that slowness is the real reason they attract less attention than a number capable of moving inside a fortnight.
Mitochondrial volume and cristae density
Stimulus. Accumulated aerobic work at intensities the muscle can repeat all day.
What it changes. How much oxidative machinery a fibre contains, and how much working surface it presents (Holloszy, 1967; Nielsen et al., 2017).
Rough timescale. Months of steady volume. Neither source gives a clean training-week number for the cristae change.
Capillary density around the fibre
Stimulus. Sustained easy volume, the low-intensity hours rather than the hard ones.
What it changes. Diffusion distance into and out of the fibre, which improves submaximal exchange and does almost nothing for peak power (Andersen and Henriksson, 1977).
Rough timescale. Eight weeks bought about a fifth more in the thigh, the shortest honest figure here.
Fat oxidation and lactate clearance
Stimulus. Months of consistent aerobic training, again mostly the easy kind.
What it changes. How much of the demand fat covers at a given submaximal intensity, and how much lactate is in the blood while you meet it (San-Millán and Brooks, 2018).
Rough timescale. Slowest of the three, and the evidence compares professionals with less-fit people rather than following a training programme.
Fuel, and the place the test is blindest
The third one is metabolic. It carries more weight than the other two once the clock runs into hours, and it is also what a maximal protocol is least able to see. Give it months of steady aerobic training and muscle gets better at two jobs at once, burning fat and clearing lactate. The consequence is that for the same submaximal effort, more of the energy comes out of fat and less has to be taken from a glycogen store that has a bottom.
Set professional endurance athletes beside less-fit individuals and the pattern sits there in the data: faster fat oxidation, less lactate in the blood, at identical workloads. That is metabolic flexibility with a number attached (San-Millán and Brooks, 2018). The design limit is worth stating plainly. The study compares two groups as they are, so it cannot tell you training produced the gap, and professional cyclists are a selected population before anybody trains them. What it establishes is that the pattern travels with being highly trained.
Over many hours it compounds until it is close to decisive, and nothing clever sits underneath it. It is arithmetic on two tanks. Glycogen has a bottom, and fat, at any distance a human being will actually race, does not. Pay for the opening hours mostly out of fat and there is still something in the account when the closing hours arrive. Pay for them out of carbohydrate and you turn up at the same moment with the account already empty.
Now consider what a maximal protocol observes. At that intensity fat cannot be oxidised anywhere near fast enough, so the body runs on carbohydrate, which is close to the reverse of what a long day needs. Good test. Pointed elsewhere.
Why so much of the training is easy
Look at what good endurance athletes actually put in their training diaries and the picture is oddly uniform across sports. Roughly four sessions in five are done at an intensity that is genuinely easy. Most of what remains is properly hard. Very little lands in the middle (Seiler, 2010). That paper is descriptive, and the distinction matters: successful athletes are the ones who survived their training rather than a random sample of everyone who tried it, and describing a pattern is a weaker claim than showing the pattern causes the result.
The mechanism does line up, which is presumably why the shape keeps reappearing. Low-intensity hours produce the three adaptations above at a fatigue price low enough that you can pay it again tomorrow. A small amount of genuinely hard work stops the top end going stale. The middle is tempting, because it feels like real training while you are doing it, and it charges a lot of fatigue for adaptations it does not specifically target. A model that keeps sessions at the two ends is what gets called polarised training.
A nine-week controlled trial in trained athletes came out the same way. The polarised group improved more on the key endurance variables than groups working around threshold, around high intensity, or around sheer volume (Stöggl and Sperlich, 2014). One trial, nine weeks, modest group sizes, one laboratory. Supporting evidence rather than a settled question.
Anyone training seriously around a demanding job meets a harsher version of this, since there is one fatigue budget and the job is drawing on it too. A hard Tuesday session gets paid for on Wednesday, by whoever is chairing the meeting. I have written about that budget as something you can learn to spend deliberately. Seen from there, keeping four sessions in five easy is how you collect a lot of stimulus for a bill the rest of your week can settle.
Why my number fell while my racing improved
Laid out in that order it stops looking like a paradox. Keeping a maximal figure pinned at its highest requires going near maximal fairly often, and an endurance block does the opposite by design. So the number slides during precisely the months when everything that matters over eleven hours is being built. Accumulated fatigue pushes the same way, and so does ordinary variation in the test.
I did not read it that way at the time. For about two weeks in July I treated the 52.4 as a fault to be located and corrected, went digging through the training for what had gone wrong, and came close to putting interval sessions back into a plan with no room for them. Right instrument, wrong question. If I had gone through with it, I would have spent August generating fatigue in defence of a figure nobody checked in October.
Which puts the personal material back to its proper size. One person, one training year, three readings, nothing to compare them against and no second run at it. Strip the published work away and I am holding three numbers off one body, useful to nobody, me included.
What is worth watching instead
The change this argues for is in what you measure, and not in how hard you work. If a test describes a ceiling your event never reaches, it belongs somewhere in the middle of the dashboard rather than at the top of it. Four other signals describe sustained capacity better.
Whether heart rate holds steady from the first hour to the last.
Whether the pace a given heart rate buys you gets faster across a block.
How much of the demand fat covers at the intensity you will really race or work at.
Whether output is still there in the closing hours, when everything is against it.
The fourth is the awkward one to measure and also the one your event cares about most. None of them reduces to a single clean number you can put in a profile, which is why they lose out to a figure that is easy to publish and easy to compare with a stranger. My own posture now is to treat maximal capacity as a threshold check, then leave it alone and spend the attention on what the distance is actually paying for.
By my own standard I am the awkward case. 51.6 in October puts me on the line I drew rather than safely above it. The convenient response would be to move the line downward until I was over it. The accurate response is the one I already gave: that threshold carries no citation, its edges are soft, and my own figure sits right at that boundary. What I follow from here is which way it is heading, over several years and not over a single season.
Sources
Seiler, S. (2010). What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance, 5(3), 276-291. DOI: 10.1123/ijspp.5.3.276
Stöggl, T., and Sperlich, B. (2014). Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. Frontiers in Physiology, 5, 33. DOI: 10.3389/fphys.2014.00033
Holloszy, J. O. (1967). Biochemical adaptations in muscle. Journal of Biological Chemistry, 242(9), 2278-2282. DOI: 10.1016/S0021-9258(18)96046-1
Nielsen, J., et al. (2017). Plasticity in mitochondrial cristae density allows metabolic capacity modulation in human skeletal muscle. Journal of Physiology, 595(9), 2839-2847. DOI: 10.1113/JP273040
San-Millán, I., and Brooks, G. A. (2018). Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports Medicine, 48(2), 467-479. DOI: 10.1007/s40279-017-0751-x
Andersen, P., and Henriksson, J. (1977). Capillary supply of the quadriceps femoris muscle of man: adaptive response to exercise. Journal of Physiology, 270(3), 677-690. DOI: 10.1113/jphysiol.1977.sp011975
Maunder, E., Seiler, S., et al. (2021). The importance of 'durability' in the physiological profiling of endurance athletes. Sports Medicine, 51(8), 1619-1628. DOI: 10.1007/s40279-021-01459-0
Questions people actually ask
Does VO2 max predict how fast you will finish a long race?
Weakly, once you are past a certain level of training. Among untrained and lightly trained people it predicts a fair amount, because the ceiling really is the binding constraint down there. Among trained endurance athletes it stops separating anybody, since everyone who turns up has already cleared the bar.
Two athletes with the same laboratory figure can finish a long-course event a long way apart. The gap sits in economy, in how much of it survives fatigue, and in what is fuelling them late on.
Why does VO2 max fall during a big endurance training block?
Because holding a maximal figure at its peak takes regular near-maximal work, and an endurance block keeps that rare on purpose. The hours go into easy volume, which builds the machinery a long event runs on while doing very little for the ceiling.
Accumulated fatigue and normal test-to-test variation push the same way. Mine dropped about three points across a year in which my racing got better, and I spent a fortnight in July convinced that was a problem.
What VO2 max is high enough for long-distance racing?
I work with roughly the low fifties in millilitres per kilogram per minute for trained endurance athletes, and this is a rule of thumb absorbed from other people rather than something I can cite. The edges are soft, and my own October reading sits on one of them.
The posture matters more than the figure. Check it, satisfy yourself it clears the bar with room to spare, then stop optimising it and watch the direction over years.
Does any of this matter if you are not training for a race?
Arguably more than the racing version does. Very little of adult life happens rested and under laboratory conditions, so what you can still do reliably on the second Thursday of a hard fortnight is a durability question in ordinary clothes.
The same adaptations underwrite a long walk on a coast path, an hour of physical work that does not write off your evening, and energy that holds through the afternoon.
Where Atlas Cove fits
The same preference runs through how we build a week at Atlas Cove. Almost any decent week can produce a peak, so the measurements worth taking are the ones taken again in month three, when conditions have stopped being ideal and normal life has resumed. A figure that slips while your endurance improves is a bill you paid deliberately, and what it bought is the capacity the distance rewards.
Tom Wuerden · Co-Founder
Engineer turned Ironman
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