Why Your Conditioning Is Not Improving. The Ultimate Guide to Energy System Training

By Ian Markow. FRCms, Kinstretch, Stick Mobility, EXOS Performance Specialist, StrongFirst SFG1, MoveMed Level 1, NASM-CPT, NCCPT. Fifteen years coaching.

“This thing, what is it in itself, in its own constitution?”
Marcus Aurelius, Meditations 8.11, translated by George Long

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Contents

1. Two people quit at minute four
2. You are looking where the light is
3. What conditioning actually is
4. How much does cardio really matter
5. Most VO2max studies never measured VO2max
6. The three limiters
7. What each limiter feels like
8. Three tests you can run this week
9. Why your heart rate spikes on heavy squats
10. What your results mean, and what they do not
11. What to do about each limiter
12. How should I condition for my sport
13. The zone 2 problem
14. What about HRV
15. The coach layer starts here
16. Why beginners improve fast and then stall
17. Why three intensity buckets
18. The stretch and the squeeze
19. Exercise selection changes the adaptation
20. Three ways to run the same interval
21. There is no magic VO2max protocol
22. Concurrent training, honestly
23. The non-responder is a prescription error
24. Limiter, bridge, performance
25. Where this gets complicated
26. How to actually start
27. Common questions
28. References

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1. Two people quit at minute four

Put two people on rowers. Same workout. Both stop at minute four.

The first one is breathing like she just surfaced from a dive. Her legs feel fine. She could keep pulling if she could just get air in.

The second one has legs full of concrete. His breathing settled thirty seconds ago. His legs will not move.

Same workout. Same time on the clock. Two completely different failures.

Now watch what happens next. They both go home and do more conditioning. More intervals, more zone 2, more of whatever their watch told them to do. One of them gets better. The other one spends six months working hard and moves almost nothing.

The difference was never effort. It was direction.

2. You are looking where the light is

There is an old joke about a man on his hands and knees under a streetlight at night.

A police officer stops and asks what he is doing. Looking for my keys, he says. The officer asks if he dropped them there. No, says the man, I dropped them over in the park.

So why are you looking here?

Because this is where the light is.

Scientists have a name for this. The streetlight effect. We study what is easy to measure instead of what matters. And this is not me reaching for a metaphor. In 2017, three of the most respected cardiovascular physiologists working published the definitive review on VO2max in Acta Physiologica. They titled it “Biology of VO2max: looking under the physiology lamp.”

They named their paper after the joke. About their own field.

So look at what we count. Steps. Zone 2 minutes. A VO2max number. Every one of them is easy to count, which is exactly why we count them. None of them tell you what actually gave out at minute four.

Your keys are in the grass.

3. What conditioning actually is

Conditioning is your body’s ability to move oxygen from the air into a working muscle and use it. That is the whole job.

It happens in three steps.

You take it in. Air moves through your airways into your lungs and oxygen crosses into your blood. Your diaphragm and the muscles around your ribs do this work, and they are muscles like any other. They fatigue.

You move it. Your heart pumps oxygenated blood out to the muscles that need it. How much it moves per beat, how much blood you have, and how much of it your red blood cells can carry all decide the ceiling here.

You use it. The muscle pulls oxygen out of the blood and burns it for energy inside mitochondria. How many mitochondria you have, how good each one is, and how many capillaries are feeding that muscle set the limit.

Three steps. One chain.

A chain does not fail everywhere at once. It fails at one link. And the link that fails first is not the same link for everyone, which is why the same workout builds one person and wastes another.

Here is the part that gets missed. You do not fail when your weak link stops coping. You fail when you run out of ways to work around it. Your body compensates for a long time before it quits. Everything you feel at minute four is the sound of those compensations running out.

4. How much does cardio really matter

More than almost anything else you could measure about a person.

In 2018 the Cleveland Clinic followed 122,007 patients through 1.1 million person-years of observation. They sorted everyone by measured cardiorespiratory fitness and then counted who died.

The least fit group were 5.04 times more likely to die than the fittest.

Numbers that big are hard to feel, so hold them against something familiar. The same study, the same patients, the same statistical model also measured the usual clinical risk factors. Smoking came in at 1.41. Diabetes at 1.40. Coronary artery disease at 1.29.

Being unfit carried a larger risk of dying than smoking did.

Not in a different study with a different population and a different adjustment. In the same one.

But you do not need to be elite

Now the part nobody selling you a program wants to lead with.

The benefit curve is steep at the start and it flattens early. Almost all of the return comes from going from doing nothing to doing something. Going from a lot to slightly more buys very little.

The guidelines recommend 150 to 300 minutes of moderate activity per week, or 75 to 150 minutes of vigorous. The dose-response curve behind those numbers has no lower threshold and falls steeply at the bottom. There is no minimum you have to clear before anything starts working.

Same shape shows up in the step count data. Eight to ten thousand gets you most of it. Twenty thousand is not twice as good.

Now the honest half, because the Cleveland Clinic data says something people leave out. They found no upper limit of benefit. Their elite performers still outlived their high performers. More fitness kept lowering the risk of dying, all the way to the top of the range, and the advantage held in patients over seventy and in patients with high blood pressure.

You do not stop benefiting. You stop getting the bargain.

The first hour a week is the best deal you will ever be offered. The tenth hour still pays, just not like the first one did. Which of those two facts matters to you depends entirely on where you are standing right now.

The uncomfortable half

The further behind you already are, the more work it takes to get the same benefit.

The dose-response work behind the guidelines is clear that risk falls steeply at the low end and keeps falling, more gradually, well past the recommended range. What it does not promise is that everyone arrives at the same place for the same effort.

It is obvious once said out loud, and it is still worth saying. Staying out of the hole is easier than climbing out of it. If you have neglected this for twenty years, you are going to watch someone who never stopped moving maintain their fitness on half the work you have to do. That is not unfair. It is just arithmetic. Start anyway.

5. Most VO2max studies never measured VO2max

This should change how you read every VO2max headline you have seen in the last three years.

To actually measure VO2max you need a mask, a metabolic cart, and gas exchange analysis. It is slow, expensive, and impossible to do on the hundreds of thousands of people in the large mortality studies.

So they did not do it.

Instead they ran a performance test and estimated. Usually a treadmill protocol where they ramp speed and incline and record how far you make it. Sometimes a submaximal cycling test where they watch how much your heart rate climbs at a fixed workload and calculate backward. No mask. No gas exchange.

An estimate is not a measurement. Good enough to sort a hundred thousand people into fitness groups. Not the same thing as putting a mask on your face.

A more honest headline for most of those studies would read: aerobic fitness performance predicts mortality.

And then it gets worse for the number

Researchers have also tested how far you can get with no exercise test at all. In the Norwegian HUNT study, a formula built from resting heart rate, waist, age and a self-reported activity level was applied to roughly 37,000 healthy adults, who were then followed for 24 years.

It predicted who would die, with an area under the curve of 0.70 to 0.77. Every individual variable inside that formula performed far worse on its own, between 0.55 and 0.63. The combination was doing real work.

No treadmill. No mask. Twenty-four years of prediction.

Be precise about what that study did and did not show. It never went head to head against a measured VO2max. So this is not a questionnaire beating the lab. It is a questionnaire getting most of the way there for nothing, which is a smaller claim and still a striking one.

There is one more wrinkle worth knowing. Some of the largest hazard ratios in this literature come from older adults doing incline treadmill tests. An 80 year old on a rising treadmill may be stopped by balance, or by leg strength, or by a technician deciding it has gone far enough. That is not necessarily an aerobic limit. Some of what looks like cardio fitness in that data is general physical capability wearing a cardio costume.

None of this means fitness does not matter. It matters enormously. It means the number is a shadow of the thing, and people have started training the shadow.

The genetics question, settled

If you were going to argue that fit people just have good genes, the genetic data closes that door.

In 2025 a Mendelian randomisation study used genetic data to separate the VO2max you inherit from the one you build. Their conclusion, in their own words: despite being a strong predictor of mortality, VO2max is not causally associated with longevity. The inherited version bought nothing. Measured fitness still predicts. And people whose fitness improves the most across ten and twenty year follow-ups have the best outcomes.

Read those together. The VO2max you were born with buys you nothing. The one you build buys you everything.

6. The three limiters

Every person has one of the three steps that gives out first. Uptake, delivery, or utilization.

Respiratory limitation is a ceiling on getting oxygen in. The diaphragm and the muscles around your ribs fatigue, or your airways cannot move air fast enough at the flow rates hard exercise demands.

This one is under-diagnosed for a specific reason. Most of your systems adapt beautifully to training. Your heart chamber enlarges. Your blood volume rises. Your mitochondria multiply. Your lungs and airways mostly do not. Highly trained athletes with enormous VO2max values often show no better lung volumes or diffusion capacity than an average sedentary adult.

The structure you were born with is roughly the structure you have.

There is a sex difference here worth stating carefully. Using CT imaging in healthy non-smokers, researchers found the large conducting airways were 26 to 35 percent smaller in women than men. Matched for height, still 20 to 30 percent smaller. It shows up after about age 14 and is absent before 12, which tells you it is structural rather than trained.

At any given ventilation, that means a higher work and oxygen cost of breathing.

Hold that loosely. It is a population average with heavy overlap between individuals. It explains why respiratory limitation shows up more often in women. It does not tell you anything about a specific woman.

Delivery limitation is a ceiling on moving oxygen. The heart’s maximum pumping capacity caps peripheral blood flow. Oxygen use in the muscle outruns oxygen supply.

This one has a face you will recognize. Strong, heavily muscled, excellent local endurance. Can do enormous sets of one thing in isolation and falls apart the moment you ask for several things at once. The problem is that a lower maximum cardiac output limits how much skeletal muscle can be dilated and supplied at any one moment. Ask for everything at once and the extremities deoxygenate hard.

Utilization limitation is a ceiling on using the oxygen that arrives. Supply is fine. The muscle cannot extract or use it fast enough.

This is the broadest category and the hardest to profile, because the causes are so different. Not enough mitochondria. Not enough capillaries. Poor recruitment. Muscle damage. Changes in blood chemistry. The common threads tend to be poor rate of force development, a low peak power relative to sustainable power, and poor recruitment under fatigue.

Nobody’s limiter is good or bad. Different sports select for different limiters. An Olympic weightlifter’s physiology is organized around a set of demands that excludes high cardiopulmonary development. A cross country skier’s is organized around the opposite. Both are correctly built for their job.

The question is never whether your limiter is good. It is whether it fits the thing you are asking your body to do.

7. What each limiter feels like

Almost nobody matches one column perfectly. You will probably see yourself in two of them, and that is normal, because by the time you stop, several systems are struggling.

Go by whichever one showed up first.

The first thing to give way is your limiter. Everything after it is your body compensating for that failure, which is why the picture gets muddy the longer you push. Think back to the earliest moment something felt wrong, not the state you were in when you quit.

Respiratory. You feel starved for air before your legs complain. Your breathing rate climbs fast and early and keeps climbing even when your pace holds steady. You catch yourself breathing shallow and high in your chest. In a hard set your breath breaks before anything else does. Afterward you recover quickly once you get your air back.

Delivery. You get pumped. Locally, brutally. Forearms, quads, shoulders, whatever is working. Your breathing is uncomfortable but manageable. You can do huge sets of a movement when it is the only thing you are doing, then fall apart when it is one of four things. Alternating between muscle groups makes it worse, not better. You need long rests to come back.

Utilization. Nothing dramatic happens. Nothing burns badly, nothing feels like suffocation. You just slow down. Power output drops off and will not come back within the set. Efforts that should feel fast feel heavy. Your top end is low relative to what you can hold, and it does not respond much to trying harder.

8. Three tests you can run this week

You do not need a lab. You need a stopwatch, a machine that gives you a number, and willingness to be honest about what actually stopped you.

Do these on separate days, rested. Write everything down. The numbers are almost useless in isolation and very useful compared against themselves in six weeks.

Test 1: the breathing test

What it finds: whether breathing gives out before your legs do, and roughly where your first ventilatory threshold sits.

You need: a rower, bike, ski erg, or a flat stretch of road. A clock.

Protocol.

1. Warm up easy for five minutes.
2. Settle into a pace that feels comfortably hard. Start talking. Say a full sentence out loud, roughly ten words.
3. Every two minutes, increase the pace one small notch. Say the sentence again at the end of each stage.
4. Stop when you cannot get a full sentence out without breaking it up to breathe. That point is your talk test threshold, and it lines up surprisingly well with the more sophisticated laboratory markers.
5. Note the pace or watts. Then count your breaths for a full minute at that intensity.
6. Hold that exact pace for three more minutes. Count your breaths again in the final minute.

Record: the pace where the sentence broke, breaths per minute at that point, breaths per minute three minutes later at the same pace, and one word for what made you want to stop. Air, or legs.

What you are looking for. Two things. First, whether your breathing broke well before your legs had anything to say. Second, whether your breathing rate kept climbing during those last three minutes while the workload never changed. A breathing rate that keeps climbing at a fixed workload means the breathing muscles are working harder to do the same job. That is fatigue, and it is a respiratory flag.

A secondary cue, useful but softer. Repeat the whole thing breathing only through your nose and note where nasal breathing breaks. It is not a validated threshold marker, so treat it as a feel, not a number.

Test 2: pump versus gassed

What it finds: whether your limit is local to one muscle or systemic across your whole body. This is the most useful of the three and almost nobody runs it.

You need: one movement you can repeat safely under fatigue. A kettlebell swing, a goblet squat, a moderate row, an assault bike. Plus one full-body cyclical piece.

A gate before you start. If the load you are using is heavier than about 30 to 40 percent of your one rep max for that pattern, stop. You do not have a conditioning problem for this movement, you have a strength problem, and no amount of conditioning fixes it. Get stronger, then come back.

Protocol.

1. Part A, isolated. Fresh and warmed up, do one set of your movement for as long as you can hold good quality. Record the reps or the time. Record what stopped you: burning muscle, or breathing.
2. Rest five full minutes. Genuinely five.
3. Part B, paired. Row 250 metres hard, or bike 30 seconds hard. Then immediately do the same movement again for as long as you can hold quality. Record the number.
4. Work out how far Part B fell short of Part A as a percentage.

What you are looking for. A big drop is the signal. If you did 25 clean reps fresh and 9 after a rower, something systemic collapsed. Your local muscular endurance clearly exists, because you demonstrated it five minutes earlier. What is missing is the ability to supply several working muscle groups at once. That is a delivery flag.

If both parts stopped at roughly the same place for the same local reason, and the number barely moved, the limit lives in that muscle. Utilization, or strength, or coordination.

Test 3: repeat effort recovery

What it finds: how well you recover between hard efforts, which is what almost every sport actually asks of you.

You need: a bike or rower with a readout.

Protocol.

1. Warm up ten minutes, including two or three short accelerations.
2. Six efforts of 30 seconds, hard but repeatable. Ninety seconds easy between each.
3. Record the number for every single effort. Distance, watts, calories, whatever the machine gives you. All six.
4. Take your best effort and your worst effort. Work out the percentage drop between them.

What you are looking for. The drop-off is the whole point, not the top number.

A large drop-off means you produce good power and cannot repeat it. That is a recovery problem between efforts, and recovery between efforts is aerobic. It points at delivery or at a missing base.

A small drop-off with low numbers throughout means the opposite. You recover fine, you never had much top end to lose. That is a power and recruitment conversation, not a conditioning one.

Note where your breathing sat during the rests too. If you were still gasping at the end of 90 seconds while your legs were ready to go, that is worth writing down.

9. Why your heart rate spikes on heavy squats

Because your muscles are squeezing so hard that blood cannot get back to your heart.

This is the single most useful thing in this article and it will change how you read your own watch.

Your heart’s total output is stroke volume multiplied by heart rate. How much blood moves per beat, times how many beats. When you contract a large amount of muscle very hard, as in a heavy set of squats, you compress the blood vessels running through that muscle. Less blood makes it back to the heart. Less blood back means less filling, which means less volume out per beat.

So your heart rate climbs. Not because the work got more aerobically demanding. Because it is trying to defend total output against a drop in stroke volume.

A high heart rate is not the same thing as a high cardiac output.

This is why your heart rate also runs high during a panic attack, during a scary film, and after too much caffeine. Same mechanism family, same misleading number.

Practically, this means two things. Your heart rate during heavy resistance training tells you almost nothing about your conditioning. And when you run the tests above, read the sensations, not just the watch.

There is a version of this that shows up in your working sets too. During a hard contraction blood flow into the muscle drops sharply and stays down until you relax. Researchers watching the low back during sustained contraction found blood volume and oxygenation fell hard at the onset and stayed there. Restricted blood flow from pressure inside the muscle turned out to be a primary driver of the fatigue. That is what the pump is. It is not the muscle running out of fuel. It is the muscle briefly cutting off its own supply.

Which is why the guy who grips the bar like he is angry at it gasses out faster than the guy who does not.

10. What your results mean, and what they do not

Now the honest part.

These tests give you a hypothesis, not a diagnosis. They tell you where to look first. They do not confirm anything.

A stopwatch cannot measure how much oxygen is actually in your muscle. Lab equipment can. Near-infrared sensors read muscle oxygenation directly and can show you the moment supply falls behind demand, in a specific muscle, in real time. Spirometry can separate weak inspiratory muscles from weak expiratory ones by comparing forced vital capacity against forced expiratory volume. A metabolic cart can tell you your true ceiling instead of an estimate.

I do not own that equipment and neither do you. So be honest about what you have. You have a pattern, not a readout.

A few more limits worth holding onto.

Your limiter moves. Train the thing that was holding you back and something else becomes the new lowest branch. A profile is a snapshot, not an identity. Do not start telling people you are a delivery-limited athlete like it is a blood type.

Training only your limiter is a mistake. Your weak link decides your ceiling, but everything around it still has to do its job, and it will end up covering for the weak link while you work on it. Train the whole system and bias toward the limiter.

And one test is one day. Bad sleep, a hot room, a big meal, a stressful week. Run each test twice before you believe it.

11. What to do about each limiter

Here is the direction for each one. Not a program. A direction, which is the thing you were missing.

If you are respiratory limited

You have two jobs. Reduce the cost of breathing, and improve the capacity of the muscles doing it.

Reducing the cost comes first and it is mostly free. Learn to breathe low and wide rather than high and shallow. Fix the positions that stop you doing that, because a rib cage that cannot expand makes every breath more expensive. Practise nasal breathing at easy intensities so the mechanics are automatic before the intensity arrives.

Then load the breathing muscles directly. Inspiratory muscle training uses a threshold device that makes you work to pull air in. Be careful with the claims here. The strongest evidence sits in clinical populations, where higher loads around 60 to 80 percent of maximum inspiratory pressure worked better than light ones, four weeks moved strength, and six to eight weeks were needed to move functional capacity. In healthy trained people the evidence is thinner and the effects are modest. One six-week trial in endurance-trained men found about a two percent improvement in a 20km time trial, with a sham group showing nothing.

Two percent is not nothing. It is also not a transformation. Sell it to yourself accordingly.

For interval work, a hard start suits you. Open the interval faster than target pace and let it settle. Starting hard drives oxygen consumption higher across the whole effort than an even pace does, which is exactly the stress you want if the goal is raising your ceiling.

If you are delivery limited

You need the aerobic base you skipped. There is no clever version of this.

Your heart’s ability to fill is the most trainable thing about it. Chamber volume can roughly double compared to untrained. What drives it is repeated exposure to a full stretch, and the way you get that is volume. Long, easy, boring work. The same logic as lengthening a hamstring. You have to spend time in the position.

The good news is that for most people the stretch is already maximal at around 50 to 60 percent of maximum heart rate, so this does not have to be hard. It has to be frequent and it has to add up.

For intervals, a gradual ramp suits you. Start below target and build through the effort. It gives your cardiac output time to catch up rather than demanding everything in the first fifteen seconds, and it puts the stress on the cardiovascular system rather than the lungs.

And accept the timeline. This is the slowest of the three to change, because you are remodelling an organ rather than adding enzymes to a cell. Months, not weeks.

If you are utilization limited

Your fix depends on which version you have, so start by narrowing it.

If your rate of force development is poor and you have no top end, you need exposure to genuinely high intensity work. Short, hard, fully recovered. This raises the quality of the mitochondria you already have rather than the quantity.

If you have top end but cannot sustain anything, you need volume, and specifically volume at an intensity high enough to recruit the fibres you actually use. Easy work only builds capacity in the fibres it recruits, which means very easy training leaves your faster fibres untouched. The middle ground matters more here than anywhere else.

If the limit is coordination or breathing mechanics under fatigue, no amount of conditioning fixes it. That is a skill problem wearing a conditioning costume. Film yourself at the end of a hard set and watch what falls apart.

12. How should I condition for my sport

Start with a question that has nothing to do with energy systems.

Why do you stop?

The best CrossFit athletes turn a workout that looks like a circuit into something that looks like a rowing race. Steady blood flow, a smooth linear decline in muscle oxygenation from start to finish, no spikes. Everybody else stops, rests, and finishes the same work in chunks.

In one comparison of two high-level competitors on the same workout, the athlete who broke it into chunks actually moved faster rep for rep. And finished more than two minutes behind. All of it went into stopping.

That principle is not about CrossFit. In basketball, in a Muay Thai round, in the last fifteen minutes of a soccer match, the person who does not have to stop wins. Not the person with the best single effort.

So your sport tells you the shape of the work. Your limiter tells you the direction. Almost everybody has the first one and almost nobody has the second. Two players on the same team need different conditioning, and they get the same conditioning.

Basketball

Forty to forty-eight minutes of clock, far longer in real time. Short repeated bursts, constant changes of direction, and an enormous braking load that most conditioning programs completely ignore.

Your aerobic capacity is what decides whether you are ready for the next possession. Not whether you can run a long way.

The classic failure is fourth-quarter legs in a big, strong, well-built player. That is the delivery-limited profile almost exactly. He needs the boring aerobic base he skipped, and more sprints will make it worse.

The other classic failure is not conditioning at all. Nobody taught him to absorb force, so his legs are gone from decelerating, not from oxygen. Test before you assume.

Soccer

Ninety minutes of near-continuous movement with a couple of hundred brief high-intensity actions scattered through it. The highest true aerobic demand of the common field sports, with repeat-sprint ability layered on top.

The interesting number is not the distance. It is the decay. Time-motion analysis of professional players found high-intensity running dropped 35 to 45 percent in the final fifteen minutes compared to the first fifteen. And after each five-minute peak in the game, output fell about 12 percent in the five minutes that followed.

Soccer does not test your top speed. It tests how fast it comes back.

That is a recovery problem, and recovery between efforts is aerobic. Which is why the sport punishes a big engine with no top end and a sprinter with no base, at different minutes of the same match.

Muay Thai

Three to five rounds of three minutes, with a minute or two between them.

Researchers put fighters in portable gas analysers through a simulated match. Oxygen uptake and heart rate sat above the anaerobic threshold for the entire fight. Excess carbon dioxide production spiked hard in round one and then declined steadily as aerobic supply took over the work.

So the picture is a violent anaerobic opening, and then three rounds paid for almost entirely by the aerobic system. If you gas in round three you are not short on power. You are short on base, or you are respiratory limited and nobody has ever told you.

The clinch deserves its own paragraph because nobody writes about it properly.

A clinch is a sustained hard isometric contraction, which compresses the vessels inside the working muscle and cuts its own blood supply. That is your forearms and shoulders dying. At the same time, a clinch loads your breathing directly, and fatigued breathing muscles pull blood away from your limbs through a reflex that raises sympathetic constriction elsewhere in the body.

Two limiters stacked, in one position, at the same moment. That is why the clinch breaks people who are otherwise in good shape, and it is why the answer is breath work and aerobic base rather than more rounds.

Tennis, pickleball, padel

Points of three to eight seconds, matches of one to three hours, often in the heat. You are almost never limited by your top end. You are limited by recovery between points and by heat, and both are aerobic.

Hyrox and CrossFit

The purest version of the cyclical question. The whole event is a test of whether you can keep moving. Go station by station and ask what makes you stop at each one, because the answer changes between stations and the fix changes with it.

The lifter who cannot climb the stairs

Five days a week under a barbell. Strong. Completely gassed carrying groceries to a second floor.

This is the most common person in my gym and the most textbook delivery limitation you will ever see. Enormous local endurance, low cardiac output, no ability to supply everything at once.

The fix is the most boring answer in this article and it works within about eight weeks.

Hiking, skiing, obstacle racing

Long duration, huge eccentric load, sometimes altitude. Usually limited by tissue tolerance to downhill and braking work before anything cardiovascular becomes the constraint. Train the descent, not just the climb.

Two things worth saying plainly

Playing your sport more is not conditioning. It is exposure. It maintains what you have. It rarely builds what is missing, because by definition your sport only ever asks for what your sport asks for.

Most recreational athletes are not limited by knowledge. They are limited by never having tested anything. You are three sessions away from knowing something about yourself that you have been guessing at for years.

13. The zone 2 problem

Zone 2 has no magic benefit over higher intensity work other than one thing. You can recover from it.

That is not a criticism. That is the entire job. Zone 2 is a load management tool. It lets you accumulate volume without digging a hole, and it lets you keep training on the days after hard sessions instead of sitting on the couch.

The problem is what happened when it became a target.

Two mistakes, in opposite directions

The first one belongs to intermediate athletes and it is the more subtle of the two. Hard session on Monday. Tuesday should be easy so you recover for Thursday. Instead Tuesday gets pushed, because it felt fine and the watch said the heart rate was still in range. Thursday arrives and you cannot hit the numbers that would have made Thursday worth doing.

You got greedy on the easy day and paid for it on the hard one. That is where keep your easy days easy and your hard days hard comes from, and the reason is not mystical. It is arithmetic about recovery.

The second mistake is the more common one and it is the one I see constantly.

Someone trains three hours a week. They read that elite endurance athletes do the large majority of their work at low intensity, which is true. So they do the large majority of three hours at low intensity, and then wonder why nothing moves.

Those athletes are training ten to fifteen times your volume. Their easy work is easy because they have to protect the recovery capacity they need for a large amount of hard work. You do not have that constraint. Volume is not your binding problem. You have room to push, and being frightened of moderate intensity when you are training three hours a week is worrying about a cost you are nowhere near paying.

The actual test for whether your easy days are easy enough

Not lactate. Not a heart rate zone on a watch that guessed your maximum from your birthday.

Get to the end of the week and ask whether you could do it again. If you feel run down and your sleep is off, you pushed the easy work too hard. Back it off and see what changes. If you feel fine, you have room.

That is a slower answer than a number, and it is a better one.

14. What about HRV

Heart rate variability answers one question well. Should I go hard today?

It does not answer the question this article is about, which is what should I be training. Those are different axes. Readiness versus direction. Your HRV cannot tell you whether it was your breathing or your heart or your mitochondria that gave out at minute four, and no amount of staring at the trend line will make it tell you.

What the evidence actually shows

It reliably improves one thing. Your HRV. Everything else crosses the line where the result could just be noise.

A systematic review and meta-analysis compared HRV-guided training against predefined training programs.

HRV-guided training was better at improving HRV. That result was clear and significant.

For maximum aerobic capacity, the effect did not reach significance. For capacity at the second ventilatory threshold, it did not reach significance. For endurance performance, it did not reach significance.

The authors’ own conclusion is the honest one. HRV-guided training may be better for maintaining and improving HRV itself, with fewer people responding badly. If it beats a predefined program on fitness and performance, the current data say it is by a small margin.

Read that again. It reliably improves the number. Its effect on the things you actually wanted is small enough that it keeps failing to clear the bar.

The finding that is actually useful

A quarter fewer hard days. The same or better result.

One randomized trial split forty recreational runners into an HRV-guided group and a traditional predefined group.

The HRV group did 13.2 hard sessions. The predefined group did 17.7. Roughly a quarter fewer.

And the HRV group’s 3000m time improved significantly while the predefined group’s did not.

Fewer hard sessions. Equal or better result. The value was never the number. The value was permission to skip a hard day on the right day.

And then this

A 2023 trial compared three groups of recreational runners. One trained according to HRV. One trained according to a self-report stress questionnaire. One followed a predefined plan.

The questionnaire group improved their 5km by 12.8 percent. The HRV group by 8.3 percent. The predefined group by 6.0 percent.

The questionnaire beat the device.

That is the second time in this article that a cheap subjective measure has matched or beaten an expensive objective one. Three survey questions and a resting heart rate outpredicted a metabolic cart on mortality. A stress questionnaire outperformed HRV on 5km time.

That is not an argument against measuring things. It is an argument that the thing worth measuring is often already free, and that buying precision on the wrong variable buys you nothing.

If you are going to use it

Trend, never a single day. One morning reading is noise.

Your own baseline only. Comparing your number to someone else’s is meaningless.

Same position, same time, before caffeine. And know that your breathing rate during the measurement moves the number, so if you slow your breathing down to relax before measuring, you are measuring your breathing.

The part I have actually watched happen

I have seen the data from a wearable cause more stress than it solved.

A client sees a low score and skips a session they would have handled fine. Another sees a good score and pushes into a day their body was already telling them to back off. Either way the number overrode the person.

That is the mechanism behind why a questionnaire beat a device in a controlled trial. Perceived readiness is real information. Handing that judgment to something strapped to your wrist does not upgrade it. Sometimes it deletes it.

We should not use data to take us further away from knowing how our own body feels.

If your HRV gets you to skip a hard session on a day you should not have trained hard, it earned its keep. If it becomes another number you chase, it has become the streetlight.

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15. The coach layer starts here

Everything above is what a person needs to find their own limiter and stop wasting months. What follows is for coaches. It is the mechanism under the advice, and the programming decisions that come out of it.

If you write programs for other people, this is the part that changes what you do on Monday.

16. Why beginners improve fast and then stall

Split everything under VO2max into two categories and most programming confusion resolves.

Central factors are the supply side. Cardiac output, blood volume, haemoglobin mass. These do not care much which muscles you use.

Peripheral factors are local to the working tissue. Capillary density, mitochondrial volume, mitochondrial quality. These care enormously which muscles you use.

Untrained people improve fast because mitochondrial adaptations arrive quickly. Give someone eight weeks of almost anything and the peripheral side moves. Then it catches up to what the heart can deliver, and progress stops.

The stall is not a motivation problem. It is a handoff from peripheral to central.

Central adaptation is slower because you are remodelling an organ rather than adding enzymes to a cell. When a client plateaus at month four, this is usually why, and the answer is almost never more intensity.

17. Why three intensity buckets, derived rather than asserted

Most models hand you three zones and tell you to trust them. Here is where they actually come from.

Mitochondrial adaptation splits in two. How many you have, and how good each one is.

Mitochondrial volume, the count, is driven almost entirely by total training volume. Intensity barely moves it. Sixty percent, seventy, eighty, roughly the same. What matters is accumulated work.

Mass-specific respiration, the quality, is the opposite. Low and moderate intensity do not touch it. It needs genuinely hard work.

So far that gives you two buckets. Here is the complication that creates the third.

These adaptations only happen in the fibres you actually recruit.

Very easy work never recruits fast twitch. So all that patient aerobic volume is building mitochondrial density in slow twitch fibres and leaving the rest of your engine untouched. Very hard work recruits everything, but you cannot accumulate much of it.

That leaves a gap. Mitochondrial volume in fast twitch fibres. Filling it requires an intensity hard enough to recruit them and sustainable enough to accumulate real time. Threshold. Sweet spot. Whatever you want to call the middle.

Three buckets, and none of them are optional:

• Easy and sustainable. Mitochondrial volume in slow twitch. Driven by volume.
• Threshold. Mitochondrial volume in fast twitch. The gap nobody trains.
• Very hard. Respiratory quality across all fibre types.

The coach who only programs easy and hard is leaving the middle bucket empty. That is the most common hole I see in otherwise decent programs.

18. The stretch and the squeeze

Cardiac output is stroke volume times heart rate. Stroke volume has two components, and they respond to completely different training.

The stretch. How much the ventricle fills between beats. This is the most trainable quality the heart has, and chamber volume can roughly double compared to untrained. What drives it is repeated exposure to a full stretch, which means volume. The same logic as lengthening a hamstring. You have to spend time in the position.

The squeeze. How completely it empties. Improves maybe ten to fifteen percent, and responds more to high intensity.

Now the detail that decides your programming.

In most people stroke volume rises with intensity up to roughly fifty to sixty percent of maximum heart rate and then plateaus. Past that point, going harder does not stretch the heart any further. So for a general population client, easy work is already maximal for this adaptation. It does not need to be hard. It needs to be frequent and it needs to add up.

In highly trained endurance athletes that plateau does not appear.

Their stroke volume keeps climbing at higher intensities. Which is exactly why advanced athletes need genuine high intensity to keep remodelling, and why prescribing an elite athlete’s session to a beginner accomplishes nothing the beginner could not have got from a long easy ride.

Same adaptation. Opposite prescriptions. The variable is training age.

19. Exercise selection changes the adaptation more than the interval does

Most conditioning arguments are about work-to-rest ratios. Almost nobody argues about movement selection, and movement selection is doing more of the work.

Classify by how much muscle mass a movement recruits.

Global, roughly sixty percent of total muscle mass or more. Rowing, skiing, running, swimming, thrusters, cleans. Under fatigue these are limited centrally, because no single muscle group gets overloaded. They stress systemic cardiovascular control hard.

Regional, roughly forty to sixty percent. Cycling, ski erg, push press, kettlebell swings, kipping pull-ups, box jumps. These sit on the fence, and cycle rate decides which side they fall on. Faster turnover pushes toward cardiorespiratory limitation. Slower and heavier pushes toward local muscular limitation. That is a dial you control.

Local, under forty percent. Arm ergometer, strict press, curls, leg extensions, strict pull-ups. Almost always limited by peripheral fatigue. These are rarely appropriate for energy system work unless the sport specifically demands it.

If your athlete is delivery limited and you program local movements, you will train the thing that was not broken.

20. Three ways to run the same interval

Same distance. Same total time. Same average pace. Three different adaptations.

Hard start. Open above target and let it settle. This drives higher mean oxygen consumption across the effort than an even pace does, which makes it a good way to accumulate time near VO2peak without adding volume. Suits a respiratory limited athlete trying to raise a ceiling.

Gradual ramp. Start below target and build. This gives cardiac output time to catch up instead of demanding everything in the first fifteen seconds, putting the stress on the cardiovascular system rather than the lungs. Suits a delivery limited athlete.

Fixed pace. The default. Predictable, easy to coach, and the least specific of the three.

None is better. But if you are coaching a group on a fixed clock and you want to individualise without rewriting the session, intra-interval pacing is the cheapest lever you have.

21. There is no magic VO2max protocol

Every year a protocol goes around and gets treated as the answer. Four minutes hard, four minutes easy, four times. Something similar in a different wrapper.

They work. That is not the problem.

The problem is that a wide range of intensities will elicit VO2max given enough duration, because oxygen demand drifts upward during sustained work. Athletes have reached VO2max at half marathon pace. They also reach it at mile pace, just sooner. It is not a point. It is a zone, and it is far wider than the protocols imply.

Which means 600m repeats at mile pace and 1500m repeats at 10k pace can both accumulate meaningful time at VO2max. The specific protocol matters less than whether you accumulate the time and can recover to do it again next week.

Every one of these protocols works for six to eight weeks and then stops.

You plateau, and you discover you neglected everything else while chasing one number. The athletes who genuinely transform their aerobic capacity do it across five and ten years of fundamentals, not in an eight week block.

Paula Radcliffe is the example worth keeping. Across years of laboratory testing her VO2max did not meaningfully change. In the same period she took over ten minutes off her marathon time and set a world record.

Program a VO2max block if the athlete needs one. Six to twelve weeks, two or three hard sessions a week, then move on. Do not build a training identity around it.

22. Concurrent training, honestly

The interference effect is real, overstated for most people, and largely a programming order problem.

Here is the honest version. You can absolutely train strength and endurance together and progress at both. But nobody has ever been an elite marathoner and an elite powerlifter simultaneously, and the best hybrid athletes would be mediocre specialists in either direction. Something is being paid.

For the general population and most recreational athletes, that cost is small enough to ignore. For anyone near the limits of human performance it is not.

Four rules that handle most of it:

• Order high skill to low skill. Technical work goes before fatigue arrives.
• Order least fatiguing to most fatiguing. Together with the first rule, this protects the quality of everything that matters.
• Do not stack high intensity inputs that drive the same adaptation. You can only absorb so much of one stimulus in a window. Spacing them is more efficient.
• Know when to break all three. A fighter needs skill under fatigue. A CrossFit competitor needs heavy lifting when tired. Breaking the rules is sometimes the point, as long as you know which rule you broke and why.

23. The non-responder is usually a prescription error

This is the section I would most like coaches to take away.

The famous family study estimated that roughly forty seven percent of the variation in VO2max trainability is genetic. That figure gets quoted constantly as evidence that some people simply do not respond.

Look at what generated it. One exercise program. One dose. Relatively low intensity, starting around fifty five percent of VO2max, about ninety minutes a week.

Imagine testing a blood pressure drug at a single dose, in a single formulation, without titrating to the patient, and concluding that half of blood pressure response is genetic.

Follow-up work found two things worth knowing. Prescribing intensity relative to lactate threshold rather than as a percentage of VO2max produced a far more uniform response across individuals. And raising the overall dose largely eliminated non-responders. Some people respond poorly to low intensity and well to high, and some the reverse.

Most non-responders are people who were given the wrong anchor.

Which is the coach-facing version of this entire article. Percentage of maximum is a convenience, not a physiological landmark. Two athletes at eighty percent of max heart rate can be on opposite sides of their threshold, doing genuinely different training while following identical instructions.

24. Limiter, bridge, performance

Most periodisation models still rest on supercompensation theory, which comes from stress physiology that has moved on considerably in the last thirty years. The models built on it assume a tidier, more uniform response than people actually have.

A more useful frame separates two kinds of adaptation.

Functional adaptations are transient. Short timescale survival responses to an overload. Real, useful, and temporary. Remove the stressor and they fade. This is why athletes on aggressive programs improve fast and then fall apart. They were riding functional adaptation with no structure under it.

Structural adaptations are the changes to muscle, bone, heart, lungs and mitochondria that let you carry load long term. They are the base that functional adaptation sits on.

The phasing that follows has three modes, and you are always doing some of each.

Limiter work drives adaptation specific to whatever is capping oxygen consumption. It raises the ceiling for later rather than maximising performance now.

Bridge work is the step almost everyone skips. It carries the athlete from developing a limiter to expressing it. Broken intervals, fast-twitch fatigue resistance work. Without it you arrive at competition fitter and less prepared.

Performance work mimics competition in every dimension. Volume, intensity, density, environment, psychological load. Time trials, simulations, scrimmages.

Two failure modes this prevents. Reduce a sport to isolated qualities and train them separately, and you raise potential while leaving the athlete underprepared. Train only the sport, and you get someone technically sharp and poorly conditioned.

The fix is not choosing. It is touching all three at all times with different emphasis depending on where the athlete is.

25. Where this gets complicated

26. How to actually start

Sedentary. Frequency before intensity before volume. Anything that raises breathing rate, most days, for as long as is comfortable. The dose-response curve is steepest here, so this is the highest return work you will ever prescribe. Do not complicate it.

Coming back from injury. Runners treat three miles as the minimum worth doing, so they rest, run three, hurt, rest, run three, hurt. Break that by making the dose absurdly small. A quarter mile a day. It keeps the daily habit intact, which is what they actually want, and it stops the boom and bust cycle that convinced them running is bad for knees.

The lifter who never conditions. Long, easy, boring, frequent. Months not weeks, because you are remodelling an organ. Expect resistance, because it feels like nothing is happening. Something is.

The runner who never lifts. Strength first, because if you are contracting at a high percentage of maximum just to move a load, you are occluding your own blood supply. Getting stronger lowers the metabolic cost of the same work.

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Common questions

What is conditioning?

Conditioning is your body’s ability to move oxygen from the air into a working muscle and use it. It happens in three steps: taking oxygen in through the lungs, moving it through the blood, and using it inside the muscle. Everyone has one of those three steps that gives out first, and that step is what your conditioning training should be aimed at.

What are the three energy systems?

The traditional model splits energy production into the ATP-PC system for very short efforts, the glycolytic system for efforts up to roughly two minutes, and the oxidative system for anything longer. It is a useful teaching model but a poor training model, because all three run at once and the model tells you nothing about which part of your own oxygen chain is limiting you.

Why am I out of breath but my legs feel fine?

That usually points to a respiratory limitation. Your breathing muscles are giving out before your legs do. It shows up as breathing that climbs early and keeps climbing even when your pace holds steady, and it recovers quickly once you get air back. Structure matters here more than people expect, because lungs and airways adapt to training far less than the heart and muscles do.

Why do my legs burn but my breathing is fine?

That points to a delivery limitation. Your heart cannot supply enough blood to all the working muscle at once, so the muscle uses oxygen faster than it arrives. The classic pattern is someone strong who can do enormous sets of one movement in isolation and falls apart the moment several things are working at the same time.

Is zone 2 training worth it?

Yes, but not for the reason it is usually sold. Zone 2 has no magic benefit over higher intensity other than recoverability. Its job is load management, letting you accumulate volume without digging a hole and keep training on the days after hard sessions. If you only train two or three hours a week, you are probably going too easy while copying athletes who train ten times your volume.

How much conditioning should I do per week?

The guidelines recommend 150 to 300 minutes of moderate activity or 75 to 150 minutes of vigorous. There is no lower threshold you have to clear before anything starts working, and the curve is steepest at the very bottom. Going from nothing to something is the best return you will ever get.

Why does my heart rate spike when I lift heavy?

Because hard muscle contraction compresses the blood vessels running through the muscle, so less blood returns to your heart. Less blood back means less volume out per beat, and heart rate rises to defend total output. A high heart rate during heavy lifting is not evidence of a hard cardio session. The same spike happens during a panic attack or after too much caffeine.

Should I use HRV to plan my training?

HRV answers whether you should go hard today. It does not tell you what you should be training. The evidence shows HRV-guided training reliably improves HRV itself, while its effect on VO2max, threshold and actual performance is small and does not clear statistical significance. If it gets you to skip a hard session on a day you should not have trained hard, it earned its keep.

Am I a non-responder to cardio?

Almost certainly not. The research behind that idea used a single program at a single relatively low dose. When intensity is prescribed relative to lactate threshold rather than as a percentage of maximum, the response across individuals becomes far more uniform, and raising the dose largely eliminates non-responders altogether.

Is playing my sport enough conditioning?

It is exposure, not conditioning. Playing maintains what you already have. It rarely builds what is missing, because by definition your sport only ever asks for what your sport asks for.

How long does it take to improve conditioning?

It depends on which link is limiting you. Mitochondrial adaptations move within weeks, which is why beginners improve quickly. Cardiac adaptations take months, because you are remodelling an organ rather than adding enzymes to a cell. If you are delivery limited, expect months and judge progress accordingly.

Do I need a heart rate monitor?

No. The talk test tracks the first ventilatory threshold closely enough for training decisions, and the best test of whether your easy days are easy enough is whether you could repeat the week. A monitor is useful. It is not a prerequisite.

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About the author

Ian Markow is a strength and conditioning coach in Boca Raton, Florida, with fifteen years of coaching experience. He runs Markow Training Systems, working with in-person and online clients, and Personal Trainer Academy, an education platform for working coaches.

His certifications include FRCms and Kinstretch through Functional Range Systems, Stick Mobility, EXOS Performance Specialist, StrongFirst SFG1, MoveMed Level 1, NASM-CPT and NCCPT.

Find him at @ianmarkow on Instagram and @ianmarkow on YouTube.

References

The limiter framework in this article draws on Evan Peikon’s work on bioenergetic limiters, and the health versus performance distinction was sharpened by Alec Blenis. Every figure below was checked against the primary source.

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