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👋🏻 Hey Reader!
Thursday September 17 · A to Z, special edition
If you run in Denver, Boulder, Flagstaff or Albuquerque — or you've got a race at elevation this autumn — this email explains something almost nobody gets right.
It isn't "the air is thinner". It's a specific, measurable mechanism, with concrete levers you can actually pull.
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What's really happening
Your legs don't fail first.
The communication between
your diaphragm and your quads does.
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⏱️ Minute 12
You arrive, do your usual warm-up, everything feels reasonable. Then 12-15 minutes in, something doesn't add up: the pace that was comfortable at home now feels like hard threshold. Heart rate spiking, breathing ragged, and legs heavy for no apparent reason.
Most runners read this as "I'm out of shape" or "the air's thinner, everyone knows that". Both are half true. What's actually happening is that your lungs are stealing blood from your legs.
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① The respiratory metaboreflex
From 4,900-6,600 feet (1,500-2,000 m) the partial pressure of oxygen drops and your body compensates by hyperventilating. Sounds fine in theory, but it carries a real mechanical cost.
The diaphragm and intercostals are skeletal muscle, same as your quads. Work them hard for long enough and they fatigue. When they do, metaboreceptors fire — type III and IV afferent fibres — sending a signal to the sympathetic nervous system.
Your body's response: prioritise blood flow to the respiratory muscles by vasoconstricting the arteries in your legs. Your nervous system decides breathing matters more than running.
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That's why altitude fatigue feels different. It isn't depleted glycogen or metabolites accumulating in the quad.
It's a central decision, made by the autonomic nervous system. Your legs feel heavy and empty not because they're damaged, but because they are literally receiving less blood. |
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🫁 The lever you can actually pull: posture
Run hunched, shoulders forward, ribcage compressed, and you reduce diaphragm expansion — forcing accessory muscles to compensate and fatigue faster. Four adjustments:
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Lift the sternum slightly, as if a thread were pulling it up and forward. It opens the intercostal space. |
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Don't shrug your shoulders toward your ears, especially at the end of reps or climbs. |
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Breathe 360° — not just into the chest, but out into the sides and lower back. Practise it at rest before trying it while running. |
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Active exhalation. At altitude many runners inhale with effort but exhale passively, leaving residual air. A slightly more forced exhale — not explosive — improves air turnover. |
This doesn't eliminate the metaboreflex — it delays the point at which respiratory muscles hit critical fatigue, which delays the peripheral vasoconstriction.
It's an advantage of minutes, not hours. But in a 10K or a half, that can be the difference between holding pace and falling apart in the last third.
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⛽ ② You burn more glycogen at an "easy" pace
Fat oxidation requires more oxygen per molecule of ATP than carbohydrate oxidation. At sea level that doesn't matter — there's plenty of O₂ to sustain that more expensive process.
At altitude your body makes an automatic adjustment: it prioritises glycolysis, which produces ATP faster with less oxygen per molecule — at the cost of depleting glycogen much faster.
The practical result: running at the same pace, or even slower than at sea level, you're burning glycogen at a higher rate.
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What changes in your fuelling plan
→ Raise carbohydrate intake during prolonged exercise, even in sessions you'd normally call low intensity.
→ Glycogen runs out earlier than expected on long runs. That can feel like a premature wall that's actually a metabolic consequence of altitude, not poor preparation.
→ If you arrive to race at elevation unacclimatised and assume your sea-level fuelling plan will be enough: it won't be.
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⌚ ③ Your watch is lying to you
Steering by heart rate or estimated VO2max alone at altitude is one of the most expensive mistakes there is.
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| ❌ Heart rate on its own |
| It rises at altitude because the heart compensates with higher cardiac output. That's an expected adaptive response, not a sign of overexertion. Run on sea-level zones and you'll brake too hard, frustrated by a number that doesn't reflect your real effort. |
| ❌ Your watch's estimated VO2max |
| These algorithms are calibrated on sea-level data and don't adjust accurately for lower atmospheric oxygen pressure. You'll see a dramatic drop that doesn't reflect real fitness loss — it reflects a limitation of the algorithm. |
| ✓ Blood oxygen saturation (SpO₂) |
This one tells you directly how well your body is transporting oxygen right now.
→ Measure it at rest on arrival and track the trend over the first days. An initial drop is normal; what matters is the recovery trajectory.
→ Check it after hard efforts too, not just at rest. Pronounced, sustained drops are a better signal than elevated heart rate for deciding whether to back off that day. |
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🧠 ④ When RPE and pace get divorced
It's common to feel an RPE of 8/10 — effort that at home would be threshold — while the GPS shows a pace that at sea level would unquestionably be a conversational 5/10 jog.
That isn't a perception failure or a sign you're flat today. Your brain is integrating chemoreceptors detecting lower SpO₂, activated respiratory metaboreceptors, and glycogen falling faster than usual. Your brain is being honest about the real physiological stress — it's the number on your wrist that isn't.
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Redefine the goal before you head out
Decide in advance that the success criterion is RPE, not pace. That removes the temptation to push for your usual number.
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Turn off the pace alert
Seeing a slow number while your body reports high effort creates unnecessary cognitive conflict. Run by feel for the first sessions.
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Accept the adaptation window
Ventilatory acclimatisation happens over the first 7-14 days. Days 3 and 4 are consistently the worst. If you have a race scheduled, that fact should decide when you arrive.
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Log the data for next time
Note RPE, pace and SpO₂ from your first sessions. Next exposure you'll have a realistic frame of reference instead of comparing to your sea-level paces.
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Where you're standing |
Leadville, CO · 10,150 ft (3,094 m) |
Mexico City · 7,350 ft (2,240 m) |
Flagstaff, AZ · 6,900 ft (2,100 m) |
Colorado Springs, CO · 6,035 ft (1,839 m) |
Boulder, CO · 5,430 ft (1,655 m) |
Denver, CO · 5,280 ft (1,609 m) |
Everything above starts applying from 4,900-6,600 ft. If you live in any of these places, this isn't travel theory — it's your everyday physiology.
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Coach's perspective
I race the 3000 m on the track as a masters athlete, and something I've learned reviewing data from athletes who train or compete at elevation is that the error is almost never effort — it's expectation.
The runner who fails at altitude usually isn't the one who ran too easy out of caution. It's the one who tried to force sea-level numbers against a physiology operating under different rules.
Altitude doesn't change your capacity. It changes the rules of the game — and it's worth playing it with the right information.
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Your question today
What elevation do you normally train at?
Reply with your city. If you're above 4,900 ft, I'll tell you what to adjust first — and if you're racing at sea level, the advantage you're carrying.
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Tomorrow: the complete beginners plan, eight weeks. |
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Coach Henri
USATF · TrainingPeaks · Certified Functional Strength Coach
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