VO₂ Max Drops in 12 Days?! How to Keep 90% of Your Fitness After Your Marathon
Estimated read time: 4.3 minutes (about as long as it takes to walk down stairs the day after a marathon). 😉
Hey Performance Nerds! Jonah here. 🤓
You lose ~7% of your VO₂ max after just two weeks off.
Is that killing your fitness… or saving your next training block? Some systems crash fast. Others hold on for months.
Here’s what we got:
✅ The Two-Phase Detraining Model 🧬 — which systems fade first (and which are surprisingly durable)
😩 Why your first run back feels awful — but doesn’t mean you’re out of shape
⚙️ How to maintain 90% of your fitness with minimal time or impact
(Augie took 2 weeks off from squirrel duty. His sprint speed crashed. His trash-talking? Never left.)
🙏 Thank You
This week, my inbox filled with your pre-race questions and nerves, and it's reminded me how lucky I am to be part of your journey. Thank you for trusting me with your training and for sharing what this race means to you.
I'm heading to Chicago to support my mom through her first marathon, and I couldn't be more excited to share the road with all of you. If there's anything I can do to help before Sunday, please just ask.
And, if you see me out there, stop me. I mean it. Meeting readers and hearing how your training went is my favorite part of race weekends. I want to know your story.
Good luck. You're ready.
🧬 Performance Sponsors:
🚀 Train Harder. Recover Smarter.
SiS—the same science-backed fuel trusted by Olympic marathoners—is now sponsoring Marathon Science.
From hydration mixes to recovery tools, every product is backed by real performance data and built for serious athletes like you.
🚀 Power Your Pace with Stryd
Stryd — the running power meter trusted by serious athletes — is now an official performance sponsor of Marathon Science, delivering real-time pacing precision so you can train smarter and race faster.
💡 See this week’s full Stryd training tip at the end of this newsletter.
🧬 The Science — Two Systems, Two Timelines
When you stop training, fitness drops in waves, not all at once.
Think of it as a Two-Phase Model:
1️⃣ Central systems (blood volume) drop fast: within weeks.
2️⃣ Peripheral systems (muscles, mitochondria, tendons) erode slowly: over months.
Phase 1 (0-4 Weeks): Blood Volume Crashes
Plasma volume can fall by up to 12% in 10–14 days.
Less plasma = less blood for your heart to pump = lower stroke volume (the amount of blood your heart pushes out each beat).
Your heart is still strong, there's just less blood to move.
VO₂ max follows:
↓ ~7% after 12 days
↓ ~10–14% by week 4
↓ ~20% by week 8 (if inactivity continues)
Your heart rate climbs 2-9 bpm at the same pace. You're working harder to deliver the same oxygen with less blood.
Here’s the good news: your muscles, tendons, and mitochondria haven’t gone anywhere.
You just feel sluggish because your delivery system (blood volume) has dipped.
It’s like having a race car engine with a pinched fuel line: horsepower’s there, but the delivery can’t keep up.
Phase 2 (4+ Weeks): Muscles Start to Fade
After a month, the deeper muscle machinery starts to decline.
Mitochondrial enzyme activity drops 18-40% over 4-8 weeks.
These are your cellular power plants. Fewer of them = less fat burning, more carb reliance, and lower efficiency.
Other changes:
Capillary density thins → fewer blood vessels delivering oxygen
Aerobic enzyme activity declines → cells get worse at turning oxygen into energy.
Fat oxidation becomes less efficient → you burn through glycogen faster and bonk sooner
But not everything fades fast:
Heart muscle size: Shrinks only after ~8 weeks (<25%)
Tendons & bones: Detrain slowly—loss happens over months
🧩 Takeaway: Systems that adapt fast, like blood and enzymes, fade fast.
Structural systems, like your heart, mitochondria, and tendons, stick around for the long haul.
💡 Why Your Comeback Feels Brutal
Your first run back feels awful, not because you lost all your fitness, but because your blood volume crashed.
The good news? It rebuilds in 2–4 weeks.
Your heart, mitochondria, and tendons never really left.
🗓️ What to Expect — The Detraining Timeline
Here's exactly what declines (and what doesn't) at each stage:
Time Off | What You Lose | How It Feels | What To Do |
1 week | Glycogen dips slightly | Fresh legs, slight rust | Full rest—no panic |
2 weeks | VO₂ max ↓ 7%, HR ↑ 2-4 bpm | Noticeably harder at pace | 1 HIIT or cross-train session |
4 weeks | VO₂ max ↓ 14%, enzymes start dropping | Working to hold paces | Cross-train 2-3x/week |
8+ weeks | VO₂ max ↓ 20%, mitochondria ↓ 18-40% | Rebuilding from scratch | Gradual return (3:1 rule) |
The pattern: Functional systems (blood volume, enzymes) drop fast.
Structural systems (heart muscle, mitochondria, tendons) hold on much longer.
🔧 How to Maintain Fitness — The Minimum Effective Dose
You can hold onto up to 90% of your fitness for up to 8 weeks with surprisingly little training.
You don’t need 60 miles a week.
You just need to keep your cardiovascular system under stress: through intensity, not volume.
✅ The recipe:
1-2 high-intensity session/week (intervals, tempo)
1–2 aerobic sessions/week (easy bike, pool, or elliptical)
2 strength sessions/week (protect tendons, bone, connective tissue)
Research shows volume can drop 40–60% without major losses, as long as intensity stays high.
Those hard sessions keep blood volume, enzymes, and heart efficiency switched on.
Cross-Training That Works
Can’t run? Maintain fitness through weightless running, cycling, or elliptical.
Aim for RPE 7–8/10 with intervals. It should feel challenging, enough for your heart and lungs to remember real work.
Sample Maintenance Week (Injured or Off-Season)
This is what staying fit looks like when you can't run:
Day | Session | Purpose |
Mon | Elliptical: 4 x 4 min hard | Maintain Aerobic Fitness |
Tue | Strength training | Reinforce muscle, tendon + bone strength |
Wed | Rest | Recovery |
Thu | Cycling: 8 x 2 min hard | Maintain Aerobic Fitness |
Fri | Strength training | Reinforce muscle, tendon + bone strength |
Sat | Easy bike or Elliptical 30–60 min | Active recovery |
Sun | Rest | Recharge |
Total training time: ~3.5 hours per week. That's it.
📅 How Long Should You Take Off After a Marathon
If you’re not injured, 2–4 weeks off is the sweet spot.
(By that point, even Augie’s tired of me pacing around the apartment—he’s practically handing me my running shoes)
Why? Around week two, blood volume and VO₂ max start to dip, but rebound quickly.
💡 Translation: 2–4 weeks off = enough recovery without losing the hard-earned adaptations.
Plan it:
Weeks 1–2: Full rest or cross-training
Weeks 3–4: Add easy runs + light tempo work
After four weeks, start running consistently again: you’re entering Phase 2, when those deeper adaptations finally start to fade.
Bottom line: Taking time off is smart. Taking too much time off means rebuilding fitness you didn’t need to lose.
🏃 Return Smart, Not Fast
When you start running again, go slow, you're building for the long game.
Follow the 3:1 rule: for every week off, plan about three weeks to rebuild.
Start at 50–60% of your usual mileage, keep one hard session per week, and increase volume by 10–15% weekly.
Your fitness returns faster than your tissues can handle load, so let them catch up.
🎯 Key Takeaways — What Actually Matters
You lose blood volume fast, but it rebuilds in weeks.
Heart, muscles, tendons, and mitochondria fade slowly—durability takes years to lose.
1–2 hard + 1–2 aerobic sessions keep you ~90% fit for a few weeks
After a marathon, 2–4 weeks off is plenty—then start rebuilding.
Slow ramp, patience now prevents re-injury later.
💬 One last thing before you go.
After taking a break, it’s easy to overthink your comeback. If you’re unsure how to rebuild, worried you’re losing fitness, or just need a sanity check on your next block, hit reply.
I read every message, and I’m happy to help however I can.
Are You a True Running Nerd? Prove it.. 🧐
Welcome to the prove you’re a nerd section. Each week, I ask a question about a common running science myth.
Answer correctly, and you’ll be entered into a weekly raffle to win a package of Jonah’s favorite supplements.
Last Week’s Results: Super Shoes, Softer Legs 👟📉
They make you fly on race day, but wear them every run, and that magic can turn on you. Most of you spotted the hidden cost.
The correct answer?
C. They offload foot and lower-leg stress, potentially weakening those tissues over time 🦶📉 ✅
Super shoes’ carbon plates and high-stack foams absorb impact so well that your calves, Achilles, and plantar tissues stop getting their usual workload. Over months, that means weaker connective tissue when you switch back to normal shoes or run long without the “assist.”
Here’s how the votes shook out:
⬜️ A. They reduce ground contact time, limiting aerobic development 🏃♂️⌛ – 1
🟨 B. They’re too unstable at slower speeds, raising injury risk 🌀🚫 – 34
🟩 C. They offload foot and lower-leg stress, potentially weakening those tissues over time 🦶📉 – 199 ✅
🟨 D. They shift mechanics so much you can’t train efficiently at easy paces 🔄🐢 – 40
Bottom line?
Save the supers for speed, long runs and race days. Let your feet and calves do their full share of work on easy runs, they’re part of your durability engine. 💪🏃♂️
🏃♂️ Stryd Training Tip — Why Accuracy Wins Your Marathon
When you’re pacing a marathon, accuracy isn’t optional, it’s everything.
Stryd’s power meter gives you real-time, precise pacing data that GPS and heart rate just can’t match. Wind, hills, temperature, fatigue, Stryd cuts through all of it by measuring your actual running power, not your watch’s guesswork.
Why it matters:
Power = consistency. Pace lies, heart rate drifts, but power always reflects true effort.
Better pacing decisions. Marathon pace usually sits at 85–93% of Critical Power (CP) — the sustainable ceiling where you can run hard without blowing up.
Reliable splits in any condition. Whether you’re on a hilly course or weaving through crowds, Stryd keeps your pacing steady and honest.
💡 Bottom line: Power pacing takes the guesswork out of marathon day. With Stryd’s accuracy, you can lock into the right effort from the first mile — and hold it all the way to the finish line.
Don’t forget: You + Science = AWESOMENESS 😎
Yours in science,
Jonah
P.S. - We have a crew of 17,450+ nerds here who are running FAST using science.
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