Why Your Heart Rate Stays High—And Why Spiking It Beats Endless Zone 2
Estimated read time: 4.2 minutes (About as long as it takes to watch two coaches argue about Zone 2 on YouTube and still feel confused 🤔)
Hey Performance Nerds! Jonah here. 🤓
Most running advice says, “Run more Zone 2 and your heart rate will drop.”
That’s only half true.
Recent research shows high intensity training produces stronger adaptation signals than moderate or low intensity work, especially when your training time is limited.
Yet the "just run easy" advice keeps spreading.
Here’s what you’ll learn today:
Why pure Zone 2 hype leaves your heart’s potential on the table
The three mechanisms that lower heart rate, and which training hits each one
How to build a hybrid training week that encourages a lower heart rate
Real timelines for change, from days to years
(Augie, my dog, thinks any heart rate above "napping" is overrated. Which explains why he's outrun by squirrels.)
💬 A quick note:
I've been reflecting on my content lately, and I want to be honest with you.
My number one goal has always been to support you through good science. But I'm not the expert on everything. There are people who've dedicated their lives to studying these topics.
They understand the nuances, the "it depends." That's their story to tell, not mine.
So I'm shifting. More long-form. More conversations with actual researchers. Less "do this, not that" from me—more connecting you directly to the people who live this work.
A community that connects you to the actual PhDs.
Thanks for being here. This is for science. This is for you.
P.S. If you have experience with YouTube—creating videos, editing, any of it—I'd love to hear from you. Just reply to this email.
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🔬 What Actually Lowers Heart Rate (3 Mechanisms)
Your heart rate drops because your body adapts in three main ways. High-intensity training gives the strongest signal for these changes.
Zone 2 helps you tolerate enough training to keep that signal going.
Think of it like a machine upgrade: more fluid in the system, a stronger pump, and eventually a rebuilt engine.
1. Blood Volume Expansion (Fast)
Training stress triggers your body to pull more water into your bloodstream.
What changes: You get about 10 to 20 percent more blood in your system.
Timeline: 3 to 14 days
Why it lowers HR: More blood returns to your heart each beat, so each squeeze sends out more blood. More blood per beat means fewer beats needed at the same pace.
This is why heart rate can drop noticeably in the first couple of weeks of consistent training, even before you feel much “fitter.”
2. Stroke Volume Increase (Medium)
This is the pump upgrade. Your heart squeezes out more blood each beat.
Quick Science: Stroke volume is how much blood your heart pumps out per beat. More blood per beat means your heart doesn’t need to beat as often.
What changes: Roughly 15 to 20 percent more mL per beat
Timeline: About 4 to 8 weeks
How it lowers HR: A heart pumping 80 mL per beat needs fewer beats than one pumping 60 mL at the same pace.
Best stimulus: Intervals at about 80 to 95 percent of max HR or an RPE of 8 to 9 (very hard effort).
At this effort your heart fills and empties as much as it can, like testing the pump at full power.
Zone 2 is too easy to create the strong pressure and stretch your heart needs to adapt.
3. Structural Remodeling (Slow)
This is the full engine rebuild. Your heart’s main chamber gets bigger and stronger so it can move more blood each beat.
What changes: Around 20 to 50 percent increase in your heart’s left-ventricular mass and chamber size
Timeline: 6 months to years
How it lowers HR: Creates the permanent low-HR profile of lifelong runners.
Best stimulus: High volume AND high intensity.
You need both a strong signal and enough time in training for the structure to remodel.
🔧 The Real Reason We Mix Intensities
Here's where most advice gets it wrong.
The myth: Zone 2 builds capillaries and mitochondria. Intensity builds the heart. Different tools for different jobs.
The truth: Hard running creates a stronger training signal for almost everything, including capillaries and mitochondria.
The problem isn't the stimulus. It's the cost.
You can’t handle hours and hours of hard running each week. Your nervous system and legs would break down.
Zone 2's superpower isn't magical physiology. It's low fatigue.
Easy running lets you:
Add time on your feet
Build more total minutes
Recover enough to run hard again
The equation:
Max Adaptation = High-intensity stimulus + Volume supported by low-fatigue Zone 2
If we could recover instantly, we'd probably do mostly intense training. Since we can't, Zone 2 fills the gap.
📋 The Training Protocol: Stimulus + Recovery
The lowest heart rates usually belong to runners who:
Protect the high-intensity signal, and
Use Zone 2 to support that signal without burning out
High Intensity = The Potent Medicine
High Intensity Intervals (30 seconds - 6+ min at 80-95% HR)
Forces maximal cardiac output
Creates the strongest heart wall stress for remodeling
Drives massive stroke volume gains
Threshold (20-40 min total at LT2)
Keeps stroke volume high for longer stretches
Trains resistance to cardiac drift
Holds the heart near its “maximum stretch” point for more time
Zone 2 = The Recovery Vehicle
Easy running (Zone 2) exists to support those harder sessions.
Adds volume at low fatigue cost
Lets you show up to hard days feeling ready
Gives you time on your feet without wearing you out
The point of running slow is simple:
So you can attack the hard sessions fresh. The easy miles are there to protect and enhance the work that actually drives the biggest heart-rate changes.
Sample Weekly Structure
Keep the logic. Adjust volume to your training age and injury history.
Day | Session | Purpose |
Mon | Easy run (45–60 min) | Recovery and volume accumulation |
Tue | 4 × 4 min @ 90–95% HR | Potent stimulus for stroke volume |
Wed | Easy run (45–60 min) | Low-fatigue volume |
Thu | Threshold block (25–40 min) | Hold strong heart effort without your HR drifting up |
Fri | Easy or Off | Nervous system and muscular recovery |
Sat | Long run (90+ min easy) | Time on feet and marathon durability |
Sun | Off or Easy | Full reset before the next intensity block |
You can tweak the specific sessions, but the pattern holds:
A few high-quality hard days, wrapped in a lot of sensible easy running.
📊 What to Expect (Timeline)
Week 1–2:
Resting HR may drop 5 to 8 bpm. That’s mostly from more blood volume, not big fitness gains.
Week 4–8:
Your HR at easy pace may drop 8 to 15 bpm. Stroke volume is improving.
Month 3–6:
Resting HR levels out. Further changes come from slow heart remodeling.
Year 1+:
You build an “athlete’s heart.” Resting HR often lands around 40 to 60 bpm.
One Important Reminder: Day-to-Day Noise
Things that can raise HR 5 to 10 bpm in a single run:
Poor sleep
Dehydration
Heat and humidity
Caffeine and overall stress
This is why you should watch 3-week trends, not single runs.
⚡ Practical Summary
Key takeaways:
High intensity drives the biggest heart adaptations.
Zone 2 lets you recover enough to repeat those hard sessions.
You run slow so you can run hard, not because Zone 2 is magic.
Plasma volume changes in days, stroke volume in weeks, structural changes in months to years.
Training often beats training hard. Consistent days matter more than big sessions.
To lower your heart rate, you need to spike it on purpose, then support that work with smart volume.
💬 One last thing before you go.
After the NFL, I wasn't sure if obsessing over performance science still mattered, until I started helping this community.
Your training goals? They brought that fire back.
So if you're stuck, second-guessing your intensity balance, or just want to share how training's going, hit reply. I read every message and I'm here to help however I can.
— Jonah
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.
Which tissue’s stiffness has the strongest proven link to faster marathon performance and better running economy?
Last Week’s Results: Two Highways, One Faster
Most of you nailed it. Once you try to fuel above ~60 g per hour, the limiter isn’t how much you eat, it’s how much your gut can absorb.
The correct answer?
B. Fructose uses a different intestinal transporter (GLUT5), allowing more total carbs to be absorbed per hour. ✅
Glucose tops out at around 60 g per hour because its transporter saturates. Add fructose and you unlock a second pathway, which lets athletes comfortably reach 90–120 g per hour with far fewer GI issues. More absorption means more usable fuel and fewer late-race slowdowns.
Here’s how the votes shook out:
⬜️ A. Fructose provides a slower, steadier release of energy – 19
🟩 B. Fructose uses a different intestinal transporter (GLUT5) – 172 ✅
⬜️ C. Fructose spares muscle glycogen by forcing fat oxidation – 6
⬜️ D. Fructose boosts insulin response to improve muscle uptake of glucose – 13
Bottom line?
If you want to push higher carb intake in a marathon, you need more than one doorway into the bloodstream. Glucose plus fructose gets you there.
🏃♂️ Stryd Training Tip — Why Your Running Efficiency Changes Hour to Hour
Running efficiency isn’t fixed. It drifts within the same run, and Stryd is one of the few tools that shows the drop before you feel it.
Why Efficiency Falls Mid-Run
1. Muscles fatigue
As your calves and quads tire, you lose tendon stiffness and elastic return. Less “free spring,” more muscular effort. Power rises even if pace doesn’t.
2. Form breaks down
Fatigue subtly alters stride length, posture, and braking forces. Tiny changes, big energy cost.
3. Fuel shifts
Lower glycogen pushes you toward less efficient fuel sources, raising the cost per step.
How Stryd Detects the Drift
Rising power at steady pace: You’re becoming less economical.
Higher form power: Stabilizers are overworking.
Leg Spring Stiffness changes: Tendons losing recoil = higher metabolic cost.
👉 Stryd turns invisible efficiency loss into clear, actionable data.
Don’t forget: You + Science = AWESOMENESS 😎
Yours in science,
Jonah
Please email me directly if you’re interested in references for this week!