Why Your $500 Smartwatch Keeps Lying During Intervals—and the Fix
Estimated read time: 4.2 minutes (about as long as it takes your wrist monitor to finally catch up to your actual heart rate during intervals). 😉
Hey Performance Nerds! Jonah here. 🤓
Is your $500 smartwatch sabotaging your training?
Your $500 watch can be 10x less accurate than a $50 chest strap. Back in the NFL, I tested tech for accuracy, and learned fast: measuring ≠ accurate.
Here’s what you’ll get today:
📉 Why wrist monitors lie when workouts matter most
❤️ The physics behind chest strap accuracy (99% vs 73%)
⌚ Which devices actually work for Zone 2, threshold, and racing
🚫 When convenience kills your adaptations
(Augie thinks all this tech talk is silly—his heart rate monitor is just putting his paw on his chest and counting. Surprisingly accurate method, though his form analysis needs work.)
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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: How Heart Rate Monitors Actually Work
Chest straps = electrocardiography (ECG): They detect the actual electrical signals your heart produces with every beat. Think plugging directly into your heart's electrical system.
Wrist monitors = photoplethysmography (PPG): They shine LED lights through your skin and measure how the light bounces back as blood pulses through your vessels.
Armbands = PPG with an upgrade. Better blood flow and less motion than your wrist.
The core problem?
Chest straps measure the signal. Wrist monitors chase the aftereffect, distorted by blood flow, temperature, and wrist bounce.
It’s like gossip: chest straps hear the story straight from the source. Wrist monitors? Third-hand version that always gets details wrong.
🎯 What Actually Works: Your Heart Rate Hierarchy
Why wrist monitors fail: Motion from your bouncing wrist overwhelms the optical sensor, so it often tracks your arm swing rhythm instead of your heartbeat.
When they're worst: High-intensity intervals, cold weather (reduced blood flow to your wrist), and that 170-180 bpm range where cadence and heart rate collide, and the watch mistakes steps for beats.
The solution: If you care about accurate heart rate data, use a chest strap or armband.
Truth: Most runners would be better off learning to run by feel and using heart rate data for post-workout analysis. Obsessing mid-run usually hurts more than it helps.
🏃♂️ How I Actually Use Heart Rate Monitoring
I don't obsess over heart rate during runs. I prefer going by feel and effort, heart rate fluctuates too much day-to-day based on sleep, stress, caffeine, and weather.
When I do use heart rate data:
Easy runs: If you struggle to keep easy days truly easy, a chest strap can help ensure your runs stay actually easy. But remember, heart rate is just one data point, not gospel.
Post-run trend analysis: I'll review average heart rate after workouts to spot patterns over weeks and months. Is the same pace getting easier? Are you seeing elevated heart rates at usual paces?
Race day insurance: Heart rate works as a late warning system to avoid blowing up in hot or hilly races. But I still pace by effort and splits first.
What I DON'T use it for: Real-time pacing during intervals. Heart rate lags behind effort, and the last thing you need during a VO2 max session is staring at your wrist instead of focusing on the work.
🔧 My Gear Recommendations
Chest Straps (99%+ accuracy):
Polar H10 – Medical-grade gold standard.
Garmin HRM-Pro – Solid alternative with extra running dynamics.
Armbands (95%+ accuracy):
Polar Verity Sense – Best-in-class optical accuracy.
COROS HR Monitor – Competitive with multi-device support.
Skip wrist monitors for workouts: Your $500 smartwatch is great for notifications, GPS, and daily activity tracking—just don't trust the heart rate during workouts.
🎯 Quick Reference Cheat Sheet
Run Type | Best Tool | Why |
Intervals | Effort (RPE) + Pace | Fast response, no lag |
Easy runs | Chest strap / armband | Keeps easy days easy |
Race day | Effort + HR backup | Insurance against blow-ups |
Post-run trends | Chest strap / armband | Spot long-term patterns |
⚡ Key Takeaways
Wrist monitors fail when accuracy matters most.
Chest straps (ECG) and armbands (upper-arm PPG) hit 95–99% accuracy.
HR is best for easy days and trends—not real-time pacing.
A $50 chest strap beats a $500 watch every time.
💬 One last thing before you go.
Back in the NFL, I learned that most tech looks slick but falls apart when you test it against reality. Smartwatches are no different, accuracy matters more than features when it comes to training.
Your goals are why I dig into the details and call out when the data doesn’t match the hype.
So if you’re second-guessing your watch’s readings, or just want to share your own “my data went wild” story, hit reply. I read every message and I’m here to help however I can.
Train smart,
Jonah
🚨 2 Science-Backed Ways I’m Reducing Injury Risk while Running 80-90 Miles a Week:
Did you miss my post about the 2 Science-Backed Ways I’m Reducing Injury Risk while Running 80-90 Miles a Week? You can find it below!
I won’t lie. These posts take me a while to make. If you find it helpful, share it on your story or with a friend. It helps me a ton!
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.
What’s the #1 biggest adaptation that raises VO₂ max in trained runners?
Last Week’s Results: Beet Power 🌱🥤
Most of you knew it: the humble beet has some serious performance science behind it. Nitrates convert to nitric oxide, which opens up blood vessels and helps muscles get more oxygen per step.
The correct answer?
C. Beetroot juice / dietary nitrates 🥤🌱 ✅
This boost isn’t about hype, it’s a measurable edge, especially for recreational and sub-elite runners. Better blood flow means lower oxygen cost at a given pace. It also makes sustaining harder paces feel easier.
At the elite level, the effect is smaller since their systems are already highly adapted, but for most marathoners it’s one of the few supplements with real, repeatable benefits.
Here’s how the votes shook out:
A. Creatine monohydrate 🏋️♂️ – 27
B. Caffeine ☕⚡ – 22
C. Beetroot juice / dietary nitrates 🥤🌱 – 186 ✅
D. Collagen peptides 💅 – 5
Bottom line?
Beets won’t turn Kipchoge into Kipchoge, but for the rest of us they’re one of the few legal performance aids that can actually move the needle. 🧪🏃♂️
🏔️ Stryd Training Tip — Master Hills with Power, Not Pace
Hills feel brutal because gravity takes an extra tax. On a climb, part of your effort goes into lifting your body upward. That vertical work costs more oxygen and energy, so even “easy” flat paces can leave you gasping when the road tilts up.
Why power wins on hills:
Pace deceives: Speed slows down on climbs even as effort skyrockets.
Gravity tax: Each meter up costs more energy than a meter forward.
Better pacing: Power shows the true effort against gravity, so you can hold steady watts instead of chasing an impossible pace.
💡 Bottom line: On hills, let power be your guide. Pace lies, gravity doesn’t. Stay steady in watts with Stryd, and your climbs stop stealing matches.
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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