Part 1: When Metrics Lie: Why Your Watch Says “Easy” but Your Legs Say “Stop”
For most of your athletic life, you were taught to trust the numbers.
Heart rate. Zones. HRV. Breathing rate. Power. Lactate.
The story was simple:
If the metrics say “easy,” the workout was easy.
If HRV is green, you’re ready.
If lactate is low, you’re recovered.
But if you’ve trained long enough—and seriously enough—you already know this isn’t always true.
You can roll home from a “Zone 2” ride with legs that feel hollowed out.
You can wake up with a great HRV score and dead quads.
You can see elevated lactate on a rest day and wonder what you did wrong.
Nothing is broken.
You’re just seeing the limits of measurement.
“The better trained you become, the more misleading single metrics can be.”
The Cardiovascular System Is Not Your Muscles
Heart rate is seductive because it’s clean. It’s continuous. It feels objective.
But heart rate measures cardiovascular strain, not muscular cost.
Stephen Seiler, one of the leading researchers on endurance intensity distribution, has repeatedly pointed out that heart rate reflects oxygen delivery—not mechanical load or tissue damage (Seiler, 2010; Seiler & Tønnessen, 2009).
Highly trained athletes develop:
Large stroke volume
Dense mitochondrial networks
Efficient oxygen extraction
Economical movement patterns
Which means the heart can stay calm while the legs quietly do expensive work.
You pedal harder.
You recruit more muscle fibers.
You generate more torque.
But your heart barely flinches.
“Your heart adapts faster than your connective tissue, your mitochondria, and your contractile proteins.”
This is why efficient athletes are uniquely vulnerable to “stealth fatigue.”
The cardiovascular system gives permission.
The musculoskeletal system pays the bill later.
Breathing Rate Isn’t Telling the Whole Story Either
Respiratory rate trackers—like Tymewear and similar tools—promise a deeper look into internal load.
And yes, breathing correlates nicely with metabolic demand.
But again: metabolism is not structure.
You can keep ventilation low while:
Depleting local glycogen
Accumulating calcium handling fatigue
Damaging Z-lines
Reducing force production capacity
Breathing reflects gas exchange.
It does not reflect tissue trauma.
Iñigo San Millán has described this disconnect clearly in his work on metabolic flexibility: muscles can be under severe energetic stress long before the cardiorespiratory system signals danger (San Millán & Brooks, 2018).
“You can be metabolically calm and mechanically cooked.”
HRV: A Nervous System Score, Not a Muscle Biopsy
HRV is powerful. It tells you about autonomic balance, stress load, and recovery of the nervous system.
Marco Altini—one of the most respected voices in applied HRV—has been explicit about this:
HRV reflects systemic stress, not localized tissue recovery (Altini, 2019).
You can normalize parasympathetic tone while:
Sarcomeres are still disrupted
Inflammation is ongoing
Glycogen is low
Tendons are irritated
The nervous system stabilizes quickly.
Muscle remodeling does not.
“HRV can say you’re calm while your legs are still rebuilding.”
This mismatch is one of the most common traps for disciplined athletes: green lights stacked on damaged legs.
Lactate: Useful, But Context-Dependent
Lactate is not the villain it was once made out to be. It is a fuel. A shuttle. A signal molecule.
But it is also stress-sensitive.
San Millán and Brooks have shown that lactate production is influenced by:
Catecholamines
Sleep deprivation
Psychological stress
Glycogen availability
Mitochondrial efficiency
Inflammation
(San Millán & Brooks, 2018)
Which means:
A higher lactate reading does not automatically mean “hard session.”
A lower reading does not guarantee “fully recovered.”
“Lactate reflects your metabolic environment, not just your training intensity.”
If your life stress is high, your resting lactate can rise.
If your fueling is off, lactate kinetics change.
If your sympathetic tone is elevated, glycolysis accelerates.
The number is real. The interpretation is fragile.
The Layered Fatigue Model Most Athletes Never Learn
Fatigue is not one thing.
Tim Noakes’ central governor model and modern integrative physiology both describe fatigue as layered (Noakes, 2012):
Central (nervous system)
Peripheral (muscle & connective tissue)
Metabolic (substrate availability, mitochondria)
Psychological (motivation, stress, perception)
Your devices mostly measure the first and third.
They barely see the second.
“Most overtraining happens in tissues your watch can’t see.”
Why Experienced Athletes Are at Higher Risk
Beginners are limited by their heart and lungs.
Experienced athletes are limited by their tissues.
Your engine improves faster than your chassis.
Power meters made this clearer. Heart rate made it subtle. Wearables made it louder—but not wiser.
Alan Couzens and Andy Coggan have both written about “durability” as the real performance limiter in trained athletes: the ability to repeatedly produce force without progressive degradation (Coggan, 2019; Couzens, 2020).
Durability does not show up in one workout.
It shows up in the fourth hard day.
The third week.
The second month.
A Coach’s Rule of Thumb
Metrics are constraints, not commandments.
They are guardrails. Not steering wheels.
Here’s what works in the real world:
1. Trust trends over snapshots
One HRV score means little. Seven days means something.
2. Separate “easy for the heart” from “easy for the legs”
They are not the same thing.
3. Track muscular signals deliberately
Heaviness. Loss of snap. Degraded coordination. Unusual soreness patterns.
4. Treat unexplained lactate as a stress signal, not a training failure
Look at sleep. Food. Life load.
5. Protect boring recovery
If training is exciting, recovery should be dull.
“Your best training tool is not your watch. It’s your ability to notice patterns.”
The Quiet Truth
The athletes who thrive are not the ones who optimize each day.
They are the ones who build by the month.
Metrics don’t make endurance sport better.
Wise interpretation does.
And that still requires something no device can measure: judgment and listening to your body.
Happy training,
Joe
Sources & Further Reading
Seiler, S. (2010). What is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes? International Journal of Sports Physiology and Performance.
Seiler, S., & Tønnessen, E. (2009). Intervals, thresholds, and long slow distance. Sportscience.
San Millán, I., & Brooks, G. A. (2018). Reexamining cancer metabolism: lactate production for carcinogenesis could be the purpose and explanation of the Warburg Effect. Carcinogenesis.
Altini, M. (2019). HRV and Training: Why HRV Is Not a Measure of Muscle Recovery. marcoalitini.com
Noakes, T. (2012). Fatigue is a Brain-Derived Emotion. Frontiers in Physiology.
Coggan, A. (2019). Durability and Performance Modeling in Endurance Sport. TrainingPeaks Blog.
Couzens, A. (2020). Why Durability Is the Missing Metric in Endurance Training. EnduranceCorner.