The idea that roughly 80% of training should be lower to moderate intensity and ~20% higher intensity is often referred to as the “80/20 rule.”

It’s not a magic formula.
And it’s not new.

It emerged from decades of observation in endurance athletes — particularly when researchers started looking at what successful athletes were actually doing, not what they said they were doing.

Across sports, levels, and distances, a consistent pattern showed up:
Most high performers spent the majority of their time at relatively easy intensities… and a smaller amount of time at very high intensity.

Why this distribution makes sense physiologically

Low-to-moderate intensity work:

  • Builds aerobic capacity

  • Improves mitochondrial density

  • Enhances fat oxidation

  • Strengthens the cardiovascular system

  • Supports nervous system stability

  • Is recoverable enough to repeat frequently

High-intensity work:

  • Improves VO₂ max

  • Raises lactate tolerance

  • Increases neuromuscular power

  • Creates a strong adaptation signal

The key difference?
Cost.

High-intensity training delivers a big stimulus — but at a high physiological price:

  • Greater autonomic stress

  • Longer recovery time

  • Higher injury risk if overused

The 80/20 approach balances stimulus vs. recoverability.


Why people get it wrong

Many people think they’re training 80/20.

In reality, they’re often doing something closer to:

  • 40% easy

  • 60% moderate-to-hard

That middle zone — “comfortably hard” — feels productive but tends to:

  • Accumulate fatigue

  • Suppress HRV over time

  • Push recovery costs higher without adding much upside

The result is stagnation, not progress.

What “80% easy” actually means

Low-to-moderate intensity should feel:

  • Sustainable

  • Conversational

  • Controlled

  • Stable from a heart rate standpoint

This is where knowing your workload for Zone 2 matters.

If heart rate drifts significantly at a given pace or power, the workload is too high for extended aerobic work — even if it feels easy at first.

What the 20% is for

The higher-intensity 20% should be:

  • Purposeful

  • Structured

  • Separated from easy days

  • Fully supported by recovery

This includes:

  • Intervals

  • Threshold work

  • VO₂ max sessions

  • Short, intense efforts

These sessions are not meant to be frequent.
They’re meant to be effective.

How to apply 80/20 in real life

Think in weekly totals, not individual sessions.

Example:

  • 5 training days

  • 4 days primarily low/moderate intensity

  • 1 day higher intensity

Or:

  • 6–8 total hours of training

  • ~5–6 hours easy/moderate

  • ~1–1.5 hours hard

The exact split doesn’t need to be perfect.
The trend is what matters.

Where HRV and RHR fit in

When the balance is right, you’ll often see:

  • Stable or improving HRV trends

  • Controlled resting heart rate

  • Recovery scores that rebound reliably

When the balance is noticeably off:

  • HRV trends downward

  • RHR creeps up

  • “Moderate” days start feeling harder

The body is very honest about this — if you’re paying attention.

The takeaway

The 80/20 approach isn’t about training less hard.
It’s about training hard when it matters and easy when it doesn’t.

Consistency comes from recoverability.
Progress comes from the right stress.
And the combination of the two is what actually moves fitness forward.