If you’ve ever looked at your watch mid-workout and thought,
“Should I trust this number?” — you’re not alone.

We track miles, splits, heart rate, cadence, elevation, sleep, recovery scores — sometimes all before breakfast. With so much information available, it’s easy to believe that more data automatically means better training.

Heart rate. Pace. Power. Rate of Pecieved Exertion.

Each metric tells a story. But none of them tells the whole story on its own.  Each of these should be used as a tool in your training and racing toolbox.

But more data isn’t better.

Better interpretation is.

Heart rate, pace, power, and rate of perceived exertion (RPE) are the four primary metrics that matter most in endurance training. Each measures something different. Each tells part of the story.

The athletes who improve consistently aren’t the ones obsessing over one number — they’re the ones who understand how the numbers work together.

Let’s break them down.  What does each one measure?

• Heart Rate = Internal Load
  How hard your cardiovascular system is working.

• Pace = Speed Over Ground
  How fast you are covering distance.

• Power = Mechanical Output
  The actual work you are producing.

• RPE (Rate of Perceived Exertion) = Subjective Effort
  How hard the effort feels to you.

Three are objective. One is subjective.

All four matter.

Heart Rate

First the most accurate way to measure this is not through the optical sensor on your device, you should use a chest strap.  Heart Rate (HR)measures how hard your cardiovascular system is working.

It reflects internal stress — how your body is responding to effort.

Why It’s Valuable

• Helps define aerobic training zones

• Keeps easy days truly easy

• Tracks aerobic development over time

• Reveals fatigue or dehydration (elevated HR at normal pace)

For endurance athletes, most training should occur in lower heart rate zones (Zone 2). This builds mitochondrial density, capillary networks, and fat oxidation — the foundation of endurance performance.

Limitations

Heart rate:

• Lags behind effort (especially during intervals)

• Is influenced by heat, caffeine, stress, sleep, hydration

• Can drift upward during long sessions (cardiac drift)

Example:
On a hot day, your heart rate may climb even if pace stays steady. That doesn’t mean you’re getting fitter — it means your body is working harder to cool itself.

Pace: Measuring Output Against Time

What it is:
Pace measures how fast you’re covering distance (minutes per mile/km).

It reflects external output in running.

Why It’s Valuable

• Simple and intuitive

• Essential for race pacing

• Useful for tempo and threshold workouts

• Helps gauge performance improvements

If you ran 8:30 pace at Zone 2 heart rate last month and now you run 8:05 at the same heart rate — that’s measurable aerobic improvement.

Limitations

Pace is influenced by:

• Hills

• Wind

• Terrain

• Heat

• Fatigue

Running 7:30 pace uphill is not the same effort as 7:30 pace downhill — but pace alone won’t tell you that.

Power: Measuring Real-Time Output

What it is:
Power measures the actual work you’re producing (watts).

It reflects true external load and responds instantly to effort changes.

Cyclists have used power meters for decades. Running power is newer but increasingly popular.

Why It’s Valuable

• Immediate feedback (no lag like heart rate)

• Accounts for hills and terrain

• Ideal for intervals

• Highly precise for race pacing

Example:
On a climb, your pace slows — but power shows whether you’re maintaining the correct effort. On a downhill, power ensures you don’t overcook your legs chasing pace.

For triathletes and cyclists, power is often the gold standard for race execution.

Limitations

• Equipment cost

• Requires testing to define zones

Data can overwhelm newer athletes

Rate of Perceived Exertion (RPE): Measuring Feel

What it is:
RPE measures how hard the effort feels on a scale (usually 1–10).

It reflects your brain’s interpretation of stress.

Why It’s Valuable

• Requires no device

• Works in any condition

• Integrates fatigue, stress, sleep, nutrition

• Builds body awareness

An RPE 3–4 effort should feel conversational.
An RPE 7–8 feels comfortably hard.
An RPE 9–10 is maximal.

Elite athletes are highly tuned into RPE — even when training with data.

Limitations

• Subjective

• Influenced by mood

• Can be misjudged by newer athletes

However, over time, athletes become remarkably accurate.

The ability to measure efforts in different ways are tools that you can use in racing and training.  The goal isn’t to chase numbers.
It’s to apply the right stimulus to become stronger, more resilient, and more efficient.  Some days the pace may feel too hard, as an athlete recognizing that and adjusting during a race or training is an important skill set.   LIkewise, if the technology fails during a race, you want to have an understanding of what the effort should feel like.   Having workouts that are written in different ways can allow you to develop these skills.