In recent years durability has become the most talked about concept I’ve seen in endurance sports science - and for good reason. As a coach, athlete and someone who's spent countless hours geeking out over training theory, I’ve found that understanding (and training for) durability might be one of the biggest unlocks for long course triathletes. And yet, it’s something many athletes aren’t paying nearly enough attention to.

What Is Durability?

Durability isn’t about how fast you are when you’re fresh, it’s about how little you slow down over time.

In more technical terms durability refers to your body’s ability to resist physiological and biomechanical decline during prolonged exercise. In other words, how well do you hold things together in the second half of an IRONMAN ride or over the final 10K of a marathon?

We all know that feeling - things start well and then at some point deep into the race heart rate rises, pace slips, form breaks down and perceived effort skyrockets. That’s not just fatigue, that’s poor durability.

Why Durability Matters More Than You Think

Endurance athletes often focus on numbers including

VO₂ Max (your aerobic ceiling)
Functional Threshold Power (FTP)
Lactate Threshold or Threshold Pace

These are important but none of them tell you how long you can actually sustain those numbers in the real world.

In a race like an IRONMAN or a marathon, what separates the winners from the chasers isn’t peak power, it’s the ability to hold strong when others start falling apart. Research is backing this up too:

A 2022 study found that durability predicted marathon performance more accurately than VO₂ Max.
Sports scientists like Stephen Seiler argue that elite endurance athletes are often made, not just by their lab numbers, but by how little they degrade over time.

A Bit of Science (but not too much)

Here’s what durability looks like in the body:

Cardiovascular drift: Heart rate rises even though power or pace stays the same.
Efficiency loss: You start burning more oxygen to do the same amount of work.
Muscle fatigue: Cadence and coordination deteriorate. Your stride shortens. Form breaks down.
RPE climbs: Everything just feels harder - even if the numbers don’t change.

All of this can happen even when you’re pacing correctly. It’s not poor planning, it’s poor durability.

How You Can Test Your Durability

You don’t need a lab to measure it. Here are two simple field-based tests any triathlete can do:

1. Heart Rate Drift Test

· Go for a steady Zone 2 ride or run - about 60–90 minutes.

· Look at your pace:HR or power:HR ratio in the first half vs. the second.

· If your heart rate climbs significantly (>5–7%) despite holding steady pace or power, that’s a sign of declining efficiency.

2. Fast-Finish Long Session

· Go long and steady, then try to hold race pace for 20–30 minutes at the end.

· If race pace suddenly feels 20 bpm harder or your form collapses, that’s durability (or lack of it) in action.

How to Train Durability

The good news? You can train it. You just have to do it on purpose. Here are some ways I build durability into my training and the athletes I coach:

· Long, Easy Endurance Work - The classic Zone 2 ride or run isn’t sexy but it builds the foundation for metabolic efficiency. Going long trains your body to burn fat, spare glycogen and resist drift.

· Back-to-Back Sessions - Especially for IRONMAN athletes: A long ride Saturday followed by a long run Sunday. Teaches your body to work under accumulated fatigue.

· Fast-Finish Long Rides/Runs - Steady for 2–3 hours then dial up to tempo or race pace for the final 20–30 minutes. Forces your body to work hard while tired, just like in a race.

· Train Low (sometimes) - Occasionally starting a session with lower glycogen availability can build fuel flexibility. Think fasted rides or evening sessions after a low-carb day. Not for everyone, but useful when managed properly.

What the Norwegians Are Doing Right

If you’ve followed athletes like Kristian Blummenfelt or Gustav Iden, you’ve seen durability in action. Their performances late in races, when others fall apart, are often unmatched.

Behind the scenes their training places a huge emphasis on building repeatable efforts under fatigue. This includes:

Double threshold days (two high-quality sessions per day)
Extremely high training volumes at low intensity
Precision tracking of efficiency loss over long sessions

The Norwegians understand that being fast isn’t enough - you need to be able to stay efficient for hours on end. That’s durability at the highest level.

Final Thoughts

Durability isn’t about being faster. It’s about being unbreakable when it counts. You don’t win or finish strong by being the best in the first hour. You win by being just as good in the last.

So, ask yourself… am I training to be fast, or am I training to stay fast?

If you’ve never considered durability before, now’s the time. Your next big race might depend on it.

Bevan McKinnon / August 2025

Durability: The Most Important Performance Factor You’re Probably Not Training