The Athlete’s Heart

Let me start with the question that brought me to Guido Claessen when I began digging deeper into my own health earlier this year.

I’m fit. But does that really mean I’m healthy?

Guido Claessen is a cardiologist who works with real athletes, not just data on a screen. He has spent years studying how the heart actually responds to training load, not what we think it does.

Much of that work comes from long-term projects like Pro@Heart and Master@Heart, where athletes are followed over years to understand how training shapes the heart. Not weeks: years.

He operates in both worlds: medicine & performance, which is rare. That’s why when he speaks, it’s not theory. It’s what he has seen, tested and questioned over time with general population, master athletes and elite athletes.

And what became clear my conversation with him is that most of us are operating under a very simple belief:

Train → Get fitter → Be healthier

And for the most part, that belief works… until it doesn’t.

• Gets people moving

• Builds discipline, resilience and sense of control

• Generate results, outputs

After enough years in health, sport, and endurance, if you’re paying attention, you start to notice that not everything fits the simple story.

You meet athletes who are incredibly fit… but not necessarily healthy.
You see coaches prescribing more and more, yet their athletes not progressing.
You hear about arrhythmias, fatigue, strange signals that don’t fit the narrative.

And that’s where this conversation with Guido becomes important.

Because he doesn’t just follow the system, he understands it and refines it. He brings something most athletes avoid: informed perspective.

Let’s get down to it!

One of the ideas that stayed with me the most is simple, but powerful:

The athlete’s heart is not just what we see on a scan. It’s the biological footprint of everything the athlete has done over time.

Your heart is not reacting to your last session, it’s not reacting to your current plan. It is adapting to your lifetime exposure.