Training Science Podcast Does Zone 1 Build a Stronger Heart Than HIIT? New MRI Data with Dr Guido Claessen & Prof Paul Laursen
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Feb 20, 2026 Dr Guido Claessen, a sports cardiologist who leads long-term MRI studies on endurance athletes. He discusses how low-intensity Zone 1–2 volume, not HIIT, most strongly links to larger cardiac size. They cover MRI methods, mechanisms from chronic wall stress, polarized training balance, recovery variability, and long-term risks like atrial fibrillation and coronary plaque.
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Wearables Made Longitudinal Cardiac Studies Possible
- Wearables enabled objective capture of thousands of sessions to link training load and MRI heart measures.
- A semi-automatic pipeline made large-scale, intensity-zone analysis feasible for longitudinal studies.
Low-Intensity Volume Builds The Athletic Heart
- Long low-intensity duration (Zone 1–2) drives cardiac enlargement more than short HIIT bouts.
- MRI-linked wearable data showed time-in-zone, not peak intensity, best predicted athlete heart size.
Moderate Intensity Drives Maximal Preload
- Cardiac chamber volumes increase most from rest to mild–moderate exercise and then plateau at high intensities.
- At very high heart rates filling time limits volume, so wall stress isn’t higher than during longer low-intensity work.