In this episode, the Athletes Compass team sits down with legendary exercise physiologist Dr. Stephen Seiler, who’s best known for popularizing polarized training. Dr. Seiler shares his latest insights into using breathing frequency as a real-time indicator of training stress—a potentially more accurate and actionable metric than heart rate or RPE. He explains the conceptual shift from traditional load-focused models to a "load–stress–strain" framework and discusses how wearables and new tech are unlocking practical ways for everyday and elite athletes to train smarter. Drawing from personal experience and collaborations with developers, he emphasizes how individual physiology, psychological state, and even life stressors all impact how we respond to training. It's an enlightening conversation for athletes and coaches alike.

Key Takeaways

• Breathing frequency may be a more sensitive, real-time indicator of training stress than heart rate or RPE.
• Seiler introduces a Load → Stress → Strain framework to better define how training affects the body.
• Heart rate often fails to capture accumulating fatigue or psychological stress, while ventilation might pick it up.
• RPE (Rate of Perceived Exertion) is often unreliable due to athlete bias and difficulty measuring it in the moment.
• New tech (e.g., Timeware and Garmin devices) is enabling field measurement of ventilation, both frequency and depth.
• Breathing frequency zones may be individualized just like heart rate zones.
• Norwegian athletes used a now-unveiled strategy of over-breathing at the start of hills to reduce oxygen deficit.
• Breathing costs can rise up to 20% of VO2max, potentially limiting leg muscle performance.

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