If you’ve listened to Fast Talk for a while, you’ve likely heard us mention “cardiac drift” or “decoupling” in several episodes. It's a favorite topic of Coach Connor's. The terms refer to cardiovascular drift, which is a "drifting" in heart rate and stroke volume over time. On the bike, we measure it by looking at a rise in heart rate relative to power. Many causes have been theorized including dehydration, muscle damage, cutaneous blood flow, and mitochondrial efficiency.  

We’re very excited to have as our featured guest today Dr. Ed Coyle, the University of Texas exercise physiology researcher who published the definitive articles on cardiovascular drift in the 1990s. In that research, Coyle, who is also the director of the Human Performance Laboratory at the university, and his colleagues demonstrated that even when hydration is maintained, CV drift can be experienced. This increase in heart rate reduces the time the heart has to fill with blood, and this is the main reason for a drop in stroke volume, or the amount of blood pushed out by the heart with each beat.   

The drop in stroke volume is a hallmark of CV drift, and historically it has been attributed to blood pooling in the skin and less blood returning to the heart. Coyle and his team found no evidence that blood flow to skin is increased or that venous return is compromised.   

In a practical sense, when a person becomes dehydrated during prolonged exercise, they also get hotter and experience a greater increase in heart rate and a lower cardiac output and circulation of blood: CV drift. The exercise becomes very hard when it should not be hard at all. Competitive cyclists interpret this to mean they are getting a “better workout” because it’s more stressful. It certainly is more stressful, but that type of cardiovascular drift is a negative stress. It does more harm than good.   

We’ll dive into all of this and much more today on Fast Talk, as we hear from Dr. Coyle and a host of other incredible guests who share their thoughts on cardiovascular drift. 

Now, let's make you fast! 

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