"But if my heart rate is down, why can't I speed back up?"
Updated: Oct 16, 2019
With 6 miles to go on the run at Ironman Maine 70.3 in 2017, I realized that my goal of going under 5 hours wasn't going to happen unless I sped up to 7:30/mile pace. I was already working pretty hard, but the course was flat and the conditions were good, so I decided to go for it. Unfortunately, after one 7:37 mile, my heart rate was unsustainably high and my pace started to slow considerably. My last 3 miles were around 9 minutes each, and I felt awful. After finishing, I was pale, nauseous, and shivering. Clearly, my body wasn’t ready to crank out those 7:30 miles. My goal of going sub-5 would have to wait.
When I went into TrainingPeaks to examine my data, something caught my eye. In those late miles on the run when my pace had slowed considerably, I noticed that my heart rate had dropped all the way down to the high 150's, which is a very sustainable range for me. But in that case, shouldn't I have been able to start running faster? In theory, yes, but it certainly didn't feel that way during the race. I spent the next year or so being confounded by this physiological paradox.
Among the many things I learned from listening to master coach Jesse Kropelnicki at the USAT coaching certification clinic in November 2018 was that there is actually a term for the very phenomenon I experienced in Maine: "aerobic decoupling." The graphs of my pace per mile (increasing as I took more time per mile) and heart rate (decreasing) diverged, or decoupled, late in the race. The real question was, why did it happen, and what could I do to prevent it from happening again?
Two years later, I was back at Ironman Maine 70.3 armed with the information that I had learned from the USAT clinic. The race would be the true test of whether I had addressed the decoupling issue. Despite trying conditions on the swim and bike, my run split was almost 5 minutes faster than 2 years prior. My last two miles were 7:20 and 6:36. The metric which measures decoupling, denoted Pa:HR in TrainingPeaks, was down from 9.22% all the way to 1.04%. I implemented what I learned from Coach Jesse, and the results were undeniable.
So what's the magic fix? Now I know, and the athletes that I coach will see it applied in their training and in the race strategy I provide them. Without letting the secret out of the bag, I can tell you that if you want to feel strong for your entire 2020 racing season, you need to start the appropriate type of training very soon. Let's get started, shall we?