Question: I do a lot of long-distance rides, including double centuries and brevets. During the first few hours, I can raise my heart rate to 160 or 170 bpm for several minutes at a time with few ramifications.
However, as the ride goes on, particularly after the first 100 miles, it’s impossible to get my heart above 150 bpm. My legs don’t burn, but I feel like I just can’t get enough air. This seems to happen even if I stay below 160 bpm early in the ride. What’s making my max heart rate decline? — Tom K.
Coach Fred Matheny Replies: Normally, heart rates fall as you get tired. We see this in stage racing where a rider can maintain a pulse of, say, 185 bpm in an early time trial. But a week later, due to accumulated fatigue, he’ll have the same feeling of perceived exertion at only 175 bpm.
In your case, I suspect heart rate decreases because of the accumulated fatigue from the first 100 miles or so.
Scientists aren’t quite sure why heart rate declines in these circumstances, but it’s probably a combination of dehydration and inadequate glycogen replacement. It’s even theorized that the heart slows down to protect itself from what you’re asking it to do.
To limit the decrease, be sure to consume plenty of carbs and fluids. And remember that the decline, while alarming, may not cause a significant drop in your power output.