I had a strange experience while doing a 200 mile ride a few weeks ago that I am hoping someone can help explain.

In the early part of the ride I was keeping a nice normal HR (for me at least) of around 130-155 depending on where I was in the paceline.

At around mile 150-175 I was not doing very well. Struggling to keep the legs turning. After that I was able to settle into a nice pace where changing my speed either slower or faster hurt.

At this point my HR was acting odd. By the way I felt it sould have been 160-175. However it was closer to 110-120. By this point I was going almost the same speed as earlier in the day. However my companions were dead and I was pulling 90% of the time.

Why late in the day was I working harder, but my HR dropped? Even if I pushed hard on a hill it was significantly lower than it should have been.

It's a well-documented phenomenon called cardiac drift. I'm sure if you Google it you'll get a lot of hits. While HR numbers will vary from person to person, a drop in HR during a long exercise session is well known. This is one of the reasons why coaches are pushing training with power meters instead of/in addition to HRMs. The HR numbers are not constant from day to day or through a long workout, so structuring your training around specific HRs is less productive than working with power information.

Regarding cardiac drift, pretty much everything I've read indicates a gradual rise in HR over long efforts due to various physiological factors requiring the heart to pump faster to sustain the same effort. mgolebiewski's post indicated a fairly significant drop in HR. So, my question is, is this the result of a very extended effort, or is something else going on? Any info is appreciated.

Typical experience for long efforts is that the HR drops throughout the ride for the same effort. We're talking long efforts (200 miles noted by the OP).

Kerry, thanks for the response. I did the google search, but had no luck uncovering an explanation for downward drift due to very extended effort. Most everything I could find dealt with triathlons or ultramarathons and indicated increased HR, which makes sense with the physiological changes. I am very interested in learning about what occurs in the body over super long efforts to cause a downward drift, and is this a potentially dangerous thing, or at the least a contraindication to continuing. If you have any info or links I'd appreciate it - thanks!

I dug back into my references, and found that indeed cardiac drift means increasing HR during an event. What I was remembering was a drop in HR with fatigue, perhaps over several days as with a stage race or long tour.

Here's a quote from coach Fred Matheny: "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."

At any rate, it is nothing to worry about, and not a signal that you need to stop, but perhaps to eat more and drink more.

In general during exercise, heart rate is a reflection of the body's metabolic demand for oxygen. In the case of ultraendurance cycling, a reduced heart rate is most likely related to the legs' inability to maintain a high power output. While your perception of exertion may remain the same or even higher than at the start of the event, I'd bet that an objective measure of work rate such as a power meter would reveal a significantly reduced power output. The reduced HR is secondary to the decreased metabolic utilization of oxygen (VO2) at the lower power output-- your heart only beats as fast as necessary in order to deliver the needed oxygen.

Upward cardiac drift is related to intravascular volume depletion (severe dehydration in layman's terms). Decreased return of venous blood to the heart results in decreased stroke volume (volume of blood pumped per heart beat)-- thus, in order to maintain the same cardiac output (in terms of volume of blood per time), the heart rate must increase in order to compensate. Not a good thing.

Kerry and supercrank - thanks for the responses. This makes a lot of sense and I think is valuable info for anyone undertaking ultraendurance efforts.