Article

While doing a web search, I found an old article from The New York Times, 2008, on heart rate:
                The Flutter Over Heart Rate
where they had the interesting note:

My interest

Why this was interesting to me concerned my "sort of" noted difficulty in getting my heart rate to higher levels comparable to what I had seen in the past.

 Yes, a year or so has passed, but not that much (I hope).  I seemed to have suddenly dropped about 5 bpm while averaging a slightly higher average speed!  It wasn't enough to be really clear on, it just "sort of" seemed that way.

Major Change: Aerobars!

One change I had been experimenting with was using aerobars. I had put them on more for a "change of position" than anything else.  It was sometimes nice to "lie down" and pedal.

As I tuned the position of the aerobars more and more, I found them to be the more comfortable and, curiously, the more powerful way to ride the bike.  Yes, the wind/air resistance was lessened a bit, but unless the relative air speed was over 15-20 mph (head wind speed + bike speed), I didn't feel much additional effort.  (Of course, it was a very different story when the trade winds at Hawi were gusting at 25-30 mph and I was thinking of pedaling towards them: I've sometimes turned around 6 miles from Hawi where the winds pick up).

Uphill but not Down

Even if I wasn't going very fast up a hill, I often felt that being on the aerobars made it easier to pedal more strongly.  Air resistance isn't much if a person is doing 5-8mph but the feeling was there.

I note that I seldom use aerobars coming down a hill at 30-34 mph!
My experience is that it takes far too long to get to the brakes from the aerobar position, so I use the drops with my one finger on each break, like I do on my mountain bike.

Prone versus Upright

The NYT article brought this all into focus: if a heart doesn't have to drive blood up an additional 2-3 feet, it doesn't have to beat as fast!   The aerobar position moved the head down a lot allowing the heart to not work as hard and allowing the legs to have more flow.

Interesting!
My heart rate, likely, really was slightly lower and my legs really were slightly better nourished.