I've been using a HRM since I got serious about this from the first of the year. This thread got me curious so I went back and reviewed the data.

My average max heart rate monthly average (162) closely agreed with the predicted maximum from the newer formula mentioned above until a few months ago when I started getting religious about getting my cadence up and most of the anomalies could be attributed to equipment issues.

Now into a couple of months of concentrating on spinning and really big hills my average HR is up only slightly but I routinely get transient spikes near 200 and they are almost always at transitions between descending and climbing.

Almost like my body is anticipating the increased work load.

This doesn't surprise me at all. I've been an avid rock climber since my 20's and you get the same effect when your brain tells you body to get ready to bust a big move.