Suzi - on a 23% grade once - my HR peaked at 220 measured by my Garmin. I'm 45. A week later, on a long hill, it peaked at 195.

Everyone's assumption is that the monitor was wrong...but in both cases, I felt like I was going to pass out. The real test is to stop and count manually.

But one thing came out when we debated this: You maximum heart rate is whatever it is...and it is THE maximum - unique to you. The formula is really a guideline.