I've noticed the same recently. I've gotten a gym membership ($10/mo, can't say no to that!!) and have been going about 5days/wk for the last 2 weeks, at least 2 days/wk are cardio and even after 45min on an elliptical machine, my heart rate rarely gets over 135bpm but on Monday, I pushed really, really hard for 20 min and I was hitting 160bpm (nearly 54 yrs old, 100% = 172bpm) for much of it, not breathing too hard (elevated yes, but not huffing and puffing) but on the bike, it doesn't take me long before my heart rate is high (I don't have an HRM so I'm not sure but it feels like it is 150+) and I'm huffing and puffing (for a while at least). I'll have to look into it.