Not me. My heart is like an auto engine, except the car is different every day. Sometimes I'm driving a high-revving, overhead cam, turbocharged 4 cylinder, other days it's like I'm driving an 8 cylinder pushrod motor with lots of low end torque. I do record heart rate, and I do stare at the graphs for hours after every ride, but I find it difficult to crack the code. Perceived effort works better for me.