I m 65 years old was a runner for 30 years, now I walk more than I run I averages about 4+ miles on a treadmill 3 x per week I walk/run on the treadmill at about a speed between 4 and 6. I also walk out side on the days away from the gym, I workout with weights as well for about 25 years. I m just curious I was a marathon/ultra marathon in my 30s 40s and 50s, but my heart beats per minute never change from around 70 to 75, no matter what I do. Is that normal? it s still at that rate now. Louse