I'm a 39 year old male and have been riding my mountain bike on the road and hike and bike trails consistantly for about 9 months. I got an HRM a few months ago. My resting heart rate gets down to about 50 bpm. I ride for about 45 min 2 to 3 times a week in a target zone of about 130-150bpm. I've noticed though that on a given day I can ride the exact same ride at about the same pace and one day I'll average around 140bpm and another day I'll average about 120bpm. My perceived effort feels around the same and if anything on the days that I average 120bpm I may feel a little more winded than on the other days.(Though not really by much) Is this variation normal, or can there be some explanation for it? Thanks.