I've been running since June, and have completed four 5K's, each time getting faster. I train anywhere from 5-18 miles a week, either on the treadmill or on the road. Recently I've noticed that my heart rate will be above 140 at the start of my run, then after a few minutes may drop to 120 or so...then slowly back up over the course of the miles. Anyone else ever have this happen? Basically the heart rate is higher than you would think it would be at the start, then drops way down. I don't feel any change in my breathing or chest...and would never have known this if not for my connected watch.
I've got a Garmin 230 that measures my pace, HR, etc.