Hi,
I had some traces like this which alarmed me so I looked online. Once I had waded past all the people telling me I was about to suffer some major cardiac event, I found some post suggesting that the HRMs aren't the most reliable at the start of a run and that they can be affected by what you wear.
So, I experimented with a few things and found the following:
- when my HRM said my HR was 160+ (from RHR of 50 after 200m of gentle jogging! but sometimes it used to do it gradually over a few minutes) my HR when taken manually was about 110
- if I was really really diligent in moistening the contact pads on the HRM it rarely seemed to happen (guessing that once you start sweating the contact is much better)
- if I wore a cotton t-shirt it never happened
- if I wore a "technical" top it happened quite frequently
So, my conclusion was that it was something to do with the strength of the contact between the pads and my chest and also the nature of the top I was wearing, presumably something to do with static and the like,
Interestingly, I bought new everything (Garmin and HRM) to see if it was better, but it was worse. Other people have confirmed this. The newer HRMs seem to be even more sensitive to this static effect than the old ones.
Not everyone suffers from this though. I guess that an individuals body composition and their trainers (how much of an earthing effect they have) and all sorts of things will make a difference.
Just my own experiences....