Tonight I ran in zone 2 with zone 2 for me being between 138 and 146bpm. My average HR for the run being 139 and I covered just on 7.5km in 50 min. The run was relaxed with me feeling comfortable for the whole run.
However at about 4.8km my HR spiked to 173bpm from about 130 for no apparent reason, I wasn't then running up hill, I did not increase my pace and there was no wind. At first I thought it was my watch so I slowed to a walk and sure enough my HR fell as it normally would and once I started running again my HR returned to 130-133bpm at the same pace before the spike.
Yesterday I ran 18km on a hilly course in high zone 3-low zone 4 with on issues.
I'd be interested if anyone has any thoughts about this.
Thank you in advance.
Sometimes heart rate monitor straps are affected by power lines and even static electricity from clothing. During wintertime, I get heart rate spikes at the beginning of my run, but after a minute or two of running, the readings settle. I've found that wearing polyester tech shirts during dry winter conditions cause the spikes. Sometimes too if the chest strap isn't tight enough, the movement of the strap while running will also cause temporary spikes.
Oh, then it's probably the watch strap itself moving around a bit, like Rich said.
strap moved i reckon!