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…