Had a terrible TH run yesterday and re-read a blog post after the run describing parasympathetic over training. I have never experienced an elevated heart rate that most see as a sign of over training or sickness. My heart rate drops 5-10 beats and perceived effort increases at all paces.
Blog Post:
https://troyshellhamer.blogspot.com/2012/03/parasympathetic-overtraining-and.html
Yesterday drove 40 minutes on the motorcycle to a flat trail around a lake and was going to have a the perfect TH run. About 5k into it at 6:00/mi I threw in the towel. I felt like I was running 5:30/mi. Drove home in the dark on a rural road with too much traffic being Friday night and people were out. Not a fun ride for the night vision disadvantaged!
Threw up last night at 1 am so maybe it isn't over training but still interesting how most runners experience elevated HRs and some lower HRs when physically overstressed.