HR lower than normal

[Monitor M]

Last week I rode and I could not get my HR up to "normal" levels. I had trained for many days in a row and understand that the HR reading meant that I was just tired and needed to take a day or so off - as I did and next time out the HR was back to normal levels.
What I don't understand is why does the HR stay low when you are tired, I mean what is the physiological basis for that observation?

 

[nightfend]

It means your heart and diaphragm muscles are weakened from overuse, so they can't push as hard. It's no different than doing a heavy session of weight lifting and having your arms feel like noodles the next day. That's why recovery days are so important.