The most bizarre thing I've experienced with HR was after my crash. For quite awhile (weeks) my HR would just race, out of all proportion to the effort. Way beyond what a reduced FTP would indicate. It was some systemic 'thing' that very suddenly stopped, like an electrical connection was snapped back together. HR dropped on a ride when I was finally able to push it a bit, and after that was essentially normal. It was truly weird, like I had to break through some block to get back to 'normal'. The other strange "electrical connection" crash effect was numbness in my chest from trauma to my ribs in back - interruption of the nerve pathways. That is finally almost entirely gone. I can now once again tell if my nipple is hard, without touching it.