I had a knee injury due to two falls on 2016 Eroica California. Damn toe clips! I was on and off the bike for a few weeks with swollen knee and previously undiagnosed arthritis. I developed a high heart rate (for me) of 155 to 165, sometimes on flat roads at slight effort. It also would not come back down when I stopped. After several diagnostics, I found I have a mild aortic stenosis (wonky heart valve). I had a couple of mild dizzy spells, but no chest pain. It took a couple of months of tests to nail down. I just turned 68, and ride almost 5K miles a year. I now have a new beta blocker (Metropolol), and a small dose aspirin a day. I'm back doing 100 miles a week, 40-45 resting heart rate, usually 120-140 on the bike. I've lost 20 lbs and feel great. Long story short, see a cardioligist, and get checked out. I feel great now, and Dr. says keep doing what I'm doing, and re-evaluate valve problem in a year. So far so good. Im convinced that part of my hr problem was due to laying off due to the knee. Good luck with yours.