When I started TR, Garmin had estimated my threshold heart rate to be 162 bpm. I thought this was pretty accurate because when I reached that level, I had nothing left. After a year of training, I can “easily” sustain 166 bpm and don’t feel like I am destroying myself until 175 bpm. Did I truly increase my Threshold Heart rate by 13 bpm through training, increased my ability to suffer more and endure higher heart rates, or a blend of the 2?
No you didn’t. In my experience Garmins estimates are way off. Ones heart rate zones don’t really change, that is physiologically you, the power you can put out at a certain range can increase and does increase as you get fit. In your case I would say the estimate was way off.