I’m 5’6. 54kg. Do HIIT 4x week & spin/combat 2x week. My Max HR (220-45=175bpm) but I NEVER reach it. Ever! My RHR is 44-45bpm. VO2max is 48.9. I push myself to absolute max but I just don’t get into my MHR zone. What am I doing wrong?
The maximum heart rate formula is just a predictive model. It aims to give an approximation of the average maximum heart rate for people by age, and it certainly isn't guaranteed to be accurate for individuals.
Here is what actual individual max heart rate measurements look like. The dots and circle are individuals, and the lines are the predictive models. You can see that in this population, the max heart rates for people in their 40s vary from around 160 to 200.
Source for image: Tanaka, H., Monahan, K. D., & Seals, D. R. (2001). Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology, 37(1), 153–156. https://doi.org/10.1016/s0735-1097(00)01054-8
If you really want to know what your max heart rate is, just use the maximum value you've ever measured when pushing yourself to your absolute max.