I’m a 47yo male who has been stair climbing for 3 years. (Fairly sedentary life prior) My cardio sessions go for about 45 minutes ,20 minutes of which are around the 165-175 bpm mark which is about as hard as I can go. I do these 3 times a week I recover from 170b to 120 in 2 mins but from 140-100 in 2 mins also Morning hr is 55 I don’t seem to make any gains if my HR is lower than this, in fact even if I take it easy I still achieve 150-160bpm. I’m disappointed with my fitness, there are older people than me who kick my butt up the same course. I’m torn between training at a lower heart rate where I can talk (140bpm) which frankly would see me take all day to finish the course, or stick with what I’m doing and push harder. I guess my max heart rate might be 180 my chest hurts (tightness)when I get up past 170 but I think this is just breathing. If I breath aggressively at home I get the same pain so I doubt it heart related. I want to push harder but I want to make gains and I wonder if I’ve reached the limit. If so how can I be sure?
Switch activity...you didn't reach a limit but your body stopped to adapt and started to accommodate to stress. This is called the biological law of accommodation. The only way to reverse it is to switch activity for a while like going from stair climbing to swimming or biking.
This applies to all types of fitness, unless you are a complete beginner you will stagnate when you keep doing the same thing over and over again.
After 3 years you are no more a beginner so obviously you won't see any progress anymore if you keep doing the same things, you have to switch things up to overcome it and progress.