I just realized that when I can't see my HR, I often push harder and up into zone 5 more than when I can see it. Like when I'm not 'exercising' but am doing something strenuous or demanding, I will often get well into zone 5, but when I'm doing cardio, only zone 5 for brief periods.
I put on 20lbs of muscle this last year and I think I'm getting close to what I can add naturally. So my goal of 'add muscle to burn calories' has basically been met at this point. So now I'm pivoting from hypertrophy, to strength building. And I am trying to figure out how to train my muscles to have more stamina. Like I understand now how strength works, it's a mix of making bigger muscles, but also your nervous system ability to activate and recruit those muscles. But I'm still hazy on what makes you able to do certain movements repeatedly without the muscle running out of energy and stopping responding, and how to train for that.
Like when I do bench press, my failure rep, the muscle still works normally, it just lacks strength to finish the rep. But when I do something like wrist roller, or grip strength, it's more like the muscle builds up lactic acid and stops responding at some point. IT's not like it's weak it's more like it stops responding. I don't know why some muscles/movements you fail from lactic acid build up, and others it's from lack of strength to continue.