@hmbrglr A -4.0 T-score is serious but it does not automatically mean you cannot exercise, though certainly you have to be careful. Using myself as an example. I was quite active doing bodyweight exercises, walking, playing pickleball and intermittently lifting weights. As my T-scores got worse and worse I made changes as I got up to a worst point of -4.1. For instance I stopped pickleball because I liked playing intensely and I did too many sudden and extreme movements. I did not stop lifting weights. I figured that I knew I had been able to say do farmer's carries with 50lbs in each hand without hurting myself. So as my scores worsened I lowered the weight somewhat. I would have liked to have increased the weights instead but as I my bones were getting less dense rapidly I figured I had to reduce. Same with squats . I was squatting with up to 50lbs (goblet squat) in the recent past so I figured I'll reduce it and do more reps.
It was not the direction I wanted to go in as I wanted to be doing around 85% of my one rep max.
After a year of Evenity and huge gains in bone density I have been steadily increasing the weights in the these exercises.
There does not seem to be any way to know how much exercise any individual can handle. How much resistance can one use. Research shows that in general more is better, if you don't hurt yourself. In my case I was able to use how much exercise I was doing before I found out I had osteoporosis as a guide. And then how much weight I was using after I knew I had it but hadn't adjusted in my mind to the reality of the situation. Then as I got worse and I woke up to my situation and I used my past exercise history as a guideline to what I could safely do. Not a perfect method but it gave me a way to go forward with exercise.
I have not fractured so far, knock on wood. Hope this helps