We Googled how to pass the Bruce Protocol stress test. If you are younger than 73 your max heart rate will be higher. 220 minus your age. The 85% is mentioned in a few places we have Googled.Can I ask where you’ve heard the maximum heart rate requirement? I can manage the 3 stages, but certainly not at under 125 bpm.
My understanding is that (as Phill D says in post #20) it’s essentially a test to check how your heart responds when placed under greater stress, to see if you’re likely to black out, and not a fitness test as such. Would it not then be more logical to set a minimum heart rate to ensure your heart is under sufficient stress? Looking back at the post #1, surely it is the second of the options you pose in the middle paragraph?
Obviously it may relate to individual health conditions, although Appendix C in the guidance , linked in post #14, doesn’t seem to refer to it (purely a lay reading of it!)