Could be Zen but it's not a phrase that I'm very familiar with and there are a lot of these floating around... aerobic threshold, anaerobic threshold, lactate threshold, lactate balance point. All of these are meant to help measure changes in metabolism from aerobic to anaerobic but I know for sure that the first 3 are slightly different (in some people the difference in BPM from one to another is 2, in others, it's much larger). I'm not sure about LBP.