There is an interesting approach coming from the muscle oxygen sensors with four training zones
https://support.humon.io/hc/en-us/articles/115002750733-What-do-the-different-colored-zones-mean-
They state that training only makes sense at the edge of these zones. Otherwise they consider it wasted energy (No man’s land)
If you are just interested in training zones then a Humon Hex might be an interesting approach for $299.-/€299.- instead of performance test.