The 3 zone system is just based on the 2 lactate thresholds, LT1 and LT2. below LT1 is typically Z1/Z2 above LT2 is Z4. the “grey area” many speak off would be somewhere between the two. A scientist will go into much finer details but there are basically 2 points on the graph where the line jumps up, they are around (not at) 2mmol/l and 4mmol/l not the best image but to give some idea
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