Chad’s spreadsheet is based partly on Fast Talk episode 54, which featured Seiler discussing how to set zones.
My notes of the podcast:
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Re: HR zones… Seiler didn’t want to give out blank numbers for HR, because individual variations are so high. He doesn’t like giving a generic answer. For example lets look at 60 min power test, puts people at maybe an average of 88%. But drift of HR is tricky on 60 min power test. Hard to decide where in 60 min effort where to find MLSS. On indoor trainer, a low-intensity session should have flat heart rate. Coach said he ballparks LT1 at 83-85% of LT2. Seiler doesn’t like percentages of percentages, so he would go with HRmax and refer percentages to that. So then, based on that, LT1 is roughly 70% of HRmax and you shouldn’t go above 75%. And Seiler would put LT2 around 85-87% HRmax.
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So back then LT1 was ~70-75% HRmax. Now it’s ~73-79%.