I've seen Seiler's article before. The biggest problem I have with this approach is "post hoc ergo propter hoc" logic. Pro athletes train 20 hours/week. They do so because they are pro athletes and they have nothing better to do. I am willing to accept that an athlete who trains 20 hours per week achieves optimal results when he spends 75% of the time in Z1. After all, there's likely diminishing returns to time you spend in higher zones. Question is (the talk alludes to this, but I don't see any evidence in its favor): is it still optimal to spend 75% of time in Z1 if you only train 8 hours a week, or is it optimal to hold Z2/Z3 time constant?