Polarized 80/20 : the winning distribution
Load80% of time in Z1-Z2 + ~20% in Z4-Z5, almost nothing in middle Z3.
Validated by Stephen Seiler (elite endurance research), polarized training avoids the "grey zone" Z3 (moderately hard) which costs recovery without delivering threshold or VO2max stimulus. The ratio is measured on weekly total time, not per session.
In TriPaced:
TriPaced targets a polarized 80/20 weekly ratio. The app raises a "polarized warn LIT<60%" warning if you stack too much Z3 without enough easy Z1-Z2 around it. Documented physiology safeguard.
Source Seiler & Kjerland, 2006 : Quantifying training intensity distribution in elite endurance athletes