Polarized training: 80% easy, 20% hard, avoid the middle
LoadTraining approach concentrating volume in Z1-Z2 and intensities in Z4-Z5, deliberately avoiding the moderate zone (Z3, 'no man's land').
Polarized training, formalized by Stephen Seiler from analysis of elite endurance athletes' training logs, observes that the world's best athletes (rowers, cross-country skiers, triathletes) distribute their volume approximately 80% at low intensity (Z1-Z2, below first threshold) and 20% at high intensity (Z4-Z5, above second threshold), with very little time in the moderate zone (Z3). The physiological hypothesis: the moderate zone (roughly at lactate threshold LT2) accumulates too much metabolic fatigue relative to adaptation gain, whereas very slow sessions develop mitochondrial density without stress, and very hard sessions stimulate VO2max and lactate tolerance. In practice for an amateur triathlete: this means making long sessions truly slow (HR <70% HRmax), reserving 20% high intensity for quality sessions done fully recovered, and resisting the urge to 'semi-push' constantly.
In TriPaced:
TriPaced plans implicitly incorporate polarization: base weeks concentrate long outings at low HR and reserve quality intervals to 1-2 sessions per week per discipline. If most of your Strava sessions cluster in Z3, this typically signals pushing 'easy' sessions too hard and 'hard' sessions not hard enough. TriPaced detects this via HR zone distribution from Strava imports.
Source Seiler KS & Kjerland GO, 2006: Quantifying training intensity distribution in elite endurance athletes, Scandinavian Journal of Medicine & Science in Sports