Polarized training is an intensity distribution where ~80% of volume is done in zones 1-2 (easy) and ~20% in zones 4-5 (intense), avoiding zone 3 'gray zone' as much as possible. This approach, studied by Stephen Seiler in elite endurance athletes, maximizes aerobic adaptations while allowing adequate recovery. It contrasts with the 'threshold' approach that accumulates time in zone 3.
In a 10-hour polarized training week: 8h in fundamental endurance (zone 2), 2h of intense intervals (zones 4-5), and almost nothing in zone 3.