The Running Week

Stripped of its motivational framing ("ego," "in Kenya we call it"), the claim is a real, widely-taught training principle: doing most of your volume at an easy, conversational effort (zone 2, below the first ventilatory threshold) to build endurance, saving hard efforts for a small slice of training — the "polarized" model. An expert-panel review of 14 sport scientists (Sitko et al., 2025) confirms this consensus on zone 2's definition and endurance value. But a more critical narrative review (Storoschuk et al., 2025, "Much Ado About Zone 2") pushes back hard on the idea that zone 2 is specifically superior for building mitochondrial density or cardiac adaptation: at matched volume, higher-intensity training often produces equal or greater mitochondrial adaptations and VO2max gains. So "stay humble, stay slow" as an endurance-building principle broadly holds up, but framing zone 2 as THE one path that "grows your heart forever" over any other approach overstates a literature that's actually mixed on this zone's specific superiority.