Deliberately running slow (zone 2, conversational effort) builds long-term aerobic base — but framing it as THE method that "grows your heart" and beats every other approach goes further than the evidence supports.

noli's read

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.

the sources

1. 1Rosenblat MA, Perrotta AS, Vicenzino B 2019
2. 2Silva Oliveira P, Boppre G, Fonseca H 2024
3. 3Menshikova EV, Ritov VB, Ferrell RE, et al. 2007