Good question: Why doesn't zone 3 lead to sustainable adaptations? So, what happens, physiologically, in zone 3? 1/ Heart rate increases/filling time decreases. Many athletes will have lower EDV (end diastolic volume) when doing z3 vs z1 work - less of that all-important "stretch" stimulus on the heart that leads to long-term cardiac remodeling. 2/ Athletes burn a lot less fat and a lot more glycogen. Because of this, training tolerance in this zone is significantly reduced. And, more importantly, the impetus placed upon the body to improve fat oxidation is not present. That is, athletes who are already "carb burners" just become better "carb burners" 3/ Zone 3 is stressful, leading to activation of/strengthening of the "Fight or flight" sympathetic nervous system. With too much z3, HRV ⬇️& the strength of the athlete's recovery system goes with it. Over time, the athlete becomes very sympathetic dominant & gets locked into "stress mode" - if this goes on too long, it leads to failing adaptation and overtraining. ###### This is why, for sustainable improvement, zone 1 should be the focus.