The article puts the target at 60%–70% of your maximum heart rate, with an effort that still lets you talk comfortably. Mito Health’s guide explains Zone 2 Cardio using heart rate and how the effort feels. It is a guide, not a new workout study. It draws on research in healthy young adults and elite endurance athletes. In its five-zone model, the second zone is light effort. The next zone runs from 70%–80% of maximum heart rate. The guide also describes the easier effort as steady movement you can sustain. Your breathing is harder than at rest, but conversation remains comfortable. US activity guidelines use a similar cue for moderate activity: you can talk, but not sing. The young-adult study helps explain why that cue matters. The hardest effort that still allowed comfortable speech matched a measured breathing turning point. That is the point where breathing starts getting harder. Both shifted together when training raised that point or blood donation lowered it. The excerpt does not give the study’s size or exact changes. For BioSocial members, the useful takeaway is to pair heart rate with how talking feels. A workout log could include heart rate, pace, and whether full sentences felt easy. Those notes offer more context than a zone label alone. Over time, they could help you compare your own sessions and share clearer notes with the community. This source explains workout effort; it does not show changes in members’ bloodwork. What heart rate do you usually see when you can still speak comfortably, and how has that changed over time?