Zone 2.
Z2 is the aerobic training intensity at which the body burns predominantly fat for fuel and produces minimal lactate. Heart rate sits at roughly 60 to 70 percent of max. Conversation is possible but breathing is noticeable.
Why it matters
Z2 is the single most trainable input for mitochondrial density and metabolic flexibility. It is the longest-leverage, most under-trained zone in modern populations. Drives the cardiovascular floor that every other intensity builds on.
Programming
Most members run four 45-minute Z2 sessions weekly. Higher volumes are common for endurance-focused protocols. Heart rate monitoring is non-negotiable; perceived effort alone is unreliable.
Diagnostics
Lactate threshold testing. VO2 max. HRV trends. The Z2 pace at a fixed heart rate is itself a useful longitudinal marker.