What Is “Zone 2 Training”?: Experts’ Viewpoint on Definition, Training Methods, and Expected Adaptations

Endurance athletes and coaches have long debated the most effective training intensities for maximizing performance. Recently, Zone 2 training has gained attention as a foundational component of endurance programs, yet definitions and implementation strategies vary. While many propose that Zone 2 training should comprise a significant portion of an athlete’s total training volume, there is no universal agreement on its precise intensity, how it should be structured, or whether it produces unique physiological adaptations. This expert commentary brings together perspectives from leading sports scientists and professional coaches to clarify the definition, preferred training methods, and expected adaptations of Zone 2 training. The following key points summarize their consensus and highlight critical insights for endurance training optimization.

Defining Zone 2 Training:

• Experts agreed that Zone 2 training occurs just below the first lactate (LT1) or ventilatory threshold (VT1).
• This intensity is characterized by stable heart rate (70-80% of max HR), low blood lactate (~1-2 mmol/L), and moderate perceived exertion (~10 on the Borg scale).

Training Methods for Zone 2:

• Continuous Training: Long sessions (>2 hours) at Zone 2 intensity, adjusting for fatigue and cardiac drift.
• Variable Continuous Training: Mixing Zone 2 with short recovery periods in Zone 1 to reduce monotony.
• Interval-Based Training: Using Zone 2 intensity as an active recovery period between high-intensity intervals.

Expected Adaptations from Zone 2 Training:

• Increased muscle capillarization and mitochondrial enzyme activity in Type I fibers, improving endurance.
• Improved metabolic efficiency, allowing athletes to sustain higher power outputs with lower physiological stress.
• Compression of LT1/VT1 toward LT2/VT2, indicating improved endurance capacity.
• Potential cognitive resilience benefits due to the mental endurance required for long Zone 2 sessions.

Challenges and Research Gaps:

• Lack of universal agreement on intensity zone models (some use 3, 5, or even 7 zones).
• Adaptations to Zone 2 training may not be unique—similar benefits might occur at slightly higher or lower intensities.
• More research is needed on long-term physiological responses, particularly in elite endurance athletes.

Practical Applications:

• Experts suggest tracking HR and RPE alongside external load (power output or pace) to optimize Zone 2 training.
• Long continuous sessions (~2+ hours) were the most favored method for endurance adaptation.

Using lactate measurements and durability tests can help assess training effectiveness.