Zone 2 training

Zone 2 training is easy aerobic work with an upper limit at the first ventilatory threshold (VT1), also called the aerobic threshold, in Viada’s zone system.^{1Viada, The Ultimate Hybrid Athlete, p. 137.} Viada describes it as the first intensity at which cardiac preload, the blood filling the heart, rises notably, with the heart stretching and strengthening over years of training.^{2Viada, The Ultimate Hybrid Athlete, p. 136.} Its low recovery cost allows large amounts of training.^{2Viada, The Ultimate Hybrid Athlete, p. 136.}

Viada says it’s unsettled whether zone 2 produces adaptations unique to that intensity or simply allows enough volume to produce them.^{3Viada, The Ultimate Hybrid Athlete, p. 138.} Those adaptations also occur at other intensities; zone 2 is where they’re the main focus and recovery costs are predictable.^{1Viada, The Ultimate Hybrid Athlete, p. 137.}

In practice

• Use the talk test to find the upper limit: Viada considers it an accurate proxy for VT1 that can replace a more complicated clinical test.^{4Viada, The Ultimate Hybrid Athlete, p. 139.}
• Favor longer sessions, since some signaling behind new mitochondria builds with duration as much as intensity.^{3Viada, The Ultimate Hybrid Athlete, p. 138.}
• Choose an activity the client can perform skillfully and without pain; disproportionate discomfort makes for poor zone 2 sessions.^{4Viada, The Ultimate Hybrid Athlete, p. 139.}

Where sources differ

ISSA’s five-zone system defines zone 2 as 61% to 70% of maximum heart rate and describes it as light activity such as slow jogging.^{5International Sports Sciences Association, Exercise Selection, Technique, and Training Instruction: Fast Track Study Guide, p. 17.} Viada uses VT1 instead, stressing that zone labels are often arbitrary and their boundaries aren’t universally agreed.^{6Viada, The Ultimate Hybrid Athlete, p. 132.},^{2Viada, The Ultimate Hybrid Athlete, p. 136.}