← Related articles

HIIT vs. Zone 2 Training: What Recent VO2max Research Actually Shows

For years, endurance coaches and exercise physiologists have debated whether short, hard efforts (high-intensity interval training, or HIIT) or long, easy sessions in "Zone 2" — roughly 60-70% of maximum heart rate, below the lactate threshold — do more to raise VO2max, the standard marker of cardiorespiratory fitness. Two recent, independent reviews add fresh data to that debate.

What a 2024 Umbrella Review Found

Published in the Scandinavian Journal of Medicine & Science in Sports in May 2024, an umbrella review by Poon and colleagues pooled data from 24 systematic reviews covering 429 primary studies and nearly 13,000 participants. The review compared HIIT against both sedentary controls and moderate-intensity continuous training (MICT), the closest laboratory equivalent to Zone 2 work. Key results:

• Compared with non-exercise control, HIIT increased cardiorespiratory fitness with a weighted mean difference of 3.25 to 5.5 mL/kg/min in VO2max (standardized mean difference range: 0.28 to 4.31).
• Compared with MICT specifically, HIIT still came out ahead, with a weighted mean difference of 0.52 to 3.76 mL/kg/min (standardized mean difference range: 0.18 to 0.99).
• The advantage held across healthy adults, people with overweight or obesity, older adults, and athletes, and across HIIT formats including low-volume, whole-body, home-based, aquatic, and sprint-interval protocols.
• The authors flagged an important caveat: most of the 24 underlying reviews scored moderate to critically low on the AMSTAR-2 quality-assessment tool, meaning the strength of the evidence base is uneven.

Why Zone 2 Still Has a Place

A separate narrative review published in Sports Medicine in July 2025 by Storoschuk, Moran-MacDonald, Gibala, and Gurd examined the popular claim that Zone 2 training is the optimal intensity for building mitochondrial capacity and fat-oxidation ability. The authors concluded that current evidence does not support Zone 2 as uniquely optimal for these adaptations in the general population — higher-intensity work appears to deliver comparable or greater mitochondrial and cardiometabolic benefits, especially when total training time is limited. However, the same authors note that elite endurance athletes, who train at very high weekly volumes, still rely heavily on large amounts of low-intensity work to manage fatigue and sustain the volume needed for long-term adaptation — a context very different from a typical recreational trainee's schedule.

Practical Takeaways

• For people with limited training time, HIIT sessions appear to raise VO2max at least as much as, and often more than, longer steady-state sessions of similar or greater total duration.
• Zone 2 / MICT training still builds a valuable aerobic base, supports recovery between harder sessions, and remains lower-risk for injury and overtraining in many populations.
• Both approaches produced meaningful VO2max gains across the reviewed literature; the "best" choice depends on time available, training history, and individual tolerance for higher-intensity effort.
• Because underlying study quality varies (per the AMSTAR-2 ratings noted above), these findings should be read as a strong signal rather than a definitive verdict, and combining both intensities is a reasonable, well-supported strategy for most people.

As always, anyone with underlying cardiovascular, metabolic, or orthopedic conditions should discuss any new interval-training program with a physician or qualified exercise professional before starting.

Source: Poon et al., Scandinavian Journal of Medicine & Science in Sports (2024)