Zone 2 vs intervals

Practical note

Graded moderate: a meta-analysis of 28 controlled trials (723 adults) found consistent gains with both approaches; downgraded from high because not all trials were randomised and the analysis used magnitude-based inference. The extra gain after interval training was small (about 1.2 mL/kg/min) and its confidence limits were wide.

Sources

• Supports“Endurance training and HIT both elicit large improvements in the VO2max of healthy, young to middle-aged adults, with the gains in VO2max being greater following HIT when compared with endurance training.” (Abstract, Conclusion) Milanović Z, Sporiš G, Weston M. 2015. Sports Medicine. Effectiveness of High-Intensity Interval Training (HIT) and Continuous Endurance Training for VO2max Improvements: A Systematic Review and Meta-Analysis of Controlled Trials. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine

Practical note

Graded low: one meta-analysis of 10 small studies (273 patients) with mixed conditions; downgraded for imprecision and risk of bias in small trials. The pooled difference was about 3 mL/kg/min in favour of interval training.

Sources

• Supports“Participants had coronary artery disease, heart failure, hypertension, metabolic syndrome and obesity. There was a significantly higher increase in the VO2peak after HIIT compared to MICT (MD 3.03 mL/kg/min, 95% CI 2.00 to 4.07), equivalent to 9.1%.” (Abstract, Results) Weston KS, Wisløff U, Coombes JS. 2014. British Journal of Sports Medicine. High-intensity interval training in patients with lifestyle-induced cardiometabolic disease: a systematic review and meta-analysis. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine

Practical note

Graded low: one meta-analysis of 13 short trials (averaging 10 weeks of 3 sessions a week); downgraded for imprecision in small trials and for risk of bias. Running-based programmes showed larger fat-mass effects than cycling-based ones.

Sources

• Supports“There were no significant differences between HIIT and MICT for any body composition measure, but HIIT required ~40% less training time commitment.” (Abstract, Results) Wewege M, van den Berg R, Ward RE, Keech A. 2017. Obesity Reviews. The effects of high-intensity interval training vs. moderate-intensity continuous training on body composition in overweight and obese adults: a systematic review and meta-analysis. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine

Practical note

Graded low: this is a narrative rather than systematic review, so study selection was not pre-specified, and one author declared an interest in an exercise-related company. It challenges a popular-media claim rather than establishing that Zone 2 training lacks benefit.

Sources

• Supports“We conclude that current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity.” (Abstract) Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ. 2025. Sports Medicine. Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine
• Context“evidence suggests prioritizing higher exercise intensities (> Zone 2) is critical to maximize cardiometabolic health benefits, particularly in the context of lower training volumes.” (Abstract) Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ. 2025. Sports Medicine. Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine

Practical note

Graded low: observational data from 3 Norwegian centres with only 3 events in 175 820 exercise hours (1 per 129 456 hours moderate-intensity, 1 per 23 182 hours high-intensity), so the comparison between intensities is very imprecise.

Sources

• Supports“The results of the current study indicate that the risk of a cardiovascular event is low after both high-intensity exercise and moderate-intensity exercise in a cardiovascular rehabilitation setting.” (Abstract, Conclusions) Rognmo Ø, Moholdt T, Bakken H, Hole T, Mølstad P, Myhr NE, Grimsmo J, Wisløff U. 2012. Circulation. Cardiovascular risk of high- versus moderate-intensity aerobic exercise in coronary heart disease patients. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine
• Context“it is also advocated that vigorous exercise could acutely and transiently increase the risk of sudden cardiac death and myocardial infarction in susceptible persons.” (Abstract, Background) Rognmo Ø, Moholdt T, Bakken H, Hole T, Mølstad P, Myhr NE, Grimsmo J, Wisløff U. 2012. Circulation. Cardiovascular risk of high- versus moderate-intensity aerobic exercise in coronary heart disease patients. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine