One Size Does Not Fit All: A Meta-Analysis of 115 Trials Comparing High-Intensity Interval and Moderate-to-Vigorous-Intensity Continuous Training Across Diverse Participants, Protocols, and Outcomes.

HIIT demonstrated significantly superior improvements in relative maximal oxygen uptake compared to MVICT.

• The standardized mean difference was g = 0.39 (95% CI [0.27, 0.51]), representing an SD of 1.30 mL·kg-1·min-1.
• 115 studies involving 3196 participants were included, with mean age range from 8 to 68 years.
• Populations spanned from untrained sedentary individuals to elite/world-class athletes.
• A three-level meta-analysis was employed to calculate standardized mean differences (SMD, Hedge's g).

HIIT demonstrated significantly superior improvements in absolute maximal oxygen uptake compared to MVICT.

• The standardized mean difference was g = 0.29 (95% CI [0.15, 0.43]), representing an SD of 0.09 L·min-1.
• This was a statistically significant finding based on confidence intervals not crossing zero.
• The meta-analysis followed PRISMA guidelines with a systematic search in PubMed, Web of Science, and CNKI databases completed on September 21, 2024.

HIIT demonstrated significantly superior improvements in maximal aerobic power/speed and mean anaerobic power compared to MVICT.

• Maximal aerobic power/speed: g = 0.31 (95% CI [0.17, 0.47]).
• Mean anaerobic power: g = 0.47 (95% CI [0.08, 0.86]).
• Both outcomes showed statistically significant differences based on confidence intervals not crossing zero.

No significant differences were observed between HIIT and MVICT for peak anaerobic power, intensity thresholds, exercise economy, or physical performance indices.

• Peak anaerobic power: g = 0.31 (95% CI [-0.06, 0.68]).
• First intensity threshold: g = 0.43 (95% CI [-0.38, 1.25]); second intensity threshold: g = 0.06 (95% CI [-0.25, 0.36]).
• Exercise economy: g = 0.26 (95% CI [-0.03, 0.54]).
• Physical performance indices: g = 0.04 (95% CI [-0.46, 0.54]).
• All confidence intervals for these outcomes crossed zero, indicating non-significant differences.

Training status significantly moderated the superiority of HIIT over MVICT for maximal oxygen uptake improvements.

• A 6-tiered participant classification framework was used to categorize training status.
• HIIT demonstrated greater improvements in VO2max among Tier 0 (inactive; g = 0.34), Tier 1 (recreationally active; g = 0.57), and Tier 3 (elite/national; g = 0.83) compared to MVICT.
• The effect size at Tier 3 (elite/national athletes; g = 0.83) was notably larger than at Tier 0 (inactive; g = 0.34).
• Subgroup analyses were used to examine training status as a potential moderator.

Sex significantly moderated the superiority of HIIT over MVICT for maximal oxygen uptake, with benefits observed in males and mixed-sex populations but not specifically reported for females alone.

• Males showed greater HIIT-vs-MVICT improvements in VO2max: g = 0.43.
• Mixed-sex populations also showed greater HIIT improvements: g = 0.42.
• Sex was identified as a significant moderator in subgroup analyses.
• Comparable effect sizes were found between males (g = 0.43) and mixed-sex groups (g = 0.42).

Interval type significantly moderated the superiority of HIIT over MVICT for maximal oxygen uptake, with both short-interval and long-interval HIIT showing advantages.

• Short-interval HIIT showed a larger effect compared to MVICT: g = 0.55.
• Long-interval HIIT also showed a larger effect compared to MVICT: g = 0.57.
• Interval type was identified as a significant moderator in subgroup analyses.
• Both short- and long-interval types showed similar magnitude of advantage over MVICT.

Exercise mode significantly moderated the superiority of HIIT over MVICT for maximal oxygen uptake, with rowing showing the largest effect.

• Rowing as exercise mode: g = 0.71.
• Running as exercise mode: g = 0.53.
• Cycling as exercise mode: g = 0.29.
• Exercise mode was identified as a significant moderator in subgroup analyses.
• The effect size for rowing (g = 0.71) was approximately 2.4 times larger than for cycling (g = 0.29).

The meta-analysis included 115 randomized controlled trials with 3196 participants spanning a wide age and fitness range.

• Studies were identified through systematic search in PubMed, Web of Science, and CNKI databases, completed on September 21, 2024.
• Eligible studies were randomized controlled trials comparing chronic effects of HIIT and MVICT.
• Mean age range across studies was from 8 to 68 years.
• Populations ranged from untrained sedentary individuals to elite/world-class athletes.
• PRISMA guidelines were followed for study selection.

Age was identified as a significant moderator of the effect of HIIT compared to MVICT on maximal oxygen uptake.

• Age was listed alongside training status, sex, interval type, and exercise mode as significant moderators in subgroup analyses.
• The paper included participants with mean ages ranging from 8 to 68 years across studies.
• Subgroup analyses and meta-regression were used to examine age as a potential moderator.