VO2max trainability and high intensity interval training in humans: a meta-analysis

Abstract

Endurance exercise training studies frequently show modest changes in VO2max with training and very limited responses in some subjects. By contrast, studies using interval training (IT) or combined IT and continuous training (CT) have reported mean increases in VO2max of up to ~1.0 L · min(-1). This raises questions about the role of exercise intensity and the trainability of VO2max. To address this topic we analyzed IT and IT/CT studies published in English from 1965-2012. Inclusion criteria were: 1)≥ 3 healthy sedentary/recreationally active humans <45 yrs old, 2) training duration 6-13 weeks, 3) ≥ 3 days/week, 4) ≥ 10 minutes of high intensity work, 5) ≥ 1:1 work/rest ratio, and 6) results reported as mean ± SD or SE, ranges of change, or individual data. Due to heterogeneity (I(2) value of 70), statistical synthesis of the data used a random effects model. The summary statistic of interest was the change in VO2max. A total of 334 subjects (120 women) from 37 studies were identified. Participants were grouped into 40 distinct training groups, so the unit of analysis was 40 rather than 37. An increase in VO2max of 0.51 L · min(-1) (95% CI: 0.43 to 0.60 L · min(-1)) was observed. A subset of 9 studies, with 72 subjects, that featured longer intervals showed even larger (~0.8-0.9 L · min(-1)) changes in VO2max with evidence of a marked response in all subjects. These results suggest that ideas about trainability and VO2max should be further evaluated with standardized IT or IT/CT training programs.

Figure 1. Schematic of our study identification and screening approach.

Figure 2. Funnel plot of standard errors observed for the reported articles by the reported difference in means.

The open circles are observed studies whereas the solid circles are imputed studies to assess the role of publication on the estimated effect. The two estimates, denoted by the open and filled diamonds for the observed and imputed studies, respectively, agree qualitatively with one another suggesting that the findings may be robust in the presence of publication bias.

Figure 3. Forest plot for the synthesis of 37 articles (40 trained cohorts) identified in the systematic review.

The estimates reported for each study are the means and 95% confidence intervals for the change in VO₂max in L · min⁻¹. The summary diamonds at the bottom of the plot represent the summarized effects using fixed and random effects models, where the random effects estimates are considered the primary findings for this study due to heterogeneity (I² = 70). The estimated increase in VO₂max as a result of interval training was 0.51 L · min⁻¹ (95% CI: 0.43 to 0.60 L · min⁻¹; p<0.001).

Figure 4. Weighted histogram and density plot of the observed effects of interval training on VO₂max.

Each of the 40 observations included in the analysis was weighted by the sample size to produce the distribution with the estimate of the percent of subjects with a given effect size noted on the Y axis.