*Review* **HRV-Based Training for Improving VO2max in Endurance Athletes. A Systematic Review with Meta-Analysis**

**Antonio Granero-Gallegos 1,**†**, Alberto González-Quílez 2,**†**, Daniel Plews 3,4 and María Carrasco-Poyatos 1,\***


Received: 29 September 2020; Accepted: 28 October 2020; Published: 30 October 2020

**Abstract:** This review aimed to synthesize evidence regarding interventions based on heart rate variability (HRV)-guided training for VO2max improvements in endurance athletes and address the issues that impact this performance enhancement. The Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE, CINAHL Complete, the Web of Science Core Collection, Global Health, Current Contents Connect, and the SciELO citation index were searched. Inclusion criteria were: randomized controlled trials; studies with trained athletes enrolled in any regular endurance training; studies that recruited men, women, and both sexes combined; studies on endurance training controlled by HRV; studies that measured performance with VO2max. A random-effects meta-analysis calculating the effect size (ES) was used. Moderator analyses (according to the athlete's level and gender) and metaregression (according to the number of participants in each group) were undertaken to examine differences in ES. HRV-guided training and control training enhanced the athletes' VO2max (*p* < 0.0001), but the ES for the HRV-guided training group was significantly higher (*p* < 0.0001; ESHRVG-CG = 0.187). The amateur level and female subgroup reported better and significant results (*p* < 0.0001) for VO2max. HRV-guided training had a small (ES = 0.402) but positive effect on endurance athlete performance (VO2max), conditioned by the athlete's level and sex.

**Keywords:** performance; heart rate variability; high-level athletes; maximal oxygen uptake
