• Other biases

The included studies appear to be free from other sources of bias.

### *4.3. Synthesis of Results*

The Kiviniemi\_2010 and Schmitt\_2017 studies were segmented for quantitative analysis according to their intervention groups. The comparisons were: Kiviniemi\_2007 a, HRV (male subgroup, HRV-guided training) vs. standard training (ST); Kiviniemi\_2010 a, HRV-1 (male subgroup, HRV-guided training) vs. standard training (ST); Kiviniemi\_2010 c, HRV-I (female subgroup, HRV-guided training) vs. standard training (ST); Kiviniemi\_2010 g, HRV-II (female subgroup, HRV-guided training tailored for women) vs. HRV-I (female subgroup, HRV-guided training); Kiviniemi\_2010 f, HRV-II (female subgroup, HRV-guided training tailored for women) vs. standard training (ST); Schmitt\_2017 a HRV (HRV-guided training) vs. N (traditional training and normoxia sleeping); Schmitt\_2017 b HRV (HRV-guided training) vs. H (traditional training and hypoxia sleeping). Therefore, the total number of individual studies analyzed were 17 (k = 7 for the experimental group; k = 10 for the control group).
