Objective

The aim of this study was to investigate the changes in 24-hour heart rate variability 1 and aerobic fitness, and their associations, in female soccer players during the preseason period.

Methods

Sixteen players were assessed (24-hour HRV and Yo-Yo Intermittent Recovery Test, level 1 [YYIR1]) before and after 4 weeks of preseason. The relationship between R-R24h length and high-frequency oscillations (HF24h) was analysed by a quadratic regression model (revealing or not saturation of vagal activity) assessed 48-h before (PRE-preseason) and 48-h after (POST-preseason) the preseason period. Additionally, the mean HF24h was calculated from the linear portion of the R–R interval versus the HF24h regression curve (HF index). The average of the corresponding R-R24h values was defined as the R–R index.

Results

In PRE-preseason, 7 players had a saturated HF24h, while in POST-preseason, 5 new cases of saturated HF24h were observed. The mean R-R24h, HF24h, R-R index, and HF index lengths significantly increased after preseason (p<0.001). Significant differences were found in YYIR1 PRE- compared to POST-preseason [930 ± 286 m (individual range: 400–1240 m) vs. 1265 ± 252 m (640–1640 m), respectively; p<0.001]. Additionally, the relative changes in HF24h and HF index were largely correlated with improvements in the distance covered during the YYIR1 (r = 0.68 and r = 0.56; respectively).

Conclusion

Enhanced vagal activity after 4-week preseason period of soccer training increased the occurrence of vagal saturation in high-level female soccer players. Additionally, the increases in HF24h and HF index were significantly correlated with aerobic fitness change.