One-min (i.e., ultra-short-term) measurements of heart rate variability (HRV) responses to internal training load (ITL) have yet to be compared to 5-minute criterion measures throughout a longitudinal training period among youth female athletes. Purpose: To verify the agreement between ultra-short-term and criterion measures when assessing weekly mean of the natural logarithm of the root mean square of the successive differences (LnRMSSDm) and its coefficient of variation (LnRMSSDcv) in response to ITL in youth female field hockey players. Methods: Eleven members (16.8 ± 1.14 years; 157.13 ± 5.15 cm; 55.21 ± 5.20 Kg) from the youth female field hockey Mexican National Team participating in a four-week training camp before the 2018 Youth Olympics in Buenos Aires, Argentina, took part in this study. Daily HRV was measured during the whole four-week camp after waking at 6:00 am. HRV data were collected using a chest-strap transmitter which was connected by Bluetooth to a Smartphone application. After a 1-minute stabilization period, a 5-minute HRV recording was initiated. RMSSD was calculated from the first min and the full 5-minute segment, and later log transformed (Ln). ITL was calculated with a modified heart rate-based training impulse method (TRIMP). Repeated measures ANOVA were used to analyze the effect of time on TRIMP, LnRMSSDm and LnRMSSDcv. For agreement between ultra-short-term and criterion LnRMSSD, paired t-tests and intraclass correlations were performed. Results: Mean and SD for all outcome parameters across time are presented in Table 1. TRIMP in week 4 was lower than all other weeks (p < 0.05). Ultra-short-term LnRMSSDm increased in week 3 relative to week 1 (p < 0.05). Despite a significant model effect, post-hoc analyses revealed no changes across time for LnRMSSD criterion (week 3 vs. week 1, p = 0.06). Pairwise comparisons for each week showed no differences between ultra-short-term and criterion LnRMSSDm (Week 1 p = 0.620, Week 2 p = 0.238, Week 3 p = 0.051, Week 4 p = 0.155) or LnRMSSDcv (Week 1 p = 0.266, Week 2 p = 0.074, Week 3 p = 0.254, Week 4 p = 0.197). Ultra-short-term and criterion measures showed very strong agreement (ICC = 0.979, p < 0.001). Conclusions: Performing ultra-short-term measures is a valid method to assess weekly LnRMSSDm and LnRMSSDcv responses to ITL among youth female athletes throughout a longitudinal training period. Practical Application: Ultra-short-term LnRMSSDm and LnRMSSDcv enhance practicality when assessing weekly responses to ITL among national level youth female field hockey players. Utilizing 1-minute rather than 5-minute recordings reduces compliance demands of athletes, making implementation of HRV monitoring more feasible for coaches.
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