This study evaluated the 7-d mean and coefficient of variation (CV) of supine and standing ultrashort log-transformed root mean square of successive R-R intervals multiplied by 20 (lnRMSSDx20) obtained with a smartphone application (app) in response to varying weekly training load (TL). In addition, the authors aimed to determine if these values could be accurately assessed in as few as 5 or 3 d/wk.
Nine women from a college soccer team performed daily heart-rate-variability measures with an app in supine and standing positions over 3 wk of moderate, high, and low TL. The mean and CV over 7, 5, and 3 d were compared within and between weeks.
The 5- and 3-d measures within each week provided very good to nearly perfect intraclass correlations (ICCs .74–.99) with typical errors ranging from 0.64 to 5.65 when compared with the 7-d criteria. The 7, 5, and 3-d supine CV and the 7-day standing CV were moderately lower during the low-load than the high-load week (P .003–.045, effect sizes 0.86–0.92), with no significant changes occurring in the other measures.
This study supports the use of the mean and CV of lnRMSSD measured across at least 5 d for reflecting weekly values. The supine lnRMssDx20 CV as measured across 7, 5, and 3 d was the most sensitive marker to the changes in TL in the 3-wk period.