Link to the publication: https://www.researchgate.net/publication/336845048_Heart_rate_variability_is_correlated_with_perceived_physical_fitness_in_elite_soccer_players

Authors of the publication say: The most commonly used time-domain (RMSSD) parameter monitored only when lying down is not sufficient to obtain relevant HRV results. Both branches of the Autonomic Nervous System (ANS) should be monitored to obtain correct results. It is important to monitor not only the commonly presented parasympathetic branch but also the sympathetic branch. Measuring only in a supine position appears inappropriate for monitoring the activity of the sympathetic branch. It is ideal to activate the sympathetic by changing position during the measurement. Using a methodology enabling to monitor the activity of both ANS branches (only HRV Spectral Analysis) in at least two positions is an approach that can capture changes in adaptation also in sports disciplines where performance does not depend almost entirely on endurance capabilities. This is the methodology used ONLY by mySASY.

mySASY was not anyhow involved in the research or/and publication. Authors do not probably know about mySASY, ... yet :)