The importance of psychological constructs for training volume and performance improvement: A structural equation model for youth speed skaters - Elferink Gemser, M. T.; De Roos, I.; Torenbeek, M.; Fokkema, T.; Jonker, L.; Visscher, C. (19th Annual Congress of the European College of Sport Science (ECSS), Amsterdam, 2. - 5. July 2014, 2014)