Sánchez-Alcaraz Martínez B. J., and Gómez-Mármol A. 2015. “Perception of Effort, Enjoyment and Learning in Secondary Students in Physical Education Lessons During a Crossfit Teaching Unit”. SPORT TK-EuroAmerican Journal of Sport Sciences 4 (1):63-68. https://doi.org/10.6018/239841.