Physical activity (PA) is defined by the American College of Sports Medicine as any movement of the body produced by the skeletal muscles that requires energy expenditure. It includes a collection of behaviors encompassing sport, conditioning exercise and life-style PA. Moderate-to-vigorous PA (MVPA) has proven to be effective in the primary and secondary prevention of many chronic diseases (see Warburton, Charlesworth, Ivey, Nettlefold, & Bredin, 2010, for a review). Despite these benefits, the population is mostly sedentary (e.g. Haskell et al., 2007; Sjöström, Oja, Hagströmer, Smith, & Bauman, 2006), and some social groups are even more inactive than others. This is particularly the case for girls, female adolescents, and women (e.g. Dumith, Gigante, Domingues, & Kohl, 2011; Hines, 2004; Knisel, Opitz, Wossmann, Keteihuf, 2009; Van Tuyckom, Scheerder, & Bracke, 2010). Indeed, females are less active (e.g. Capersen, Pereira, Curran, 2000; Trost, Pate, Sallis, Freedson, Taylor, Dowda, & Sirard, 2002), participate less in motor activities (Hines, 2004), and quit PA more than males (e.g. Dumith et al., 2011). Given that sports participation in childhood significantly predicts participation during adulthood (Perkins, Jacobs, Barber, & Eccles, 2004; Telema, Yang, Viikari, Valimaki, Wanne, & Raitakari, 2005), early differences observed between boys and girls could lead to sex inequalities in health in adulthood. It is thus crucial to identify factors likely to explain such differences in youth.