Socio-economic factors and active commuting to school in urban Spanish adolescents: the AVENA study - Chillon, Palma; Ortega, Francisco B.; Ruiz, Jonatan R.; Perez, Isaac J.; Martin-Matillas, Miguel; Valtuena, Jara; Gomez-Martinez, Sonia; Redondo, Carlos; Rey-Lopez, Juan P.; Castillo, Manuel J.; Tercedor, Pablo; Delgado, Manuel;AVENA Study Grp (European Journal Of Public Health, 2009)