1 Institute of Biomedicine (IBIOMED), University of León, Campus Universitario, 24071, León, Spain; University of the Basque Country, Spain; Spanish Back Pain Research Network.
2 Research Department, Kovacs Foundation, Paseo de Mallorca 36, 3°, 1(a). 07012, Palma de Mallorca, Spain; Spanish Back Pain Research Network.
3 Iberoamerican Cochrane Center-Servei d'Epidemiologia Clínica i Salut Pública, Institut d'Investigació Biomèdica Sant Pau, Sant Antoni M. Claret 171, Barcelona, Catalonia 08041, Spain; CIBER Epidemiología y Salud Pública (CIBERESP), Spain.