Levels of Physical Activity That Predict Optimal Bone Mass in Adolescents The HELENA Study - Gracia-Marco, Luis; Moreno, Luis A.; Ortega, Francisco B.; Leon, Francisco; Sioen, Isabelle; Kafatos, Anthony; Martinez-Gomez, David; Widhalm, Kurt; Castillo, Manuel J.; Vicente-Rodriguez, German;HELENA Study Grp (American Journal Of Preventive Medicine, 2011)