title: Impact of physical activity, sedentary behaviour and muscle strength on bone stiffness in 2-10-year-old children-cross-sectional results from the IDEFICS study.
abstract: BACKGROUND
  Physical activity (PA), weight-bearing exercises (WBE) and muscle strength contribute to skeletal development, while sedentary behaviour (SB) adversely affects bone health. Previous studies examined the isolated effect of PA, SB or muscle strength on bone health, which was usually assessed by x-ray methods, in children. Little is known about the combined effects of these factors on bone stiffness (SI) assessed by quantitative ultrasound. We investigated the joint association of PA, SB and muscle strength on SI in children.
  
  
  METHODS
  In 1512 preschool (2- < 6 years) and 2953 school children (6-10 years), data on calcaneal SI as well as on accelerometer-based sedentary time (SED), light (LPA), moderate (MPA) and vigorous PA (VPA) were available. Parents reported sports (WBE versus no WBE), leisure time PA and screen time of their children. Jumping distance and handgrip strength served as indicators for muscle strength. The association of PA, SB and muscle strength with SI was estimated by multivariate linear regression, stratified by age group. Models were adjusted for age, sex, country, fat-free mass, daylight duration, consumption of dairy products and PA, or respectively SB.
  
  
  RESULTS
  Mean SI was similar in preschool (79.5 ± 15.0) and school children (81.3 ± 12.1). In both age groups, an additional 10 min/day in MPA or VPA increased the SI on average by 1 or 2%, respectively (p ≤ .05). The negative association of SED with SI decreased after controlling for MVPA. LPA was not associated with SI. Furthermore, participation in WBE led to a 3 and 2% higher SI in preschool (p = 0.003) and school children (p < .001), respectively. Although muscle strength significantly contributed to SI, it did not affect the associations of PA with SI. In contrast to objectively assessed PA, reported leisure time PA and screen time showed no remarkable association with SI.
  
  
  CONCLUSIONS
  This study suggests that already an additional 10 min/day of MPA or VPA or the participation in WBE may result in a relevant increase in SI in children, taking muscle strength and SB into account. Our results support the importance of assessing accelerometer-based PA in large-scale studies. This may be important when deriving dose-response relationships between PA and bone health in children.
authors.affiliation: Department of Epidemiological Methods and Etiological Research, Leibniz Institute for Prevention Research and Epidemiology - BIPS, Achterstr. 30, 28359, Bremen, Germany. herrmann@bips.uni-bremen.de.
authors.forename: Diana
authors.initials: D
authors.lastname: Herrmann
authors.affiliation: Department of Biometry and Data Management, Leibniz Institute for Prevention Research and Epidemiology - BIPS, Achterstr. 30, 28359, Bremen, Germany. buck@bips.uni-bremen.de.
authors.forename: Christoph
authors.initials: C
authors.lastname: Buck
authors.affiliation: Department of Public Health, Ghent University, 4K3, De Pintelaan 185, 9000, Ghent, Belgium. Isabelle.Sioen@UGent.be.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Sioen
authors.affiliation: Research and Education Institute of Child Health, 138 Limassol Ave, #205, 2015, Strovolos, Cyprus. kourides@cytanet.com.cy.
authors.forename: Yiannis
authors.initials: Y
authors.lastname: Kouride
authors.affiliation: Department of Paediatrics, Queen Silvia Children's Hospital, University of Gothenburg, Rondvägen 15, 41685, Gothenburg, Sweden. staffan.marild@pediat.gu.se.
authors.forename: Staffan
authors.initials: S
authors.lastname: Marild
authors.affiliation: Department of Pediatrics, Medical Faculty, University of Pecs, Jozsef A. u. 7, 7623, Pecs, Hungary. molnar.denes@pte.hu.
authors.forename: Dénes
authors.initials: D
authors.lastname: Molnár
authors.affiliation: GENUD (Growth, Exercise, Nutrition and Development) Research Group, University of Zaragoza, C/Domingo Miral s/n, 50009, Zaragoza, Spain. Theodora.Mouratidou@ec.europa.eu.
authors.forename: Theodora
authors.initials: T
authors.lastname: Mouratidou
authors.affiliation: Centre for Sport and Exercise Science and Medicine (SESAME), University of Brighton, Welkin House, 30 Carlisle Road, Eastbourne, BN20 7SN, UK. Y.Pitsiladis@brighton.ac.uk.
authors.forename: Yannis
authors.initials: Y
authors.lastname: Pitsiladis
authors.affiliation: Institute of Food Sciences, National Research Council, Via Roma 64, 83100, Avellino, Italy. prusso@isa.cnr.it.
authors.forename: Paola
authors.initials: P
authors.lastname: Russo
authors.affiliation: Department of Chronic Diseases, Centre of Behavioural and Health Sciences, National Institute for Health Development, Hiiu 42, 11619, Tallinn, Estonia. toomas.veidebaum@tai.ee.
authors.forename: Toomas
authors.initials: T
authors.lastname: Veidebaum
authors.affiliation: Department of Epidemiological Methods and Etiological Research, Leibniz Institute for Prevention Research and Epidemiology - BIPS, Achterstr. 30, 28359, Bremen, Germany. ahrens@bips.uni-bremen.de.|Faculty of Mathematics and Computer Science, Bremen University, Bibliothekstraße 1, 28359, Bremen, Germany. ahrens@bips.uni-bremen.de.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Ahrens
mesh_terms: D061725:Accelerometry / Q000706:statistics & numerical data; D015992:Body Mass Index; D015519:Bone Density / Q000502:physiology*; D002111:Calcaneus / Q000000981:diagnostic imaging; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D005060:Europe; D015444:Exercise / Q000502:physiology; D005260:Female; D018737:Hand Strength; D006801:Humans; D008297:Male; D009043:Motor Activity / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D011446:Prospective Studies; D057185:Sedentary Behavior; D014463:Ultrasonography