title: Effects of physical activity and muscle quality on bone development in girls.
abstract: BACKGROUND
  Poor muscle quality and sedentary behavior are risk factors for metabolic dysfunction in children and adolescents. However, because longitudinal data are scarce, relatively little is known about how changes in muscle quality and physical activity influence bone development.
  
  
  OBJECTIVE
  In a 2-yr longitudinal study, we examined the effects of physical activity and changes in muscle quality on bone parameters in young girls.
  
  
  METHODS
  The sample included 248 healthy girls age 9-12 yr at baseline. Peripheral quantitative computed tomography was used to measure calf and thigh muscle density, an indicator of skeletal muscle fat content or muscle quality, as well as bone parameters at diaphyseal and metaphyseal sites of the femur and tibia. Physical activity was assessed using a validated questionnaire specific for youth.
  
  
  RESULTS
  After controlling for covariates in multiple regression models, increased calf muscle density was independently associated with greater gains in cortical (β = 0.13, P < 0.01) and trabecular (β = 0.25, P < 0.001) volumetric bone mineral density and the bone strength index (β = 0.25, P < 0.001) of the tibia. Importantly, these relationships were generalized, as similar changes were present at the femur. Associations between physical activity and changes in bone parameters were weaker than those observed for muscle density. Nevertheless, physical activity was significantly (all P < 0.05) associated with greater gains in trabecular volumetric bone mineral density and the bone strength index of the distal femur.
  
  
  CONCLUSIONS
  These findings suggest that poor muscle quality may put girls at risk for suboptimal bone development. Physical activity is associated with more optimal gains in weight-bearing bone density and strength in girls, but to a lesser extent than changes in muscle quality.
authors.affiliation: 1College of Medicine, Mayo Clinic, Rochester, MN; 2Department of Nutritional Sciences, University of Arizona, Tucson, AZ; and 3Department of Physiological Sciences, University of Arizona, Tucson, AZ.
authors.forename: Joshua N
authors.initials: JN
authors.lastname: Farr
authors.forename: Deepika R
authors.initials: DR
authors.lastname: Laddu
authors.forename: Robert M
authors.initials: RM
authors.lastname: Blew
authors.forename: Vinson R
authors.initials: VR
authors.lastname: Lee
authors.forename: Scott B
authors.initials: SB
authors.lastname: Going
mesh_terms: D015502:Absorptiometry, Photon; D000293:Adolescent; D001130:Arizona; D015519:Bone Density / Q000502:physiology; D001846:Bone Development / Q000502:physiology*; D002648:Child; D005260:Female; D006801:Humans; D007867:Leg Bones / Q000254:growth & development*; D008137:Longitudinal Studies; D009043:Motor Activity / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D012044:Regression Analysis; D011795:Surveys and Questionnaires; D014057:Tomography, X-Ray Computed