title: Muscle volume is related to trabecular and cortical bone architecture in typically developing children.
abstract: BACKGROUND
  Muscle is strongly related to cortical bone architecture in children; however, the relationship between muscle volume and trabecular bone architecture is poorly studied. The aim of this study was to determine if muscle volume is related to trabecular bone architecture in children and if the relationship is different than the relationship between muscle volume and cortical bone architecture.
  
  
  METHODS
  Forty typically developing children (20 boys and 20 girls; 6 to 12y) were included in the study. Measures of trabecular bone architecture [i.e., apparent trabecular bone volume to total volume (appBV/TV), trabecular number (appTb.N), trabecular thickness (appTb.Th) and trabecular separation (appTb.Sp)] in the distal femur, cortical bone architecture [cortical volume, total volume, section modulus (Z) and polar moment of inertia (J)] in the midfemur, muscle volume in the midthigh and femur length were assessed using magnetic resonance imaging. Total physical activity and moderate-to-vigorous physical activity were assessed using an accelerometer-based activity monitor worn around the waist for four days. Calcium intake was assessed using diet records. Relationships among the measures were tested using multiple linear regression analysis.
  
  
  RESULTS
  Muscle volume was moderately-to-strongly related to measures of trabecular bone architecture [appBV/TV (r=0.81), appTb.N (r=0.53), appTb.Th (r=0.67), appTb.Sp (r=-0.71); all p<0.001] but more strongly related to measures of cortical bone architecture [cortical volume (r=0.96), total volume (r=0.94), Z (r=0.94) and J (r=0.92; all p<0.001)]. Similar relationships were observed between femur length and measures of trabecular (p<0.01) and cortical (p<0.001) bone architecture. Sex, physical activity and calcium intake were not related to any measure of bone architecture (p>0.05). Because muscle volume and femur length were strongly related (r=0.91, p<0.001), muscle volume was scaled for femur length (muscle volume/femur length(2.77)). When muscle volume/femur length(2.77) was included in a regression model with femur length, sex, physical activity and calcium intake, muscle volume/femur length(2.77) was a significant predictor of appBV/TV, appTb.Th and appTb.Sp (partial r=0.44 to 0.49, p<0.05) and all measures of cortical bone architecture (partial r=0.47 to 0.54; p<0.01).
  
  
  CONCLUSIONS
  The findings suggest that muscle volume in the midthigh is related to trabecular bone architecture in the distal femur of typically developing children. The relationship is weaker than the relationship between muscle volume in the midthigh and cortical bone architecture in the midfemur, but the discrepancy is driven, in large part, by the greater dependence of cortical bone architecture measures on femur length.
authors.affiliation: Department of Kinesiology and Applied Physiology, University of Delaware, Newark, DE 19713, USA.
authors.forename: Deepti
authors.initials: D
authors.lastname: Bajaj
authors.affiliation: Department of Kinesiology and Applied Physiology, University of Delaware, Newark, DE 19713, USA.
authors.forename: Brianne M
authors.initials: BM
authors.lastname: Allerton
authors.affiliation: Department of Kinesiology and Applied Physiology, University of Delaware, Newark, DE 19713, USA.
authors.forename: Joshua T
authors.initials: JT
authors.lastname: Kirby
authors.affiliation: Department of Orthopedics, Nemours AI duPont Hospital for Children, Wilmington, DE 19803, USA.
authors.forename: Freeman
authors.initials: F
authors.lastname: Miller
authors.affiliation: School of Psychological Sciences and Health, University of Strathclyde, Glasgow G1 1XQ, UK.
authors.forename: David A
authors.initials: DA
authors.lastname: Rowe
authors.affiliation: Biostatistics Core Facility, University of Delaware, Newark, DE 19713, USA.
authors.forename: Ryan T
authors.initials: RT
authors.lastname: Pohlig
authors.affiliation: Department of Kinesiology and Applied Physiology, University of Delaware, Newark, DE 19713, USA. Electronic address: modlesky@udel.edu.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Modlesky
mesh_terms: D015519:Bone Density / Q000502:physiology*; D002648:Child; D003430:Cross-Sectional Studies; D005260:Female; D005269:Femur / Q000254:growth & development*; D006801:Humans; D008297:Male; D009043:Motor Activity / Q000502:physiology; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000254:growth & development*; D009929:Organ Size / Q000502:physiology
keywords: Bone microarchitecture; Bone strength; Bone structure; Calcium; Femur length; MRI; Muscle; Physical activity