title: Muscle strength and soccer practice as major determinants of bone mineral density in adolescents.
abstract: OBJECTIVE
  To analyse the relationship between isokinetic strength of the lower limb muscles and bone mineral density and content (BMD, BMC) of adolescent male soccer players and age-matched controls not involved in sport (12-15 years).
  
  
  METHODS
  A random sample of 151 young males was divided into soccer players (SG; n=117) and control subjects (CG; n=34). Peak torque of knee extensors (PTE) and flexors (PTF) was measured during isokinetic knee joint movement (90°/s) of the dominant and non-dominant lower limbs. BMD and BMC of the whole-body, lumbar spine, dominant/non-dominant lower limb were determined by dual-energy X-ray absorptiometry. Physical activity was monitored with accelerometers during 5 days. Estimated maturity offset was used as an indicator of biological maturity status.
  
  
  RESULTS
  Whole-body BMD (1.03±0.01 vs. 0.98±0.01 g/cm2, P=0.003) and dominant (1.09±0.01 vs. 1.02±0.01 g/cm2, P<0.001) and non-dominant (1.09±0.01 vs. 1.01±0.01 g/cm2, P<0.001) lower limb BMD was greater in SG compared to CG. No significant differences were found for BMC. Compared to CG, SG performed better in the YY-IE2 test (780±40 vs. 625±31 m), exhibited higher PTE (dominant limb: 155.2±30.3 vs. 123.4±37.0N m; non-dominant limb: 156.2±36.1 vs. 120.4±41.1 N m) and PTF muscles (dominant limb: 79.0±25.3 vs. 57.1±25.3 Nm; non-dominant limb: 73.3±20.7 vs. 57.0±24.2N m). Moreover, the PTE, soccer participation and maturity status were positively associated with the BMD at all body sites (r2=0.57-0.73, P<0.05).
  
  
  CONCLUSIONS
  Muscle strength of knee extensors is associated with BMD and BMC at all body sites. Muscle-skeletal structures respond positively to the weight-bearing and impact-loading imposed by soccer practice. Soccer seemed to be a multilateral balanced sport activity.
authors.affiliation: Faculty of Sport, University of Porto, Centre of Research, Education, Innovation and Intervention in Sport, Porto, Portugal. aseabra@fade.up.pt
authors.forename: André
authors.initials: A
authors.lastname: Seabra
authors.forename: Elisa
authors.initials: E
authors.lastname: Marques
authors.forename: João
authors.initials: J
authors.lastname: Brito
authors.forename: Peter
authors.initials: P
authors.lastname: Krustrup
authors.forename: Sandra
authors.initials: S
authors.lastname: Abreu
authors.forename: José
authors.initials: J
authors.lastname: Oliveira
authors.forename: Carla
authors.initials: C
authors.lastname: Rêgo
authors.forename: Jorge
authors.initials: J
authors.lastname: Mota
authors.forename: António
authors.initials: A
authors.lastname: Rebelo
mesh_terms: D000293:Adolescent; D001823:Body Composition; D015519:Bone Density / Q000502:physiology*; D002648:Child; D006801:Humans; D007537:Isometric Contraction; D007717:Knee / Q000502:physiology; D035002:Lower Extremity; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000378:metabolism*; D008297:Male; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011859:Radiography; D016059:Range of Motion, Articular / Q000502:physiology; D012420:Running / Q000502:physiology*; D012916:Soccer / Q000502:physiology*; D019415:Torque