title: Effect of muscle length on voluntary activation level in children and adults.
abstract: OBJECTIVE
  The aim of the present study was to compare the effect of muscle length on the level of voluntary activation (VA) at short and long muscle lengths between children and adults.
  
  
  METHODS
  Thirteen prepubertal boys (10.2 ± 1.1 yr) and 10 men (23.9 ± 2.9 yr) performed 5-s maximal isometric voluntary contractions of the knee extensor muscles at three muscular angles (20°, 90°, 100°; 0°, full extension) interspersed with at least 60-s passive recovery periods. Single magnetic stimulations were delivered to the femoral nerve during maximal isometric voluntary contractions to determine the level of VA using the twitch interpolation technique. The specific torque was calculated as the absolute torque divided by thigh muscle mass, as assessed using dual-energy x-ray absorptiometry. Finally, the theoretical specific torque that could be produced with a complete (i.e., 100%) activation level (specific torque at 100% VA) was estimated from the values of specific torque and VA.
  
  
  RESULTS
  Results showed a higher specific torque in adults at 90° and 100° but not at 20°. Accordingly, VA was significantly higher in adults at 90° (94% ± 4% vs 88% ± 8%, P < 0.05) and 100° (93% ± 6% vs 86% ± 8%, P < 0.05), whereas no significant difference was observed at 20°. Interestingly, the specific torque at 100% VA was not different between groups whatever the joint angle.
  
  
  CONCLUSIONS
  The lower ability of children to fully activate their motor units at long muscle length could account for their lower specific torque because no difference in theoretical specific torque was observed between groups at 90° and 100°.
authors.affiliation: 1EA 3533, Laboratory of Metabolic Adaptations during Exercise in Physiological and Pathological Conditions (AME2P), Clermont University, Blaise Pascal University, Aubière, FRANCE; 2Department of Movement Sciences, Oxylane Research, Villeneuve d'Ascq, FRANCE; 3Clermont University Hospital, Clermont-Ferrand, FRANCE; 4INRA, Human Nutrition Unit (UMR 1019), CRNH Auvergne, Clermont-Ferrand, FRANCE; and 5Clermont University, Auvergne University, Clermont-Ferrand, FRANCE.
authors.forename: Virginie
authors.initials: V
authors.lastname: Kluka
authors.forename: Vincent
authors.initials: V
authors.lastname: Martin
authors.forename: Sebastian Garcia
authors.initials: SG
authors.lastname: Vicencio
authors.forename: Anne-Gaëlle
authors.initials: AG
authors.lastname: Jegu
authors.forename: Charlotte
authors.initials: C
authors.lastname: Cardenoux
authors.forename: Cédric
authors.initials: C
authors.lastname: Morio
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Coudeyre
authors.forename: Sébastien
authors.initials: S
authors.lastname: Ratel
mesh_terms: D000293:Adolescent; D000328:Adult; D000886:Anthropometry; D002648:Child; D004576:Electromyography; D006801:Humans; D007537:Isometric Contraction; D007719:Knee Joint / Q000502:physiology; D008297:Male; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000033:anatomy & histology* / Q000294:innervation / Q000502:physiology*; D011999:Recruitment, Neurophysiological; D012741:Sexual Maturation; D019415:Torque; D055815:Young Adult