title: Effect of knee position on quadriceps muscle force steadiness and activation strategies.
abstract: BACKGROUND
  In this study we investigated the effect of knee position on quadriceps force steadiness and activation strategies.
  
  
  METHODS
  Quadriceps force steadiness was evaluated in 22 volunteers at two knee positions by testing their ability to regulate submaximal force. Muscle activation strategies were studied in both time and frequency domains using surface electromyography.
  
  
  RESULTS
  Quadriceps force fluctuations and the associated agonist and antagonist activity were significantly higher at 90° than at 30° of flexion (P < 0.05). The quadriceps median frequency recorded at 30° was significantly higher than at 90° of flexion (P < 0.05). Regression analyses revealed that force steadiness was related to quadriceps activation and median frequency (P < 0.001), but not to hamstring coactivation (P > 0.05).
  
  
  CONCLUSIONS
  The results indicate that knee position significantly affects quadriceps force steadiness and activation strategies. This finding may have important implications for designing a force control testing protocol and interpreting test results.
authors.affiliation: Searle Laboratory, Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, Illinois, USA. ckrishnan@ric.org
authors.forename: Chandramouli
authors.initials: C
authors.lastname: Krishnan
authors.forename: Eric J
authors.initials: EJ
authors.lastname: Allen
authors.forename: Glenn N
authors.initials: GN
authors.lastname: Williams
mesh_terms: D000328:Adult; D004576:Electromyography / Q000379:methods; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000502:physiology*; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D052097:Quadriceps Muscle / Q000502:physiology*; D019415:Torque; D055815:Young Adult