title: Modifying center of mass trajectory during sit-to-stand tasks redistributes the mechanical demand across the lower extremity joints.
abstract: OBJECTIVE
  Sit-to-stand tasks are commonly facilitated by modifying the initial position of the center of mass relative to the feet. It was hypothesized that modifications in the center of mass trajectory during sit-to-stand tasks altered the total body momentum at seat departure and redistributed the lower extremity net joint moments.
  
  
  METHODS
  Between-task within-subject comparison was employed using a robust statistical method to accommodate for small sample size.
  
  
  METHODS
  Six individuals performed four sit-to-stand tasks with systematic modifications in the initial center of mass position by varying the orientation of the lower extremity segments. The momentum of the center of mass and lower extremity net joint moments were quantified and compared.
  
  
  RESULTS
  Reducing the horizontal center of mass displacement significantly reduced horizontal total body momentum required at seat departure. Sit-to-stand tasks initiated with more horizontal shank and thigh positions required significantly greater knee and hip extensor net joint moments than those with more vertical shank and thigh positions. Sit-to-stand tasks initiated with vertical shank positions also required significantly greater hip extensor net joint moments as compared to those with more horizontal shank orientations.
  
  
  CONCLUSIONS
  When changes in initial center of mass position are made, alteration in center of mass horizontal momentum and the orientation of the lower extremity segments relative to the reaction force are observed. Consequently, mechanical demand imposed on the ankle, knee, and hip joint is redistributed. The magnitude of the net joint moments is dependent on the segment orientation, the reaction force, and the adjacent net joint moment.
authors.affiliation: Biomechanics Research Laboratory, Department of Kinesiology, University of Southern California, 3560 Watt Way, PED 107, Los Angeles, CA 90089-0650, USA.
authors.forename: W
authors.initials: W
authors.lastname: Mathiyakom
authors.forename: J L
authors.initials: JL
authors.lastname: McNitt-Gray
authors.forename: P
authors.initials: P
authors.lastname: Requejo
authors.forename: K
authors.initials: K
authors.lastname: Costa
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D001696:Biomechanical Phenomena / Q000379:methods; D005260:Female; D006801:Humans; D007596:Joints / Q000502:physiology*; D035002:Lower Extremity / Q000502:physiology*; D008297:Male; D009068:Movement / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D013314:Stress, Mechanical; D019415:Torque; D016474:Weight-Bearing / Q000502:physiology*