title: Limb collapse, rather than instability, causes failure in sit-to-stand performance among patients with parkinson disease.
abstract: BACKGROUND
  Previous studies focused on describing successful sit-to-stand (STS) performance in patients with Parkinson disease (PD). Little is known about why these patients fail to perform this transfer activity. Objective This study aimed to determine the role of dynamic stability and limb support in governing successful STS performance in patients with PD and to determine the limits of recovery for discriminating between successful and failed STS trials.
  
  
  METHODS
  This was a cross-sectional study.
  
  
  METHODS
  Twenty-eight patients with PD were instructed to perform the STS task. Kinematic data on 18 successful trials and 14 failed trials (when the patients fell backward) were collected with a motion analysis system. Dynamic stability was determined by the anteroposterior position of the body's center of mass (COM) relative to the base of support (BOS) and by the anteroposterior velocity of the COM relative to the BOS (Velocity(COM/BOS,AP)). Limb support was characterized by the hip height (Height(hip)).
  
  
  RESULTS
  /b> The findings revealed no between-group ("risers" versus "fallers") differences in dynamic stability. The fallers shifted their COM in a significantly more anterior position to compensate for their lower Velocity(COM/BOS,AP) at seat-off. It was in the vertical direction that the fallers had significantly reduced peak COM velocity (Velocity(COM,vertical)) and lower corresponding Height(hip) than the risers. Results of a stepwise regression model showed that Velocity(COM/BOS,AP) and Height(hip) at the instant of peak Velocity(COM,vertical) could best predict the STS outcome (success versus failure), with an overall prediction accuracy of 87.5%. The limit differentiating successful from failed STS trials was: Height(hip)=-0.814 Velocity(COM/BOS,AP) + 0.463. Limitations All of the patients were community dwelling and had a moderate level of disease severity. The results cannot be generalized to those who are institutionalized or with advanced PD.
  
  
  CONCLUSIONS
  Limb support and ill-timed peak forward COM velocity, rather than dynamic stability, play the dominant roles in determining successful STS performance in patients with PD.
authors.affiliation: Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Hong Kong. rsmmak@inet.polyu.edu.hk
authors.forename: Margaret K Y
authors.initials: MK
authors.lastname: Mak
authors.forename: Feng
authors.initials: F
authors.lastname: Yang
authors.forename: Yi-Chung
authors.initials: YC
authors.lastname: Pai
mesh_terms: D000368:Aged; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D035002:Lower Extremity / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology*; D010300:Parkinson Disease / Q000503:physiopathology*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture; D015203:Reproducibility of Results; D013647:Task Performance and Analysis