Research Article| Volume 357, ISSUE 1-2, P146-151, October 15, 2015

Postural instability in Parkinson Disease: To step or not to step


      • Parkinson subjects and Controls required the same force to be pulled off-balance
      • All Controls took a step in direction of pull to maintain balance without falling
      • 59% of the time Parkinson subjects did not take a step to maintain balance and fell
      • Parkinson subjects do not defend against sudden gravitational instability
      • Parkinson gait disorder may be due to subjects avoiding gravitational instability


      Postural instability is a key feature of Parkinson Disease that is associated with falls and morbidity. We designed a pull apparatus to quantitatively measure the force needed to pull subjects off-balance. Thirteen Controls and eight individuals with Parkinson Disease (PD) were evaluated. All individuals with PD reported subjective symptoms of postural instability and were symptomatic for approximately 9.4 years when tested. No significant differences were found between Controls and PD subjects in the magnitude of force required to pull them off-balance. None of the Controls fell and all took a step into the direction of pull to maintain their balance. 59% of the time PD subjects fell because they did not take a step in the direction of pull to maintain their center of mass (COM) over their feet, thus indicating a deficiency in postural reflexes. If they fell on the first pull, PD subjects did not show a learning effect when pulled multiple times in the same direction. The utility of the Pull Test to detect postural instability is related to the subject's behavioral response, not the force needed to pull them off balance. Our findings may also help explain certain features of the PD gait as an attempt by subjects to avoid postural instability by not placing their COM in gravitationally unstable positions.


      COM (center of mass), PD (Parkinson Disease), PI (postural instability), CON (controls)


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