Background.Rigidity is one of the cardinal signs of Parkinsons disease (PD), which has been associated with the reduced ability to adapt movement coordination patterns to changes in task performance (reduced flexibility), such as changes in walking speed or stride frequency (Winogrodska et al., 2005).It has been suggested that the reduced flexibility in PD coincides with an increased resistance of walking patterns to perturbations (increased stability; van Emmerik & Wagenaar, 1999; Wagenaar & van Emmerik, 1994).Objective.To investigate the effects of rigidity on the flexibility and stability of walking patterns in individuals with PD.It is hypothesized that compared to healthy elderly controls (HC), the individuals with PD with rigidity will have a reduced ability to change amplitudes of arm, leg and trunk movements and phase relations between these movements when instructing participants to walk at a wide range of velocities.This reduced flexibility in PD walking patterns will coincide with a smaller variability in movement amplitudes and phase relations (increased stability).It is expected that these changes will result in an increased stride frequency and a reduced stride length in the PD participants.Methods.Participants were instructed to walk on a treadmill while systematically increasing and then decreasing the walking speed between 0.22 and 1.30 m/s in steps of about 0.23 m/sec.The kinematic data were collected by means ofa 3D movement recording system.The dependent variables included stride frequency, stride length, stride time, amplitude of limb and trunk movements, and the phase relation between arm and leg movements as well as between transverse pelvic and thoracic rotations.Results.Although the trunk coordination showed a decreased variability in PD compared to HC, an increased variability in relative phase between left and right arm swing was observed.PD participants also showed smaller amplitudes with less variability for arm, leg, thoracic and pelvic rotations as well as the phase relation between thoracic and pelvic rotations.In addition, an increased stride frequency was observed in PD compared to the HC.Conclusions.The increased variability (or decreased stability) of the phase relation between left and right arm swing may be related to the reduced out-of-phase forcing of the arm movements at the shoulders as a result of the smaller amplitude of thoracic rotation and reduced counterrotation between pelvic and thoracic rotation (axial rigidity) in PD.Physical therapy aimed at improving the forcing of arm movements at the shoulders may have a positive impact on Parkinsonian gait.