Postural stability and rigidity are two of the major motor symptoms of Parkinson\u27s disease (PD). Falls in PD tend to happen mainly during tasks involving turning and bending motions of the upper body. Although correlations between clinical measures of postural stability and axial rigidity have been reported, the nature in which axial rigidity affects postural control mechanisms in PD remains undetermined. The purpose of this dissertation was (1) to investigate changes in control of the upper body due to aging and PD and (2) to study the impact of these changes on the interaction of posture and manual control. In two experimental series, patients with PD as well as healthy younger and healthy older controls were tested under manipulations of upper body motion and proximity to stability boundaries while standing. Overall, PD patients were able to adapt their postural strategies to changing task demands and adequately performed the required tasks without falling. Across both experiments, a lack of adaptability in pelvis-trunk coordination as well as reduced segmental ranges of motion were observed in patients with PD. Patterns of center of pressure and trunk center of motion suggested that aging alone mainly affects control of reactive forces at the foot-support surface interface, while PD patients show additional impairments in controlling the upper body. PD patients showed both increased and decreased variability in movement patterns compared to controls, undermining the traditional notion that increased variability characterizes movement pathology and equates with postural instability. The results stress the importance of incorporating analysis of three dimensional upper body motions into the study of balance impairments. The findings overall suggest that in PD changes in adaptive control of upper body motion are related to changes in postural control mechanisms
