Pathare, N, Brunt, D, Bishop, MS, Ko, MS, Choi, BS, Lyons, JM; Departments of Physical therapy and Neurology, University of Florida, Gainesville, FL. USA.neeti@ufl.edu.

PURPOSE: To determine the effect of velocity on force modulation during initiation of gait in healthy elderly subjects and persons with Parkinson’s disease (PD). BACKGROUND: Hypokinesia is a characteristic of idiopathic Parkinsonism. It is also used to describe the gait in these patients where velocity is primarily decreased due to short step lengths. But the mechanisms underlying this are not well understood. The ground reaction forces that occur prior to swing toe off determine the velocity of gait initiation. Analysis of these forces that dictate the velocity of gait initiation may help explain the mechanisms of hypokinesia in a gait related activity. SUBJECTS: Subjects with PD (n = 14, mean age = 65.35 years) and healthy elderly subjects (n =12, mean age = 66.07 years) participated in this study. The subjects with PD had a disease duration ranging from 1- 25 years and belonged to stages I – III of the Hahn and Yahr classification. All the subjects were on medication during the study. METHODS: Subjects were asked to initiate gait in three different tasks (self paced, fast and to a target). Peak acceleration ground reaction forces (Fx), slopes to peaks, time to peaks and time to toe off were measured for both the stance and swing limbs using two force plates embedded in a walkway. ANALYSIS: Two way (mixed model) ANOVA (within - task; between - group) was used. The level of significance was set at 0.05. RESULTS: A significant interaction effect for group x task was observed for peak Fx and slope to Fx for both the limbs. The elderly group generated much greater Fx and at a higher rate of Fx generation than the group with PD during the fast task. The self-paced and target tasks did not differ significantly for these variables. Significant main effects for task were noted for the temporal events studied in both the groups, with longest durations for the target task and least for the fast task. CONCLUSIONS: Both the groups of subjects were able to modulate their initiation of gait to the velocity instructed. Corcos et al used time and rate of rise of torque to explain how we program for a given force amplitude. Time and rate of rise could either be regulated (remain invariant) or controlled (allowed to vary). The changes in both the time and slope of peak Fx (both limbs) shows that subjects controlled both the time to force and rate of rise in force to achieve the desired initiation velocity. The difference between the two groups lies in the generation of an acceleration force and rate of force production. Deficits in rate of force production in persons with PD have been noted as a mechanism of hypokinesia in the upper extremity research. Results of this study indicate similar mechanisms to underline hypokinesia during gait initiation. FUNDING SOURCE: None.

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