The GDI-Kinetic: a new index for quantifying kinetic deviations from normal gait.

This article introduces a new index, the GDI-Kinetic; a direct analog of the GDI based on joint kinetics rather than kinematics. The method consists of: (1) identifying "features" of the raw gait kinetic data using singular value decomposition, (2) identifying a subset of features that account for a large percentage of the information in the raw gait kinetic data, (3) expressing the raw data from a group of typically developing children as a linear combination of these features, (4) expressing a subject's raw data as a linear combination of these features, (5) calculating the magnitude of the difference between the subject and the mean of the control, and (6) scaling and transforming the difference, in order to provide a simple, and statistically well-behaved, measure. Linear combinations of the first 20 gait features produced a 91% faithful reconstruction of the data. Concurrent and face validity for the GDI-Kinetic are presented through comparisons with the GDI, Gillette Functional Assessment Questionnaire Walking Scale (FAQ), and topographic classifications within the diagnosis of Cerebral Palsy (CP). The GDI-Kinetic and GDI are linearly related but not strongly correlated (r(2)=0.24). Like the GDI, the GDI-Kinetic scales with FAQ level, distinguishes levels from one another, and is normally distributed across FAQ levels six to ten, and among typically developing children. The GDI-Kinetic also scales with respect to clinical involvement based on topographic CP classification in Hemiplegia types I-IV, Diplegia, Triplegia, and Quadriplegia. The GDI-Kinetic complements the GDI in order to give a more comprehensive measure of gait pathology.