Quantifying complexity and variability in phase portraits of gait

Background: Injuries to the lower extremity often cause limitations to joint motion and alter movement patterns of limb segments during gait. We hypothesized that complexity and variability of limb segment motion during gait would increase in both limbs due to unilateral injury. Using simulated injury to generate asymmetric gait, we developed new methods to quantify changes in the complexity and variability of limb segment angular phase portraits. Methods: To simulate reduced range of motion associated with knee injury, the right knee was constrained to full extension by an external brace. Thigh, shank and foot segment angular phase portraits were generated from 20 healthy male subjects walking for 3-minute trials with and without the brace. Using Fourier-based methods, complexity was quantified by the number of harmonic frequencies suitable for fitting the phase-portrait shape - with a larger number of harmonics indicating greater complexity. Variability was characterized by the drift and confidence area generated by the inter-cycle excursion of the phase-portrait centroid. Findings: Significant differences were found in complexity and variability measures due to bracing. Phase-portrait shape complexity and variability increased in the right (braced) limb, compared to the unbraced condition; while only variability increased for the left (contralateral) limb during bracing. Interpretation: These new methods proved successful at quantifying changes in the complexity and variability that have been visually observed in phase portraits during asymmetric gait. This work provides a method that can be incorporated into clinical assessments to provide quantifiable measures of more precise differences in gait dynamics.