Method for characterization of dynamic ankle stiffness in patients with spasticity

Background: Dynamic ankle stiffness has been quantified as the slope of the ankle joint moment-angle curve over the gait interval of the second rocker, defined explicitly as the period of the gait cycle from the first relative maximum plantar flexion in early stance to maximum dorsiflexion in midstance. However, gastrocnemius spasticity may interfere with the second ankle rocker in patients with spasticity. This gait disruption results in stiffness calculations which are misleading. Current dynamic stiffness metrics need to be modified. Research Question: The main goal of this study was to develop and test a new method to better evaluate dynamic ankle stiffness in individuals with pathologic gait who lack a second rocker interval. Methods: Twenty unimpaired ambulators (10/20 female, 26.7 ± 5.0 years, BMI: 23.2 ± 2.2) and 9 individuals with cerebral palsy (5/9 female, 5.7 ± 1.7 years, BMI: 14.6 ± 2.1, GMFCS Levels: I – 2, II – 5, III - 2) participated in this study. Dynamic ankle stiffness was evaluated using the previous kinematic method, defined by the interval of maximum plantar flexion to maximum dorsiflexion angle in midstance, and the proposed kinetic method, defined by the interval from the maximum dorsiflexion moment to first peak plantar flexion moment. Stiffness was quantified as the linear slope between the sagittal plane ankle angle and moment. Method differences were explored using an equivalence test (α = 0.05). Results and Significance: There was equivalence between the methods for unimpaired ambulators (p = 0.000) and a lack of equivalence for patients with spasticity (p = 0.958). The new method was successfully applied to all 9 pediatric ambulators with CP and demonstrated increased stiffness in patients with spasticity as compared to the previous method. The ability to objectively calculate ankle stiffness in pathologic gait is critical for determining change associated with clinical intervention.