Ankle kinematics during a drop-vertical jump in patients with chronic ankle instability and healthy controls: A bivariate confidence interval comparison

Background: Chronic ankle instability (CAI) has been associated with frontal and sagittal plane kinematic differences compared to patients with no history of ankle sprain during landing tasks. Single plane kinematics are often statistically compared to identify group differences, however, the complex, multi-planar motions of the ankle allow for unique kinematic adaptations at the joint and univariate waveform analysis may be limited in the assessment of joint motion. Bivariate confidence interval analysis allows for statistical comparisons to be made when considering the simultaneous frontal and sagittal plane kinematics of the ankle.Research question: Can the bivariate confidence interval analysis identify unique joint coupling differences during a drop-vertical jump in patients with CAI?Methods: Subjects with CAI and matched healthy controls performed 15 drop-vertical jump maneuvers while kinematics were collected using an electromagnetic motion capture system. An imbedded forceplate was used to determine ground contact timing. Kinematics were analyzed using a bivariate confidence interval from 100 ms pre-ground contact to 200 ms post-ground contact. Any region where group confidence intervals did not overlap was considered statistically different.Results: Prior to initial contact participants with CAI had greater plantar flexion from 6 ms to 21 ms and 36-63 ms prior to landing. After making ground contact differences were found from 92 ms to 101 ms and 113-122 ms. Greater plantar-flexion and eversion was identified in the patients with CAI prior to ground contact and following landing patients with CAI had greater inversion and plantarflexion compared to healthy controls.Significance: The bivariate analysis identified unique group differences compared to univariate analysis including group differences prior to landing. These unique findings indicate that comparing groups using a bivariate analysis may provide important information on the kinematic differences of patients with CAI and how multiple planes of motion compensate during dynamic landing tasks.