Individuals with motor incomplete spinal cord injuries (iSCI) often have impaired abilities to maintain upright balance. For able-bodied (AB) individuals, the ankle and hip joint accelerations are in anti-phase to minimize the postural sway during quiet standing. Here we investigated how inter-joint coordination between the ankle and hip joints was affected in individuals with iSCI leading to their larger postural sway during quiet standing. Data from sixteen individuals with iSCI, fourteen age- and sex-matched AB individuals and thirteen young AB individuals were analyzed. The participants performed quiet standing during which kinematic and kinetic data were recorded. Postural sway was quantified using center-of-pressure velocity and center-of-mass acceleration. Individual ankle and hip joint kinematics were quantified, and the inter-joint coordination was assessed using the cancellation index (CI), goal-equivalent variance (GEV), non-goal-equivalent variance (NGEV) and uncontrolled manifold (UCM) ratio. Individuals with iSCI displayed greater postural sway compared to AB individuals. The contribution of ankle angular acceleration towards one's sway was significantly greater for those with iSCI compared to AB groups. CI and the UCM ratio were not statistically different between the groups, while GEV and NGEV were significantly greater for the iSCI group compared to the AB groups. We demonstrated that individuals with iSCI show larger postural sway compared to the AB individuals during quiet standing, primarily due to larger ankle joint acceleration. We also demonstrated that the inter-joint coordination between ankle and hip joint is not affected in individuals with iSCI, which is not successfully able to reduce the large COM acceleration.
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