Feedforward postural muscle modes and multi-mode coordination in mild cerebellar ataxia.

The purpose of this study was to investigate postural muscle synergies (M-modes) and quantitative multi-mode coordination to ensure reproducible center of pressure (COP) in anterior-posterior trajectories associated with voluntary-induced perturbations in patients with mild cerebellar ataxia. We applied the framework of the uncontrolled manifold hypothesis for the patients with ataxia. Nine patients diagnosed with spinocerebellar degeneration (SCD) and nine healthy adults stood on a force plate performed the voluntary unloading task. Ground reaction forces and surface electromyogram signals of ten trunk and leg muscles were recorded. Total variance of the first three principal components in the SCD group was similar to the control group. The co-contraction M-modes, uniting muscle pairs with opposing actions at major leg joints, were observed more frequently in the SCD group than in the control group during anticipatory postural adjustments. The quantitative multi-mode coordinations to ensure stable COP trajectories prior to and after motor actions were smaller in the SCD group than in the control group. We conclude that individuals with mild cerebellar ataxia organize feedforward muscle modes and show more co-contraction modes and impaired coordination during feedback and feedforward postural control.