Given the impaired motor output of the lower limb post-stroke, we argue that cortical damage alters the modular control structure underlying muscle activation patterns during both static and dynamic tasks. Muscle synergies were extracted from EMG collected during isometric hip torque production performed by chronic stroke and control subjects. The stroke group presented a reduction in muscle synergies, consistently displaying only two of the four characteristic synergies identified in the control group. These findings do not support a generalized co-contraction of muscles underlying motor impairments. Our data also indicated that the activation coefficients of the muscle synergies retained by the stroke group were altered. Finally, muscle synergies extracted from the isometric torque production tasks were able to account for a large portion of the variance in the gait EMG at similar limb postures in both subject groups. We therefore propose that muscle activation patterns during the swing phase of gait are constrained by the set of muscle synergies identified under the isometric conditions.
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