A Comprehensive Understanding of Postural Tone Biomechanics: Intrinsic Stiffness, Functional Stiffness, Antagonist Coactivation, and COP Dynamics in Post-Stroke Adults.

Objective: To analyse the relationship between traditional stiffness and muscle antagonist coactivation in both stroke and healthy participants, using linear and non-linear measures of coactivation and COP during standing, stand-to-sit, and gait initiation.

Methods: Participants were evaluated through a cross-sectional design. Electromyography, isokinetic dynamometer, and force plate were used to calculate coactivation, intrinsic and functional stiffness, and COP displacement, with both linear and non-linear metrics. Spearman's correlations and Mann-Whitney tests were applied ( < 0.05).

Results: Post-stroke participants showed higher contralesional intrinsic stiffness ( = 0.041) and higher functional stiffness ( = 0.047). Coactivation was higher on the ipsilesional side during standing ( = 0.012) and reduced on the contralesional side during standing and transitions ( < 0.01). Moderate correlations were found between intrinsic and functional stiffness ( = 0.030) and between coactivation and intrinsic stiffness (standing and stand-to-sit: = 0.048) and functional stiffness (gait initiation: = 0.045). COP displacement was reduced in post-stroke participants during standing ( < 0.001) and increased during gait initiation ( = 0.001). Post-stroke participants exhibited increased gastrocnemius/tibialis anterior coactivation during gait initiation ( = 0.038) and higher entropy and stability across tasks ( < 0.001).

Conclusion: Post-stroke participants showed higher contralesional intrinsic and functional stiffness, reduced coactivation in static tasks, and increased coactivation in dynamic tasks. COP and coactivation analyses revealed impaired stability and random control, highlighting the importance of multidimensional evaluations of postural tone.