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- Intrinsic and functional stiffness vs. antagonist co-activation: a correlational analysis in stroke patientsPublication . Ribeiro, Edgar; Silva, Augusta; Pinho, Liliana; Rubim, Santos; Pinho, Francisco; Pinheiro De Sousa, Andreia SofiaTonus has been considered the basis of central nervous system (CNS) functional adaptation to fulfill the dual purpose of movement and postural control [1]. Among the multiple pathologies of CNS, stroke has been the most investigated in the context of tonus dysfunction, typically through measures involving the evaluation of muscle resistance against stretching at rest [2]. However, the recent evidence demonstrating that the increased muscle resistance obtained under these conditions is more related to intrinsic compo- nents, being not associated to movement dysfunction [3], turn relevant the assessment of biomechanical variables that could reflect tonus dysfunction in a highly demanding postural control functional context, like upright standing [4] and postural phases of gait initiation and stand-to-sit [5, 6]. By representing muscle synergies organization within postural tone to increase joint stiffness and postural stability, antagonist co- activation at the ankle level would reflect tonus dysfunction in poststroke subjects [7]. Furthermore, the evaluation of ankle stiffness through the analysis of stabilometric data obtained by a force platform during a functional task such as upright standing [8], here called functional stiffness, may also be used as a func- tional alternative to intrinsic stiffness assessment. Based on the previously exposed, it can be hypothe- sized that functional stiffness, rather than intrinsic stiffness, would best reflect the muscle synergies in- volved in the maintenance of postural stability during functional tasks. The main aim of the present study was to evaluate the level of correlation between intrinsic stiffness at rest, functional stiffness in upright standing and antagonist co-activation in upright standing and postural phases of gait initiation and stand-to-sit, at the ankle joint of post-stroke subjects.