Ankle spasticity is inversely correlated with antagonist voluntary contraction in hemiparetic subjects

The correlation between the severity of spasticity and residual muscular activity is unclear, yet the latter is often used to investigate the effects of therapeutic interventions in spastic movement disorders. Our objectives were to compare the EMG and force generated by the ankle plantar- and dorsi-flexors in normal and spastic hemiparetic subjects, and to investigate their reproducibility and their correlation with clinical spasticity. Thirteen spastic hemiparetic and seven age-matched normal subjects generated maximal isometric ankle plantar- and dorsi-flexing contractions. Maximal force was significantly decreased to 33% of the non-affected leg during dorsiflexion and to 59% during plantarflexion. Measurements of force parameters (onset and magnitude) during both dorsi- and plantar-flexion and the EMG co-contraction ratios during dorsiflexion were highly reproducible (r = 0.78 to 0.99). Our most interesting finding was that the EMG co-contraction ratio was inversely correlated with the amount of force produced by the paretic dorsiflexors (r = -0.91), and that the latter was in turn, inversely related to plantarflexor spasticity (r = -0.65). The high reproducibility of the force measurements suggested that they could be used to evaluate the long-term effects of therapeutic interventions. Furthermore, our findings demonstrated that the voluntary motor deficit in the paretic dorsiflexors but not the spastic plantarflexors was a reliable and valid indicator of the severity of ankle spasticity.