Contribution of passive stiffness to ankle plantarflexor moment during gait after stroke

Objective: To measure the contribution of passive stiffness to the ankle pl antarflexor moment during gait in subjects with hemiparesis early after str oke. The relationship of passive stiffness with gait speed was also examine d. Design: Cross-sectional, descriptive. Patients and Other Participants: A sample of convenience of 14 patients (54 .7 +/- 10.9yrs) with a hemiparesis for less than 5 months and 11 healthy co ntrols (50.6 +/- 11.6yrs). Main Outcome Measures: The contribution of passive stiffness to the plantar flexor moment during gait was obtained using moment-angle slope (stiffness) values. Total plantarflexor stiffness was measured during gait, and passiv e stiffness was measured during passive dorsiflexion imposed by an isokinet ic dynamometer at velocities and ranges of movement matched with values rec orded during the plantarflexor lengthening period of the stance phase. The contribution of passive stiffness was obtained by dividing the passive stif fness (dynamometer) by the total plantarflexor stiffness (gait). Results: On the paretic side, passive stiffness contributed more (16.8%; ra nge 2.9% to 49.6%) to total plantarflexor stiffness during gait compared (p < .01) with both the nonparetic side (7.3%) and control values (5.9%). Thi s increased contribution on the paretic side resulted from a large muscle-t endon passive stiffness, a decreased active muscle contribution, or both. A lthough in some patients the increased passive component led to the develop ment of a total plantarflexor stiffness that was within normal values, it d id not in others either because the active component was very small or beca use limited dorsiflexion during the stance phase prevented the passive comp onent tension to develop. The contribution of passive stiffness was not sig nificantly (p > .05) related to gait speed in both the patients and the con trols. Conclusions: The increased contribution of passive stiffness to total plant arflexor moment during gait likely acts as an adaptation for a defective mu scle active component, helping ankle push-off at the end of the stance phas e. Although this mechanism is effective in most of the patients, it cannot come into action if the dorsiflexion movement during the stance phase is pr evented, for instance, by enhanced stretch reflexes.