Behavior of medial gastrocnemius motor units during postural reactions to external perturbations after stroke

Abstract

ObjectiveThis study investigated the behavior of medial gastrocnemius (GM) motor units (MU) during external perturbations in standing in people with chronic stroke. MethodsGM MUs were recorded in standing while anteriorly-directed perturbations were introduced by applying loads of 1% body mass (BM) at the pelvis every 25–40s until 5% BM was maintained. Joint kinematics, surface electromyography (EMG), and force platform measurements were assessed. ResultsAlthough external loads caused a forward progression of the anterior–posterior centre of pressure (APCOP), people with stroke decreased APCOP velocity and centre of mass (COM) velocity immediately following the highest perturbations, thereby limiting movement velocity in response to perturbations. MU firing rate did not increase with loading but the GM EMG magnitude increased, reflecting MU recruitment. MU inter spike interval (ISI) during the dynamic response was negatively correlated with COM velocity and hip angular velocity. ConclusionsThe GM utilized primarily MU recruitment to maintain standing during external perturbations. The lack of MU firing rate modulation occurred with a change in postural central set. However, the relationship of MU firing rate with kinematic variables suggests underlying long-loop responses may be somewhat intact after stroke. SignificancePeople with stroke demonstrate alterations in postural control strategies which may explain MU behavior with external perturbations.