Quantification of reflex activity in stroke survivors during an imposed multi-joint leg extension movement

The goal of this study was to compare short- and long-latency reflex responses in eight major lower-extremity muscle groups following an imposed multi-joint leg movement between a group of 14 chronic (> 1 year) stroke survivors and 10 healthy age-matched controls, and to investigate the influence of joint velocities and muscle excitation levels on these reflex responses in each respective group. Subjects were seated with their foot anchored to a sliding footplate that could extend their leg. Prior to the leg being moved, subjects were instructed to pre-activate hip and knee flexors and extensors. Feedback of joint torque was used to help subjects activate muscles over a range of excitation levels. Following pre-activation, the subject's leg was passively extended so the knee or hip joint rotated at one of three different speeds (30, 60, and 120 degrees/s). In general, it was found that the magnitude of stroke survivors' reflex response was greater compared to controls' in certain biarticular muscles, notably the gastrocnemius and medial hamstring, and the uniarticular adductor longus, and that the long-latency reflex component (between 40 and 150 ms post-movement) accounted for most of the observed differences. Furthermore, while reflex response amplitudes increased in both groups with increasing movement speed, the rate of increase was significantly larger in stroke subjects than in controls. Clinically, these findings may help explain why stroke survivors walk slowly since it is under these conditions that reflex responses better emulate those of their able-bodied counterparts.