Opportunities and Challenges Assessing Stretch Reflex Impairments Following Stroke

Spasticity and discoordination are hallmarks of the motor impairments that often follow a stroke. The stretch reflex has been implicated as a contributor to both, rationalizing the many studies that have sought to quantify changes to the stretch reflex resulting from stroke. The stretch reflex is typically evoked in humans using controlled perturbations to alter the length of a single muscle (e.g. using a tendon tap) or groups of muscles by displacing a joint or limb with a robot. Interpreting the results of these studies can be challenging since multiple mechanisms contribute to stretch-evoked responses, and which mechanisms and therefore experimental protocols are most relevant can change with the level of impairment. Elegant, highly controlled laboratory experiments have often been used to mitigate these challenges but since the stretch reflex is known to vary with task, it is unclear how many of these studies extrapolate to more functional conditions. This presentation will review previous and recent approaches to quantifying the stretch reflex after stroke, with an emphasis on how the measured responses depend on the experimental protocol used and the behaviors being studied. The objective is to provide a framework for designing future studies that target mechanisms of interest and for interpreting the results of past work in a coherent manner.