A method for quantification of stretch reflex excitability during ballistic reaching

Stretch reflexes are crucial for performing accurate movements and correcting for unpredictable environments at low energy costs. Stretch reflexes are modulated by supraspinal structures via corticofugal pathways. Neural activity in these structures is difficult to observe directly, but characterization of reflex excitability during volitional movement can be used to study how these structures modulate reflexes and how neurological injuries impact this control. We have developed a novel protocol to quantify stretch reflex excitability during ballistic reaching. This novel method relies on a custom haptic device (NACT-3D) capable of applying high-velocity (> 270 °/s) joint perturbations in the plane of the arm while participants perform 3D reaching tasks in a large workspace. We assessed its performance on two control participants and four participants with chronic hemiparetic stroke. Participants reached ballistically from a near to a far target, with elbow extension perturbations applied in random catch trials. Perturbations were applied before movement, at movement onset, or at peak movement velocity. Stretch reflexes were elicited in the biceps muscle in the stroke cohort as measured by electromyographic (EMG) activity both before (pre phase) movement and near movement onset (early phase). Significant reflexive EMG was also seen in the pectoralis major in the pre motion phase. In the control cohort, no significant reflexive EMG was seen. This novel protocol can capture increased stretch reflex excitability in post-stroke individuals when compared to controls and quantify the modulation of stretch reflexes during movement.