Title: 1 Neck Rotation Modulates Flexion Synergy Torques Indicating an Ipsilateral Reticulospinal Source 2 for Impairment in Stroke. 3 4 Authors: Contact Information

41 The effect of reticular formation excitability on maximum voluntary torque (MVT) 42 generation and associated muscle activation at the shoulder and elbow was investigated through 43 natural elicitation (active head rotation) of the asymmetric tonic neck reflex (ATNR) in 26 44 individuals with stroke and 9 age-range-matched controls. Isometric MVT generation at the 45 shoulder and elbow was quantified with the head rotated (face pointing) contralateral and 46 ipsilateral to the paretic (stroke) and dominant (control) arm. Given the dominance of abnormal 47 torque coupling of elbow flexion with shoulder abduction (flexion synergy) in stroke and well 48 developed animal models demonstrating a linkage between reticular formation and ipsilateral 49 elbow flexors and shoulder abductors, we hypothesized that constituent torques of flexion 50 synergy, specifically elbow flexion and shoulder abduction, would increase with contralateral 51 head rotation. The findings of this investigation support this hypothesis. Increases in MVT for 3 52 out of 4 flexion synergy constituents (elbow flexion, shoulder abduction, and shoulder external 53 rotation) were observed during contralateral head rotation only in individuals with stroke. EMG 54 data of the associated muscle co-activations were non-significant however are presented for 55 consideration in light of a likely underpowered statistical design for this specific variable. This 56 study provides evidence for not only the re-emergence of ATNR following stroke, but also 57 indicates a common neuroanatomical link, namely an increased reliance on ipsilateral 58 reticulospinal pathways, as the likely mechanism underlying the expression of both ATNR and 59 flexion synergy that results in the loss of independent joint control. 60 61 62