Intrusion of pathological synergies does not explain impaired 3D arm movements in subacute stroke

It has long been of interest what the contribution of pathological synergies is to abnormal motor control of the arm after stroke. The flexor synergy is defined as unwanted co-activation of flexion at the shoulder and elbow joints. Here we used a video-based, marker-less 3D pose-estimation kinematic model to examine two reaching tasks with different requirements for elbow flexion. Twenty-eight sub-acute (2-12 weeks) post-stroke subjects and sixteen healthy controls performed both a cup to mouth task, requiring shoulder and elbow flexion (within flexor synergy), and a reaching task, requiring shoulder flexion and elbow extension (outside of flexor synergy). Using kinematic analysis of the hand and of elbow/shoulder joint angles, we assessed both overall task performance and intrusion of pathological synergies. Motor impairment, strength and spasticity were measured using established clinical scales. Performance in both tasks was impaired to a similar degree in the patients compared to controls. This lack of performance difference for the in- vs. out-of-flexor synergy tasks was consistent with our finding of no evidence for intrusion of a flexor synergy in the reaching task. Specifically, for the reaching task there was no difference between patients and controls either in time spent within-movement in flexor synergy or in the correlation between shoulder and elbow angles when the shoulder was flexing. A regression analysis indicated that the only significant predictor of poor task performance was degree of weakness. Notably, even though we found no kinematic evidence for post-stroke intrusive flexor synergies, the Fugl-Meyer Assessment (FMA), which was devised to quantify post-stroke synergies, was markedly abnormal. This seeming contradiction is resolved by the observation that abnormal coupling at the shoulder and elbow can occur due to weakness alone, and it is this "synergy mimic" that leads to a low FMA score. The finding that FMA can be abnormal for two qualitatively distinct forms of impaired inter-joint coordination has implications for the interpretation of longitudinal studies that use this single measure. In the sub-acute stage of stroke, intrusion of abnormal synergies is not the reason for impaired reaching. ### Competing Interest Statement The authors have declared no competing interest.