Flexion synergy overshadows flexor spasticity during reaching in chronic moderate to severe hemiparetic stroke

Objective: Pharmaceutical intervention targets arm flexor spasticity with an often-unsuccessful goal of improving function. Flexion synergy is a related motor impairment that may be inadvertently neglected. Here, flexor spasticity and flexion synergy are disentangled to determine their contributions to reaching dysfunction. Methods: Twenty-six individuals participated. A robotic device systematically modulated shoulder abduction loading during ballistic reaching. Elbow muscle electromyography data were partitioned into windows delineated by elbow joint velocity allowing for the separation of synergy-and spasticity-related activation. Results: Reaching velocity decreased with abduction loading (p < 0.001) such that velocity was 30% slower when lifting the arm at 50% of abduction strength compared to when arm weight was supported. Abnormal flexion synergy increased with abduction loading (p < 0.001) such that normalized activation ranged from a median (interquartile range) of 0.07 (0.03-0.12) when arm weight was supported to 0.19 (0.12-0.40) when actively lifting (large effect size, d = 0.59). Flexor spasticity was detected during reaching (p = 0.016) but only when arm weight was supported (intermediate effect size, d = 0.33). Conclusion: Flexion synergy is the predominant contributor to reaching dysfunction while flexor spasticity appears only relevant during unnaturally occurring passively supported movement. Significance: Interventions targeting flexion synergy should be leveraged in future stroke recovery trials. (C) 2017 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.