Alterations in upper limb muscle synergy structure in chronic 1 stroke survivors

40 Previous studies in neurologically intact subjects have shown that motor coordination can be 41 described by task-dependent combinations of a few muscle synergies, defined here as a fixed 42 pattern of activation across a set of muscles. Arm function in severely impaired stroke survivors 43 is characterized by stereotypical postural and movement patterns involving the shoulder and 44 elbow. Accordingly, we hypothesized that muscle synergy composition is altered in severely 45 impaired stroke survivors. Using an isometric force matching protocol, we examined the spatial 46 activation patterns of elbow and shoulder muscles in the affected arm of 10 stroke survivors 47 (Fugl-Meyer < 25/66) and in both arms of 6 age-matched controls. Underlying muscle 48 synergies were identified using non-negative matrix factorization. In both groups, muscle 49 activation patterns could be reconstructed by combinations of a few muscle synergies (typically 50 4). We did not find abnormal coupling of shoulder and elbow muscles within individual muscle 51 synergies. In stroke survivors, as in controls, two of the synergies were comprised of isolated 52 activation of the elbow flexors and extensors. However, muscle synergies involving proximal 53 muscles exhibited consistent alterations following stroke. Unlike controls, the anterior deltoid 54 was coactivated with medial and posterior deltoids within the shoulder abductor/extensor 55 synergy and the shoulder adductor/flexor synergy in stroke was dominated by activation of 56 pectoralis major, with limited anterior deltoid activation. Recruitment of the altered shoulder 57 muscle synergies was strongly associated with abnormal task performance. Overall, our results 58 suggest that an impaired control of the individual deltoid heads may contribute to post-stroke 59 deficits in arm function. 60 61 62 63 64 65 66 67