In many patients, when weakness and spasticity are treated effectively, or resolve spontaneously, motor dysfunction remains severe ( Landau, 1980 Hesse et al., 1996 Dewald et al., 2001). Overall, our results suggest that alterations in the shoulder muscle synergies underlying isometric force generation appear prominently in mild and moderate stroke, as in most cases of severe stroke, in an impairment level-dependent manner.īroadly speaking, disturbances of motor function following stroke can be attributed to three primary impairments: reduced corticospinal drive to agonist muscles (weakness), altered reflex activity (e.g., spasticity), and impaired motor coordination ( Twitchell, 1951 Brunnstrom, 1970 Knutsson and Mårtensson, 1980). Subsequent analysis confirmed that the incidences of three elbow-related synergies were independent of impairment level, while the incidences of four shoulder-related synergies were systematically correlated with impairment level. Cluster analysis of pooled synergies for the 30 subjects identified seven distinct clusters (synergies). In contrast, the shoulder-related synergies of some stroke survivors with mild and moderate impairment differed from controls, in a manner similar to severely impaired subjects. Broadly speaking, elbow-related synergies were conserved across stroke survivors, regardless of impairment level. For all groups, EMG spatial patterns were well explained by task-dependent combinations of only a few (typically 4) muscle synergies. We applied non-negative matrix factorization to identify the muscle synergies and compared their structure across groups, including previously collected data from six age-matched control subjects and eight severely impaired stroke survivors. Accordingly, we examined muscle synergies underlying spatial patterns of elbow and shoulder muscle activation recorded during an isometric force target matching protocol performed by 16 chronic stroke survivors, evenly divided across mild and moderate impairment levels. However, whether similar alterations are evident in stroke survivors with lesser impairment remains unclear. Our recent study of severely impaired chronic stroke survivors showed that some muscle synergies underlying isometric force generation at the hand are altered in the affected arm. Previous studies indicate that motor coordination may be achieved by assembling task-dependent combinations of a few muscle synergies, defined here as fixed patterns of activation across a set of muscles. 4Department of Biomedical Engineering, Northwestern University, Chicago, IL, USA.

3Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, IL, USA.2Department of Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA.1Department of Kinesiology, Temple University, Philadelphia, PA, USA.

Muscle synergy upper lower