Abstract
Muscle synergies describe common patterns of co- or reciprocal activation that occur during movement. After stroke, these synergies change, often in stereotypical ways. The mechanism underlying this change reflects damage to key motor pathways as a result of the stroke lesion, and the subsequent reorganization along the neuroaxis, which may be further detrimental or restorative to motor function. The time course of abnormal synergy formation seems to lag spontaneous recovery that occurs in the initial weeks after stroke. In healthy individuals, motor cortical activity, descending via the corticospinal tract (CST) is the predominant driver of voluntary behavior. When the CST is damaged after stroke, other descending pathways may be up-regulated to compensate. The contribution of these pathways may emerge as new synergies take shape at the chronic stage after stroke, as a result of plasticity along the neuroaxis. The location of the stroke lesion and properties of the secondary descending pathways and their regulation are then critical for shaping the synergies in the remaining motor behavior. A consideration of the integrity of remaining descending motor pathways may aid in the design of new rehabilitation therapies.
Highlights
Muscle synergies describe common patterns of co- or reciprocal activation that occur during movement
Non-negative matrix factorization (NNMF) does not capture inhibitory relationships, which may be a limitation of the method
We adopt the general definition of the term synergy given above, reference will be made to clinically abnormal synergies as well as synergies identified by matrix factorization, and the caveats with regard to the definitions of each should be borne in mind
Summary
Muscle synergies describe common patterns of co- or reciprocal activation that occur during movement. Cheung et al (2009) reported that the underlying structure of muscle synergies is preserved after stroke when patients performed upper limb reaching tasks. An informative approach undertaken by García-Cossio et al (2014) reported differences in the pattern of upper limb synergy expression that were dependent upon the affected anatomical structures (i.e., preservation of sensorimotor cortex).
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