Why it matters: Previous work in animals and humans has shown that groups of muscles tend to be co-activated as a unit, in predicable patterns, or synergies, across a wide range of movements.
These synergies are thought to represent the fundamental building blocks from which the brain constructs complex movements. The new findings support this concept and also suggest new approaches to the rehabilitation of stroke patients. Stroke is a leading cause of long-term disability in the US, with about 700,000 new or recurrent cases each year.
How they did it: The researchers, led by Emilio Bizzi, an MIT Institute Professor and a member of the McGovern Institute for Brain Research and the Department of Brain and Cognitive Sciences, used electromyographic (EMG) recording to measure activity in arm and shoulder muscles of 8 stroke patients as they performed a variety of reaching movements. The patients had stroke damage in one cortical hemisphere only, so one arm was impaired while the other was largely unaffected. The researchers used computational methods to identify groups of muscles whose activation was correlated across movements. In 7 out of 8 patients, these correlations, or synergies, were largely identical between the affected and unaffected arms, even though the actual movements were very different between the two arms. The results support the view that the synergies are encoded in the brainstem or spinal cord, areas that were unaffected in these patients. “We show that descending neural signals from the motor cortex select, activate and combine a small number of muscle synergies that are specified by networks in the spinal cord or brainstem,” Bizzi explains, “and different movements emerge as these synergies are recruited to various degrees.”
Next steps: The findings suggest a new approach to the rehabilitation of stroke patients. By identifying synergies whose activations are affected following a stroke, it may be possible to develop focused rehabilitation methods that specifically train the impaired synergies. As a first step toward this goal, the researchers plan to monitor a group of stroke patients as they undergo rehabilitation therapy, to determine whether the post-stroke improvements in motor function can be explained as changes in the activation pattern of specific synergies.
Source: Cheung VC, Piron L, Agostini M, Silvoni S, Turolla A, Bizzi E. (2009). Stability of muscle synergies for voluntary actions after cortical stroke in humans. Proc Natl Acad Sci (USA). Oct 19 2009.
Funding: McGovern Institute for Brain Research at MIT and the Italian Ministry of Health
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