![]() To retrieve the structures of muscle synergies from the variability of muscle activations, we and others have utilized a computational procedure, the non-negative matrix factorization algorithm (NMF), originally proposed by Lee and Seung (1999, 2001). Most voluntary movements are the result of the simultaneous activation of a few muscle synergies via descending or afferent pathways which produces a complex electromyographic (EMG) pattern in the limb’s muscles. These results together argue that the experimentally derived force fields are generated by discrete groups of muscles activated as individual units, or muscle synergies, from whose linear combination a vast repertoire of movement and posture could be generated. A subsequent simulation study showed that the combinations of frog hind-limb muscles that produced stable force fields were similar to the muscle groups observed to be co-activated in spinal microstimulation studies ( Loeb et al., 2000). Vector summation of force fields led to the hypothesis that generation of movement and posture may be based on the combination of a few discrete motor primitives. Similar results were also obtained by Hogan and colleagues ( Lemay et al., 2001), Tresch and Bizzi (1999), and Kargo and Giszter (2000). When the pattern of forces resulting from co-stimulation were compared with those computed by linear summation of the two individual fields, the co-stimulation fields and the summation fields were found to be equivalent in 83% of the cases ( Mussa-Ivaldi et al., 1994). ![]() Following the initial description of the force field, Mussa-Ivaldi and others found that co-stimulation of two spinal sites led to vector summation of the forces generated at each site separately. In the spinal frog and rat, different groups of force field were activated as the stimulating electrode was moved to different loci of the lumbar spinal cord in the rostro-caudal and medio-lateral directions. The output of an activated module can be characterized as a force field, or the collection of isometric muscle forces generated at the limb’s endpoint over different locations of the workspace. A spinal module is a functional unit of spinal interneurons that generates a specific motor output by imposing a specific pattern of muscle activation. With these approaches, we and others were able to provide the experimental basis for a modular organization of the spinal cord circuitry in the frog (with the studies cited above), rat ( Tresch and Bizzi, 1999), and cat ( Lemay and Grill, 2004). ![]() The core argument for the neural origin of motor modules rests on studies of the spinal cord in several vertebral species, conducted using a variety of techniques such as microstimulation ( Bizzi et al., 1991 Giszter et al., 1993), N-methyl-D-aspartate (NMDA) iontophoresis ( Saltiel et al., 2001, 2005), and cutaneous stimulation ( Tresch et al., 1999). EXPLICIT ENCODING OF MUSCLE SYNERGIES IN THE NERVOUS SYSTEM
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