Modulation of Muscle Synergy Recruitment in Primate Grasping

Abstract

In grasping, the CNS controls a particularly large number of degrees of freedom. We tested the idea that this control is facilitated by the presence of muscle synergies. According to the strong version of this concept, these synergies are invariant, hard-wired patterns of activation across muscles. Synergies may serve as modules that linearly sum, each with specific amplitude and timing coefficients, to generate a large array of muscle patterns. We tested two predictions of the synergy model. A small number of synergies should (1) account for a large fraction of variation in muscle activity, and (2) be modulated in their recruitment by task variables, even in novel behavioral contexts. We also examined whether the synergies would (3) have broadly similar structures across animals. We recorded from 15 to 19 electrodes implanted in forelimb muscles of two rhesus macaques as they grasped and transported 25 objects of variable shape and size. We show that three synergies accounted for 81% of the electromyographic data variation in each monkey. Each synergy was modulated in its recruitment strength and/or timing by object shape and/or size. Even when synergies were extracted from a small subset of object shape and size conditions and then used to reconstruct the entire dataset, we observed highly similar synergies and patterns of modulation. The synergies were well conserved between monkeys, with two of the synergies exceeding chance structural similarity, and the third being recruited, in both animals, in proportion to the size of the object handled.

Electromyogram pattern recognition for control of powered upper-limb prostheses: State of the art and challenges for clinical use

“…Although the re have be en attempts to accomplish this with EMG [56,[75][76][77], none of these approaches have been found to be clinically reliable. It is unlikely that robust, simulta neous, independent control of multiple degrees of free dom is attainable for the person with transradial ampu tation using surfa ce EMG because of the complex nature of forearm muscle synergies [78][79], the inherent cross talk in the surface signal [75,80], and the displacement in these muscles that occurs during c ontraction. Various approaches have attempted to decompose the multimuscle syner gies into the activity of consti tuent muscles, but the models are very sensitive to anatomical and electrophysiological factors that may change during use.…”

Evidence of muscle synergies during human grasping

“…A similar line of research has been so far pursued, as far as we know, only in primates and only loosely in humans. Primates have been shown to generate consistent-across-subject sEMG patterns in [7,27] when engaged in simple grasping actions; in those works up to 19 needle (invasive) sEMG electrodes were used on (in both papers) two animals. In particular in [27] it was shown that three synergies accounted for 81% of the sEMG variance, but the analysis performed was time-dependent, meaning that synergies are short temporal profiles of activation rather than single sEMG samples.…”

“…Primates have been shown to generate consistent-across-subject sEMG patterns in [7,27] when engaged in simple grasping actions; in those works up to 19 needle (invasive) sEMG electrodes were used on (in both papers) two animals. In particular in [27] it was shown that three synergies accounted for 81% of the sEMG variance, but the analysis performed was time-dependent, meaning that synergies are short temporal profiles of activation rather than single sEMG samples. In this work we concentrate on a simpler PCA-based dimensionality reduction (which is the type effectively used in [7] as well as, e.g., in [29]) and obtain quantitatively similar results, that is, as far as the amount of variance is concerned.…”

Modeling discrete and rhythmic movements through motor primitives: a review

“…It is not known yet if the concept of force fields can be extended to higher vertebrates, but it has been shown that a finite set of (time-variant) synergies of muscles could account for the movement generation in humans during fast reaching movements (d'Avella et al (2006)) as well as in primate grasping (Overduin et al (2008)), providing evidence for the existence of motor primitives.…”