Patterns of Muscle Activity Underlying Object-Specific Grasp by the Macaque Monkey

Abstract

During object grasp, a coordinated activation of distal muscles is required to shape the hand in relation to the physical properties of the object. Despite the fundamental importance of the grasping action, little is known of the muscular activation patterns that allow objects of different sizes and shapes to be grasped. In a study of two adult macaque monkeys, we investigated whether we could distinguish between EMG activation patterns associated with grasp of 12 differently shaped objects, chosen to evoke a wide range of grasping postures. Each object was mounted on a horizontal shuttle held by a weak spring (load force 1-2 N). Objects were located in separate sectors of a "carousel," and inter-trial rotation of the carousel allowed sequential presentation of the objects in pseudorandom order. EMG activity from 10 to 12 digit, hand, and arm muscles was recorded using chronically implanted electrodes. We show that the grasp of different objects was characterized by complex but distinctive patterns of EMG activation. Cluster analysis shows that these object-related EMG patterns were specific and consistent enough to identify the object unequivocally from the EMG recordings alone. EMG-based object identification required a minimum of six EMGs from simultaneously recorded muscles. EMG patterns were consistent across recording sessions in a given monkey but showed some differences between animals. These results identify the specific patterns of activity required to achieve distinct hand postures for grasping, and they open the way to our understanding of how these patterns are generated by the central motor network.

Evidence of muscle synergies during human grasping

“…Such a force configuration might effectively be planned in advance using the sEMG signal of a human subject, for instance during teleoperation, mapped onto the dynamics of the robotic gripper. Indeed, in [6] it was shown that different EMG signals related to different objects differed not only during grip but also during the reaching movement towards the object;…”

“…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.…”

“…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.…”

Relation between object properties and EMG during reaching to grasp

“…We investigate the usability of this information from forearm sEMG signals. Brochier et al (2004) were able to distinguish between different objects by using EMG-signals recorded from electrodes implanted in the hand, arm, and finger muscles of monkeys. They showed that the recorded signals were different for each object during a reaching movement as well as during a grasp.…”

Spatiotemporal distribution of location and object effects in the electromyographic activity of upper extremity muscles during reach-to-grasp

“…The problems of relating the activity of various muscles to the mechanics of the grasping hand are complex (Schaffelhofer et al 2015;Towles et al 2013;ValeroCuevas 2005;Valero-Cuevas et al 2003). Nevertheless, when 6 -8 objects, which differ in size and shape, are all presented at the same location, the object that is being grasped can be decoded from the EMG activity of 8 -12 forearm and intrinsic hand muscles (Brochier et al 2004;Fligge et al 2013). The decoding accuracy increases progressively during the movement, approaching 100% correct object classification midway through the reach, indicating that patterns of activation across the set of distal muscles become more object specific as the movement evolves.…”