Hand Synergies During Reach-to-Grasp

Abstract

An emerging viewpoint is that the CNS uses synergies to simplify the control of the hand. Previous work has shown that static hand postures for mimed grasps can be described by a few principal components in which the higher order components explained only a small fraction of the variance yet provided meaningful information. Extending that earlier work, this study addressed whether the entire act of grasp can be described by a small number of postural synergies and whether these synergies are similar for different grasps. Five right-handed adults performed five types of reach-to-grasps including power grasp, power grasp with a lift, precision grasp, and mimed power grasp and mimed precision grasp of 16 different objects. The object shapes were cones, cylinders, and spindles, systematically varied in size to produce a large range of finger joint angle combinations. Three-dimensional reconstructions of 21 positions on the hand and wrist throughout the reach-to-grasp were obtained using a four-camera video system. Singular value decomposition on the temporal sequence of the marker positions was used to identify the common patterns ("eigenpostures") across the 16 objects for each task and their weightings as a function of time. The first eigenposture explained an average of 97.3 +/- 0.89% (mean +/- SD) of the variance of the hand shape, and the second another 1.9 +/- 0.85%. The first eigenposture was characterized by an open hand configuration that opens and closes during reach. The second eigenposture contributed to the control of the thumb and long fingers, particularly in the opening of the hand during the reach and the closing in preparation for object grasp. The eigenpostures and their temporal evolutions were similar across subjects and grasps. The higher order eigenpostures, although explaining only small amounts of the variance, contributed to the movements of the fingers and thumb. These findings suggest that much of reach-to-grasp is effected using a base posture with refinements in finger and thumb positions added in time to yield unique hand shapes.

Patterns of Hand Motion during Grasping and the Influence of Sensory Guidance

“…This is in contrast to a previous procedure in which we treated the data from each joint as a separate "trial," yielding weighting coefficients that differed across joint angles (Soechting and Flanders, 1997). It is also in contrast to the procedure used recently by Mason et al (2001). In their analysis, the PC s in Equation 4 were constants in time, but the weighting coefficients a in varied with time.…”

Unvealing the Principal Modes of Human Upper Limb Movements through Functional Analysis

“…Not surprisingly, a lot of studies have been devoted to model how the nervous system can cope with the complexity of hand sensory-motor architecture (Mason et al, 2001;Todorov and Ghahramani, 2004;Zatsiorsky and Latash, 2004;Thakur et al, 2008;Gabiccini et al, 2013;Santello, 2014). These studies have led to the definition of the so-called synergies, broadly intended as covariation patterns that can be represented at different levels (Santello et al, 2016).…”

“…At the muscle level, different works explored patterns of muscle activity whose timing and/or amplitude modulation enables the generation of different movements, see d ' Avella and Lacquaniti (2013) for a review. Synergies have also been identified and defined as covariation patterns of joint angles, e.g., hand postural synergies (Santello et al, 1998(Santello et al, , 2013Mason et al, 2001), or covariation patterns among digit forces [for a review see Zatsiorsky and Latash (2004)]. However, to correctly understand human manipulation, in addition to hand analysis, the role of whole upper limb movements should be also taken into account.…”

Biologically inspired kinematic synergies enable linear balance control of a humanoid robot

“…This suggests that a set of muscles (multiple degrees of freedom) are controlled by a low dimensional (possibly even one-dimensional) variable. Other experiments suggest that this principle of kinematic synergies is present over a wide range of different movements like reaching and grasping (Mason et al 2001), upper-arm movement (Sabatini 2002), and making a step (Wang et al 2005). Hence, kinematic synergies seem to present a general strategy biological organisms apply.…”