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Todd C. Pataky, Greg P. Slota, Mark L. Latash and Vladimir M. Zatsiorsky

During power grasp, the number of local force maxima reflects either the central nervous system’s preferential use of particular hand regions, or anatomical constraints, or both. Previously, both bimodal and trimodal force maxima have been hypothesized for power grasp of a cylindrical handle. Here we measure the number of local force maxima, with a resolution of 4.8°, when performing pushing and pulling efforts in the plane perpendicular to the cylinder’s long axis. Twelve participants produced external forces to eight targets. The number of contacts was defined as the number of local maxima exceeding background variance. A minimum of four and a maximum of five discrete contacts were observed in all subjects at the distal phalanges and metacarpal heads. We thus reject previous hypotheses of bimodal or trimodal force control for cylindrical power grasping. Since we presently observed only 4–5 contacts, which is rather low considering the hand’s kinematic flexibility in the flexion plane, we also reject hypotheses of continuous contact, which are inherent to current grasping taxonomy. A modification to current grasping taxonomy is proposed wherein power grasp contains separate branches for continuous and discrete contacts, and where power and precision grasps are distinguished only by grasp manipulability.

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Elaine Leonezi Guimarães, Andréa Baraldi Cunha, Daniele de Almeida Soares and Eloisa Tudella

The aim of this systematic literature review was to examine and discuss studies that investigated reaching in preterm infants during the first year of life. Databases were searched using keywords such as reaching, grasping, preterm, and premature, in addition to specific terms from the Medical Subject Headings (MeSH) (motor skills, infant, movement, premature birth, hands) regardless of year of publication. One hundred thirty-five studies were identified, 9 of which were selected. The results showed that preterm infants adopt strategies (bimanual reaches and reaches with less rectilinear trajectories toward an object in motion, reaches with semi-open and open hand, reaches at lower speeds, with increased movement units, and variable postural muscle activity) compared with full-term infants. However, the results on how intrinsic factors (e.g., prematurity) and extrinsic factors (e.g., body position, physical properties of the object) influence early reaching are still limited.

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Paola Cesari and Karl M. Newell

The experiment reported examined: (a) the role of the geometrical body scaled informational invariant for the transition of human grip configurations; (b) whether the same invariant can be scaled considering also the force applied during the grasp phase; and (c) how the temporal duration of the grasp and displacement phases of prehension are scaled to the object properties of size and mass. Adult subjects performed a series of trials in reaching, grasping, and displacing spheres that varied in size and mass. The grip transitions were described by the body scaled relation:

where Ls and Ms are, respectively, the diameter and the mass of the spheres grasped and Lh and Mh are the length and the mass of the hand. The impulse during the grasp phase was linearly related with the mass of the spheres within each density. The temporal durations of the grasp and displacement components were scaled coherently to the object properties. These findings provide support to the hypothesis that the grasp and displacement components of prehension are organized coherently within a single action.

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Joel R. Martin, Mark L. Latash and Vladimir M. Zatsiorsky

This study investigated the effects of modifying contact finger forces in one direction—normal or tangential—on the entire set of the contact forces, while statically holding an object. Subjects grasped a handle instrumented with finger force-moment sensors, maintained it at rest in the air, and then slowly: (1) increased the grasping force, (2) tried to spread fingers apart, and (3) tried to squeeze fingers together. Analysis was mostly performed at the virtual finger (VF) level (the VF is an imaginable finger that generates the same force and moment as the four fingers combined). For all three tasks there were statistically significant changes in the VF normal and tangential forces. For finger spreading/squeezing the tangential force neutral point was located between the index and middle fingers. We conclude that the internal forces are regulated as a whole, including adjustments in both normal and tangential force, instead of only a subset of forces (normal or tangential). The effects of such factors as EFFORT and TORQUE were additive; their interaction was not statistically significant, thus supporting the principle of superposition in human prehension.

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Geert J.P. Savelsbergh, John van der Kamp and Walter E. Davis

Twenty-one children with Down syndrome (DS) and 20 without disability, ages 3 to 11 years, completed the experiment in which they were asked to grasp and lift cardboard cubes of different sizes (2.2 to 16.2 cm in width). Three conditions were used: (a) increasing the size from the smallest to the largest cube, (b) decreasing the size from the largest to the smallest, and (c) a random order of sizes. Children with DS were found to have smaller hand sizes in comparison to age-matched children without DS. In addition, the shift from one-handed to two-handed grasping appeared at a smaller cube size for children with DS than for children without DS. However, when the dimensionless ratio between object size and hand size was considered, the differences between groups disappeared, indicating that the differences in grasping patterns between children with and without DS can be attributed to differences in body size.

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Markus Janczyk and Wilfried Kunde

According to the Perception-Action-Model (PAM), the human visual cortex consists of the ventral vision-for-perception and the dorsal vision-for-action streams. Performance decrements with increasing variation of nominally task-irrelevant stimulus features (Garner-Interference) was suggested as an empirical tool for identifying contributions of these streams: vision-for-perception, but not visionfor-action, should suffer from Garner-Interference, but inconsistencies in this argument were revealed by several studies. We here used a new manipulation to induce Garner-Interference in a dorsal task: The stimulus objects did not differ in their lengths but in the side to which they were weighted. In Experiment 1, Garner-Interference was found in a ventral perceptual judgment task. Notably, we did also find Garner-Interference in skilled right-handed grasping in Experiment 2. These findings suggest that the presence or absence of Garner-Interference does not consistently index the contribution of different processing streams for perception and action, but the coprocessing of nominally task-irrelevant stimulus features in general, be it for perception of action.

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Pietro G. Morasso, Vittorio Sanguineti and Francesco Frisone

Although it is true that the specific research on grasping has been dominated in recent years by the canonical transport + grip model originally formulated by Jeannerod (1984), still one can find in the research on reaching a number of links and anticipations to the new view on grasping made explicit by the authors of the target article. This paper reviews some of the relevant concepts and outlines a modeling framework that aims at biological plausibility.

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Bert Steenbergen

The present reaction on the paper of Smeets and Brenner focuses on two premises of their proposed model The first is that grasping is nothing more than pointing with two fingers. It is argued that this assumption cannot be upheld in light of the differences between both actions with respect to neuromuscular structures, muscular innervation, use of visual feedback, and basic function The second premise of the model is that the velocity profile of the transport component is symmetrical and independent of intrinsic object properties. It is shown that the symmetrical velocity profile represents a boundary condition and is influenced by intrinsic object properties. Given these concerns, it is doubtful that the model in its present form will add much to our understanding of the control of grasping.

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Jeroen B.J. Smeets and Eli Brenner

We begin our response by discussing the commentators' arguments concerning our proposal to abandon the classical distinction between transport and grip. In the second section, we argue that the minimum-jerk model is not fundamental to our approach, but very convenient. In the third section, we discuss how the experimental results that the commentators mention fit into our new approach. We conclude that the predictive capacity of our model, combined with its simplicity, makes it very useful for understanding grasping.

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Edwin Van Thiel and Bert Steenbergen

In this study, we examined the degree and timing of shoulder displacements during hitting, reaching, and grasping movements performed by young adults with hemiparetic cerebral palsy. The participants performed unimanual and bimanual arm movements towards targets and objects of different sizes. On the basis of the assumption that shoulder displacement due to trunk translation and rotation is a successful, adaptive reaction to reduced joint mobility in the affected arm, the fluency of hand displacements was expected to remain invariant under variations of shoulder displacement as is also the case in healthy participants. The results point in this direction. With respect to the timing of shoulder displacement, prior research suggested that hemiparetic movements can be characterized by inconsistent motion-timing patterns—that is, the timing of shoulder and hand-displacement onsets varied between trials. Therefore, the within-subject variability of the movement-onset asynchrony between hand and ipsilateral shoulder displacement was expected to be larger on the impaired side than on the unimpaired side. This prediction was not confirmed. which challenges these earlier conclusions. Additionally. we also examined the peak-velocity asynchrony of the hand and shoulder. Contrary to the onset asynchrony, the peak asynchrony varied between the hitting and reaching task and between the hitting and grasping task. For the reaching and grasping tasks, there were also significant differences between hands. Again, variability of the (peak-velocity) asynchrony was not significantly increased when comparing the impaired hand with the unimpaired hand. The results suggests that the hemiparetic participants were capable of flexibly recruiting and sequencing the various degrees of freedom of their impaired side required for successful task completion, albeit in different magnitudes and sequenced differently.