Internal forces during object manipulation

Gao, Fan; Latash, Mark L.; Zatsiorsky, Vladimir M.

doi:10.1007/s00221-005-2282-1

Cited by 52 publications

(77 citation statements)

References 41 publications

(35 reference statements)

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“…The Varignon theorem of classical mechanics was used to calculate the VF and thumb moment arms with respect to the handle's center of mass; then the distances were subtracted from each other to calculate the couple arm, D, Equation (5) (5) where is the normal force of the finger f and d f is the moment arm of the same finger; also, is the normal force of the thumb and is the moment created at the thumb fingertip about the z-axis. The couple arm can be seen as a projected vertical distance between the points of application of the VF and thumb normal forces.…”

Section: Modelingmentioning

confidence: 99%

Digit force adjustments during finger addition/removal in multi-digit prehension

2008

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We explored adjustments in multi-digit coordinated action on a hand-held object with finger addition and removal. The subjects (n= 7) kept a vertically oriented handle at rest using a prismatic grasp as if holding a glass of liquid and then either added one finger to the grasp-the index (I) or little (L) finger-or removed one finger. Three external torques were applied on the apparatus: clockwise, counterclockwise, and no torque. The individual digit forces and moments were recorded with 6-component sensors. The change in grasping force depended on the function of the manipulated finger, i.e. on whether the finger resisted external torque (torque agonist) or assisted it (torque antagonist). There was a significant increase of the grasping force when an antagonist was added or when an agonist was removed. These force increases were not necessary for slipping prevention: the normal forces prior to the manipulation were large enough to prevent slipping. All other finger manipulations exhibited no significant change in the grip force, except for the antagonist removal during the supination efforts (after removing the I finger the grasping force decreased). In contrast, the changes in the tangential force depended on the manipulated finger, not on the finger function with respect to external torque. There was a significant thumb force increase when the I finger was added or when the L finger was removed; opposite changes were seen when the L finger was added or the I finger was removed. The changes of the virtual finger (VF) tangential force were equal and opposite to the thumb tangential force alterations; these opposite changes caused changes in the moments these forces generated. The changes in the moments of the tangential forces were counterbalanced by the opposite changes in the moments of normal forces such that the total moment remained constant and the handle orientation was maintained. At the level of individual finger (IF) forces two strategies of error compensation were found: (a) local error compensation -the opposite action of the neighboring finger, i.e. force decrease in response to a force increase (finger addition), and vice versa, and (b) distant error compensation -similar action by a finger that is a torque antagonist to the manipulated finger. During the transient periods, the changes in the thumb and VF forces were simultaneous and equal in magnitude. The normal forces increased or decreased concurrently while the changes in the tangential forces were opposite in direction. The data support the existence of chain effects in the digit force adjustments to finger addition or removal. We conclude that the digit force adjustments during the object manipulation are controlled mainly in a feed-forward manner. The obtained data agree with the principle of superposition reported previously. The findings agree with earlier reports on the limited ability of CNS to organize synergies at two levels of a control hierarchy simultaneously.

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Section: Modelingmentioning

confidence: 99%

Digit force adjustments during finger addition/removal in multi-digit prehension

2008

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“…One can discuss the issue of the interfinger force compensation mathematically by invoking the concept of null spaces. The null space of an m × n matrix G is the set of all vectors f in a state space R n such that Gf = 0 (for a more detailed discussion of the null spaces in multifinger prehension, see Gao, Latash, & Zatsiorsky, 2005). Let f be a 4 × 1 vector of normal finger forces, F be a 2 × 1 vector of the resultant consisting of the VF normal force and M n , and G be a 2 × 4 grasp matrix.…”

Section: Local and Synergic Friction Effectsmentioning

confidence: 99%

Adjustments to Local Friction in Multifinger Prehension

Aoki

Latash

Zatsiorsky

2007

Journal of Motor Behavior

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The authors studied the effects of surface friction at the digit-object interface on digit forces and moments when 12 participants statically held an object in a 5-digit grasp. The authors changed lowfriction contact (LFC) with rayon and high-friction contact (HFC) with sandpaper independently for each digit in all 32 possible combinations. Normal forces of the thumb and virtual finger (VF), an imagined finger with a mechanical effect equal to that of the 4 fingers, increased with the thumb at LFC or with an increase in the number of fingers at LFC. When the thumb was at LFC, the thumb tangential force decreased. The VF tangential force decreased when the number of fingers at LFC increased. The interaction of the local responses to friction and the synergic responses necessary to maintain the equilibrium explain the coordination of individual digit forces.Keywords finger forces; friction; grasp stability; grasping; prehension When grasping an object, people adjust digit forces to the friction: More-slippery objects are grasped more strongly, resulting in a higher ratio of grip force (digit force normal to the grip surface) to load force (digit force tangential to the grip surface; Cadoret & Smith, 1996;Cole & Johansson, 1993; Johansson, 1996;Johansson & Westling, 1984b, 1987. Some researchers have examined participants' force adjustments when they used two or three digits to grasp objects that had a different friction for each digit (Burstedt, Edin, & Johansson, 1997;Burstedt, Flanagan, & Johansson, 1999;Edin, Westling, & Johansson, 1992;Quaney & Cole, 2004). The researchers found that a grip-load force ratio for individual digits was scaled on the basis of current local friction at those digits. Participants made the adjustments by modulating normal (grip force) and tangential (load force) digit forces. Similar force adjustments occur when two individuals do the grasping (Burstedt et al., 1997). It has therefore been concluded that coordination of individual digit-tip forces during human manipulation emerges from independent neural networks that control each engaged digit. That low level of control must be subordinate to a higher level of control related to other aspects of manipulative tasks. The investigators mentioned such aspects of the higher level control as (a) temporal synchronization of digit actions before substantial manipulative forces are applied to the object, (b) selecting grasp configurations, and (c) coping with various constraints imposed by the physical properties of the object, such as its weight and size. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptCompared with the two-digit and tripod grasps, in grasps performed by all five digits, performers undoubtedly have greater freedom to select interdigit force sharing. During multifinger prehension, however, the forces and moments exerted on the object by individual digits always change in a highly coordinated manner (Baud-Bovy & Soechting, 2001; Iberall, 1987;Kinoshita, Kawai, & Ikuta, 1995;Santello & Soecht...

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“…On the other hand, there is a well-established coupling between the grip force and the load force in the pinch grasp, when the load (gravitational or inertial forces) is perpendicular to the line defined by the contact points that define the grip force [27]. Such a coupling has also been evidenced in multi-fingered grasps when the load acted in a direction that was orthogonal to the null space of the grasp [17]. The grasping force was also found to covary with the load when the net force was not orthogonal to the null space of the grasp but, in this case, the role of the CNS is more difficult to assess because variations of the internal forces can at least in part be ascribed to the visco-elastic properties of the fingers [48].…”

Section: Mefs and Motor Synergiesmentioning

confidence: 98%

“…They also raise the question of whether the grip and net forces are governed by different neurophysiological mechanisms. As a matter of fact, various studies have considered the possibility that contact forces can be decomposed in a grasping and manipulative component, each being controlled independently as in some robotic manipulators (e.g., [55,17,48]). In the context of our study, the idea is that areas upstream with respect to the point of stimulation would control the manipulative component and maintain the motor cortex in a dynamic state whereby the finger forces produced the desired net force.…”

Section: Mefs and Motor Synergiesmentioning

confidence: 99%

Contact forces evoked by transcranial magnetic stimulation of the motor cortex in a multi-finger grasp

Baud-Bovy

Prattichizzo

Rossi

2008

Brain Research Bulletin

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Transcranial magnetic stimulation (TMS) is a tool of choice to study the functionality of the corticospinal pathway. In this study, we used single-pulse TMS at different intensities and during different levels of grasping force, to stimulate the hand area of the left primary motor cortex (M1). We measured, the TMS-evoked forces, or motor evoked forces (MEFs) in a multi-fingered three-point grasp in addition of the conventional motor evoked potentials (MEPs) from the right forearm and intrinsic hand muscles. This paper aims at presenting the viability of this innovative approach and some preliminary results. The timing (i.e., latencies and peak times), amplitudes and directions of the MEF were analyzed. We found that the TMS evoked synergistic increases of the force magnitudes, akin to those observed when participants voluntarily increased the grip force. The MEF sizes and MEP amplitudes increased with TMS intensity in most cases. The grip force (which measures the overall force involved in the grasp) and the net force (which measures the net effect of all contact forces exerted on the object) seem to be differently affected by single TMS pulses of the motor cortex.

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Internal forces during object manipulation

Cited by 52 publications

References 41 publications

Digit force adjustments during finger addition/removal in multi-digit prehension

Digit force adjustments during finger addition/removal in multi-digit prehension

Adjustments to Local Friction in Multifinger Prehension

Contact forces evoked by transcranial magnetic stimulation of the motor cortex in a multi-finger grasp

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