The nature of human prehension: Three dextrous hands in one

Iberall, Thea

doi:10.1109/robot.1987.1087813

Cited by 114 publications

(58 citation statements)

References 15 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…In such a grasp, the thumb and the fingers oppose each other. Control of the prismatic grasp is frequently considered as being based on a two-level hierarchy: at the upper level, the task is shared between the thumb and a virtual finger (VF), an imagined finger that produces the same mechanical effect (wrench) as several fingers combined (Arbib et al 1985;Iberall 1987;Santello and Soechting, 2000); at the lower level, action of the VF is shared among the actual fingers. During the performance, there are four mechanical constraints that must be satisfied: (i) no-slip constraint: the normal forces of the thumb and VF should be sufficiently large to prevent slipping ( (ii) the sum of the thumb and VF normal forces should equal zero; (iii) the tangential forces of the thumb and VF should equal the supported load; and (iv) the moment of force generated by the performer should be equal and opposite to the external torque acting on the handle.…”

Section: Introductionmentioning

confidence: 99%

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

2008

View full text Add to dashboard Cite

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.

show abstract

Section: Introductionmentioning

confidence: 99%

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

2008

View full text Add to dashboard Cite

show abstract

“…Further analysis revealed that these synergies could be organized in the CNS hierarchically with at least two levels (Shim et al 2003(Shim et al , 2005Zatsiorsky and Latash 2004;Zatsiorsky et al 2003b). At the higher level, force and torque constraints are distributed between the thumb and virtual finger (VF), an imagined finger that generates the same mechanical effect as the four actual fingers (Arbib et al 1985;Baud-Bovy and Soechting 2001;Iberall 1987;Santello and Soechting 1997). At the lower level, the force and torque of the VF are distributed among the four fingers.…”

Section: Introductionmentioning

confidence: 99%

Effects of friction at the digit-object interface on the digit forces in multi-finger prehension

et al. 2006

View full text Add to dashboard Cite

The effects of surface friction at the digit-object interface on digit forces were studied when subjects (n = 8) statically held an object in a five-digit grasp. The friction conditions were SS (all surfaces are sandpaper), RR (all are rayon), SR (S for the thumb and R for the four fingers), and RS (the reverse of SR). The interaction effects of surface friction and external torque were also examined using five torques (−0.5, −0.25, 0, +0.25, +0.5 Nm). Forces and moments exerted by the digits on a handle were recorded. At zero torque conditions, in the SS and RR (symmetric) tasks the normal forces of the thumb and virtual finger (VF, an imagined finger with the mechanical effect equal to that of the four fingers) were larger for the RR than the SS conditions. In the SR and RS (asymmetric) tasks, the normal forces were between the RR and SS conditions. Tangential forces were smaller at the more slippery side than at the less slippery side. According to the mathematical optimization analysis decreasing the tangential forces at the more slippery sides decreases the cost function values. The difference between the thumb and VF tangential forces, ΔF t , generated a moment of the tangential forces (friction-induced moment). At non-zero torque conditions the friction-induced moment and the moment counterbalancing the external torque (equilibrium-necessitated moment) could be in same or in opposite directions. When the two moments were in the same direction, the contribution of the moment of tangential forces to the total moment was large, and the normal forces were relatively low. In contrast, when the two moments were in opposite directions, the contribution of the moment of tangential forces to the total moment markedly decreased, which was compensated by an increase in the moment of normal forces. The apparently complicated results were explained as the result of summation of the friction-related (elemental) and torque-related (synergy) components of the central commands to the individual digits.

show abstract

“…The following terminology is used: virtual finger (VF) is an imaginary finger that produces a wrench (the force and moment) equal to the sum of wrenches produced by all the fingers (Arbib et al 1985;Cutkosky 1985;Iberall 1987;Iberall et al 1989;Santello and Soechting 2000;Baud-Bovy and Soechting 2001). The handle orientation is defined as the angle between the longitudinal axis of the handle and the vertical.…”

Section: Introductionmentioning

confidence: 99%

Internal forces during object manipulation

2005

View full text Add to dashboard Cite

Internal force is a set of contact forces that does not disturb object equilibrium. The elements of the internal force vector cancel each other and, hence, do not contribute to the resultant (manipulation) force acting on the object. The mathematical independence of the internal and manipulation forces allows for their independent (decoupled) control realized in robotic manipulators. To examine whether in humans internal force is coupled with the manipulation force and what grasping strategy the performers utilize, the subjects (n=6) were instructed to make cyclic arm movements with a customized handle. Six combinations of handle orientation and movement direction were tested. These involved: parallel manipulations (1) VV task (vertical orientation and vertical movement) and (2) HH task (horizontal orientation and horizontal movement); orthogonal manipulations (3) VH task (vertical orientation and horizontal movement) and (4) HV task (horizontal orientation and vertical movement); and diagonal manipulations (5) DV task (diagonal orientation and vertical movement) and (6) DH task (diagonal orientation and horizontal movement). Handle weight (from 3.8 to 13.8 N), and movement frequency (from 1 to 3 Hz) were systematically changed. The analysis was performed at the thumb-virtual finger level (VF, an imaginary finger that produces a wrench equal to the sum of wrenches produced by all the fingers). At this level, the forces of interest could be reduced to the internal force and internal moment. During the parallel manipulations, the internal (grip) force was coupled with the manipulation force (producing object acceleration) and the thumb-VF forces increased or decreased in phase: the thumb and VF worked in synchrony to grasp the object more strongly or more weakly. During the orthogonal manipulations, the thumb-VF forces changed out of phase: the plots of the internal force vs. object acceleration resembled an inverted letter V. The HV task was the only task where the relative phase (coupling) between the normal forces of the thumb and VF depended on oscillation frequency. During the diagonal manipulations, the coupling was different in the DV and DH tasks. A novel observation of substantial internal moments is described: the moments produced by the normal finger forces were counterbalanced by the moments produced by the tangential forces such that the resultant moments were close to zero. Implications of the findings for the notion of grasping synergies are discussed.

show abstract

The nature of human prehension: Three dextrous hands in one

Cited by 114 publications

References 15 publications

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

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

Effects of friction at the digit-object interface on the digit forces in multi-finger prehension

Internal forces during object manipulation

Contact Info

Product

Resources

About