Effects of Object Compliance on Three-Digit Grasping

Abstract

Compared with rigid objects, grasping and lifting compliant objects presents additional uncertainties. For any static grasp, forces at the fingertips depend on factors including the locations of the contact points and the contact forces must be coordinated to maintain equilibrium. For compliant objects, the locations and orientations of the contact surfaces change in a force-dependent manner, thus changing the force requirements. Furthermore, every force adjustment then results in additional changes in object shape. This study characterized force and muscle activation patterns in this situation. Fingertip forces were measured as subjects grasped and lifted a 200-g object using their thumb, index, and ring fingers. A spring was sometimes placed under the index and/or ring finger contact surface. Surface electromyographic activity was recorded from ten hand muscles and one proximal arm muscle. The patterns of grip (normal) force and muscle activity were similar across conditions during the load and lift phases, but their amplitude depended on whether the contact surface was compliant. Specifically, the grip force increased smoothly during the load phase of the task under all conditions. To the contrary, the tangential contact (load) force did not increase monotonically when one or more of the contact surfaces were compliant, resulting in a decoupling of the grip and load forces.

Analysis of the effects of surface stiffness on the contact interaction between a finger and a cylindrical handle using a three-dimensional hybrid model

“…In addition, the dimensions of the handle may alter a person's gripping behaviors. However, the stiffness of the handle covering material is much higher than that of the soft tissues and typical handle diameters are around 40 mm, based on previous ergonomic studies [5,8,38]. Indeed, the adjustment of the joint moment ratio for different gripping force levels was observed to be so minor that they did not produce measurable contact pressure variations, at least in the current measurements using a pressure sensor film.…”

“…This assumption may not be strictly correct physiologically since people may tend to grip in a manner to minimize the grip effort for different gripping force levels, resulting in varying joint moment ratios as a function of the gripping force [37]. The joint moment ratio in fingers is subject specific and may also be dependent on the contact stiffness [38]. In addition, the dimensions of the handle may alter a person's gripping behaviors.…”

Manipulation of a fragile object by elderly individuals

“…The issue of manipulating fragile objects has been barely touched upon in studies of prehension. Recently, two studies have focused on the mechanics and digit interaction during manipulation of deformable objects (Winges et al 2009; Gorniak et al 2010). The former study focused on grip and load force coupling during tripod grasp of a spring-like object.…”

Force prediction by fingernail imaging using active appearance models

“…When the fingers are instrumented [2,11], the potential exists for loss of haptic sense. When the object is instrumented [17,12], the finger contact points are constrained. Although it is possible to perform unconstrained grasping with instrumented objects [5], some constraints must still be placed on the contact locations.…”

“…Two contours are used to represent the finger: the outer edge (points 1-15) and the fingernail edge (points[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. First, the finger direction (represented by the vector along the middle of the finger) is estimated.…”