Multidigit Control of Contact Forces During Transport of Handheld Objects

Abstract

When an object is lifted vertically, the normal force increases and decreases in tandem with tangential (load) force to safely avoid slips. For horizontal object transport, horizontal forces at the contact surfaces can be decomposed into manipulation forces (producing acceleration/deceleration) and grasping forces. Although the grasping forces must satisfy equilibrium constraints, it is not clear what determines their modulation across time, nor the extent to which they result from active muscle contraction or mechanical interactions of the digits with the moving object. Grasping force was found to increase in an experimental condition where the center of mass was below the contact plane, compared with when it was in the contact plane. This increase may be aimed at stabilizing object orientation during translation. In another experimental condition, more complex moments were introduced by allowing the low center of mass to swing around a pivot point. Electromyographic (EMG) activity recorded from several intrinsic and extrinsic hand muscles failed to reveal active feedback regulation of contact force in this situation. Instead, in all experimental conditions, EMG data revealed a strategy of feedforward stiffness modulation. Multiple regression analysis revealed that muscle activity at remote digits (e.g., the index and ring fingers) was highly correlated with the contact force measured at another digit (e.g., the thumb). The data suggest that to maintain grasp stability during horizontal translation, predictable as well as somewhat unpredictable inertial forces are compensated for by controlling the stiffness of the hand through cocontraction and modulation of hand muscle activity.

Grip forces during fast point-to-point and continuous hand movements

“…By dissociating these two components, the authors reported that during the initial accelerating phase of the movement the grasping force increased slowly if at all, reaching its maximum value only midway during the movement, i.e., at peak velocity (zero acceleration). A temporal coincidence between the peak of grasping force and the peak of velocity for horizontal movements was also reported by Gao et al (2005) and by Winges et al (2007). A dissociation between manipulating and grasping components of the grip was adopted in a double-step experiment (Danion and Sarlegna 2007) to ascertain whether grasp force corrections precede or follow arm movement corrections.…”

“…More specifically, Winges et al (2007) and Slota et al (2011) questioned the notion of model-based predictions by suggesting that the modulation of grasp force can be explained more simply by taking into account the elastic forces arising from muscle co-contraction. Along similar lines we ask whether a parsimonious account of the experimental findings may be derived from the assumption that the forces generated in both point-to-point and circular movements can be described as the response of viscoelastic systems to appropriately tailored driving inputs.…”

“…Thus, load and grasp forces are programmed mostly to cope with the expected weight of the object. Inertial forces may instead become a significant component of the total load when a grasped object is moved around Wing 1993, 1995;Flanagan and Tresilian 1994;Gao et al 2005;Kinoshita et al 1993Kinoshita et al , 1995Smith and Soetching 2005;Winges et al 2007;Zatsiorsky et al 2005). investigated the relation between grip and load forces during vertical and horizontal point-to-point arm movements.…”

Proximal arm kinematics affect grip force-load force coordination

“…Authors suggested that coupling between proximal and distal muscles played a role in this directional tuning. Subsequent EMG studies showed that horizontal transport was achieved by feedforward stiffness modulation (Winges et al 2007a), which was potentially mediated by coupling between proximal arm muscles and intrinsic hand muscles (Winges et al 2007b). These coordination patterns extend to more complex circular movements, where grip force reflects the additive effects of the vertical and horizontal movement components (Slota et al 2011).…”

Coordination of Contact Forces During Multifinger Static Prehension

“…In robotics, the manipulation and internal forces are controlled independently (Kerr & Roth, 1986; Yoshikawa & Nagai, 1991). In contrast, people modulate the internal forces with the manipulation force (Smith & Soechting, 2005; Gao et al, 2005; Winges et al, 2007; Gorniak et al, 2010). For example, during vertical object manipulation the grasping pinch forces scale linearly with the effective load force, which depends on the acceleration (Johansson et al, 1992; Flanagan et al, 1993; Flanagan & Tresilian, 1994; Smith & Soechting, 2005).…”