Adaptive control of grip force to compensate for static and dynamic torques during object manipulation

Abstract

Manipulating a cup by the handle requires compensating for the torque induced by the moment of the mass of the cup relative to the location of the handle. In the present study, we investigated the control strategy of subjects asked to perform grip-lift movements with an object with center of mass located away from the grip axis. Participants were asked to lift the manipulandum with a two-fingers precision grip and stabilize it in front of a visual target. Subjects showed a gradual and slow adaptation of the grip-force scaling across trials: the grip force tended to decrease slowly, and the temporal coordination between grip-force and load-torque rates displayed gradually, better-coordinated patterns. Importantly, this adaptation was much slower than the stabilization of the same parameters measured either when no torque came into play or after previous adaptation to the presence of a torque. In contrast, the maximum rotation induced by the torque was controlled efficiently after only few trials, and an unexpected decrease in the tangential torque produced significant overcompensation. An unexpected increase in torque produced a consistent opposite effect. This shows that the compensation for the dynamic torque was based on an anticipatory, dynamic counter-torque produced by the arm and wrist motor commands. The comparatively slow stabilization of grip-force control suggests a specific adaptation process engaged by the presence of the torque. This paradigm, including tangential torques, clearly constitutes a powerful tool to extract the adaptive component of grip control during object manipulation.

Biomechanics of Finger Pad Response under Torsion

“…Trial-to-trial adaptation of grip force is slower in the presence of torque, possibly explained by differences in cutaneous feedback received (Crevecoeur et al 2011). In the present study, we confirm that local deformation patterns are qualitatively different under torsion than translation.…”

Not only perception but also grasping actions can obey Weber’s law

“…The precision of grasp contact positions definitely did not improve over successive repetitions even though inaccurate grasps provided direct visual and haptic feedback in the form of visually perceived object rotation and haptically perceived tangential torque which requires grip force adjustments to prevent rotational slip (Goodwin et al, 1998; Kinoshita et al, 1997). One possible reason for the lack of calibration could be linked to the slow adaptation that the grip force undergoes in the presence of torque load which might, in this particular case, negatively affect the ability to use haptic inputs effectively (Crevecoeur et al, 2011). Another reason that might have hindered or slowed down calibration was the randomized positioning of objects which required participants to assume slightly different arm postures for each grasping action.…”

How tactile afferents in the human fingerpad encode tangential torques associated with manipulation: are we better than monkeys?

“…Skin mechanoreceptors, and their corresponding afferents, contribute to the control of dexterous object manipulation by signalling gripped object features, fingertip forces, and grip safety (Johansson and Westling, 1987; Goodwin et al, 1998; Nowak et al, 2003; Johansson and Flanagan, 2009; Crevecoeur et al, 2011; Khamis et al, 2014; Delhaye et al, 2021; Schiltz et al, 2021). Sensory inputs can rapidly update internal representations of hand-object manipulations, which are used to counteract unexpected events and build predictive strategies (Johansson and Westling, 1988; Johansson and Cole, 1992; Miall and Wolpert, 1996; Birznieks et al, 1998; Witney et al, 2004; Flanagan et al, 2008) which might also include planned slips (Birznieks et al, 1998).…”