Force coordination in static manipulation tasks performed using standard and non-standard grasping techniques

Abstract

We evaluated coordination of the hand grip force (GF; normal component of the force acting at the hand-object contact area) and load force (LF; the tangential component) in a variety of grasping techniques and two LF directions. Thirteen participants exerted a continuous sinusoidal LF pattern against externally fixed handles applying both standard (i.e., using either the tips of the digits or the palms; the precision and palm grasps, respectively) and non-standard grasping techniques (using wrists and the dorsal finger areas; the wrist and fist grasp). We hypothesized (1) that the non-standard grasping techniques would provide deteriorated indices of force coordination when compared with the standard ones, and (2) that the nervous system would be able to adjust GF to the differences in friction coefficients of various skin areas used for grasping. However, most of the indices of force coordination remained similar across the tested grasping techniques, while the GF adjustments for the differences in friction coefficients (highest in the palm and the lowest in the fist and wrist grasp) provided inconclusive results. As hypothesized, GF relative to the skin friction was lowest in the precision grasp, but highest in the palm grasp. Therefore, we conclude that (1) the elaborate coordination of GF and LF consistently seen across the standard grasping techniques could be generalized to the non-standard ones, while (2) the ability to adjust GF using the same grasping technique to the differences in friction of various objects cannot be fully generalized to the GF adjustment when different grasps (i.e., hand segments) are used to manipulate the same object. Due to the importance of the studied phenomena for understanding both the functional and neural control aspects of manipulation, future studies should extend the current research to the transient and dynamic tasks, as well as to the general role of friction in our mechanical interactions with the environment.

Delayed Visual Feedback Affects Both Manual Tracking and Grip Force Control When Transporting a Handheld Object

“…For instance, when we manipulate a handheld object, grip force is largely determined by predictive mechanisms, as evidenced by its modulation preceding or being in synchrony with the destabilizing load force resulting from the hand movement (i.e., tangential load at the object-finger interface; Danion and Sarlegna 2007;Davare et al 2007;Johansson and Westling 1988a;White et al 2005;Zatsiorsky et al 2005). Moreover, such anticipatory adjustments are observed irrespectively of the number of hands or fingers used to hold the object (de Freitas and Jaric 2009;Flanagan and Tresilian 1994). The leading hypothesis is that the nervous system anticipates the load force using internal representations of the body and the object in conjunction with a copy of the motor commands (Flanagan and Wing 1997;Kawato 1999).…”

Grip and load force coordination in cyclical isometric manipulation task is not affected by the feedback type

“…This coupling has been widely investigated during manipulation tasks that involve lifting a grasped object [1], moving a handheld object upward and downward discretely [7] or continuously [5,8], and isometrically applying sinusoidal LF profiles on an externally fixed object [4,6,9]. This highly coupled relationship is characterized by a parallel change of GF and LF with virtually no time delay.…”

“…This highly coupled relationship is characterized by a parallel change of GF and LF with virtually no time delay. During tasks in which LF changes (e.g., lifting, transporting, and shaking a handheld object) this parallel change of GF and LF ensure an economical exertion of GF [1,4,5,7,9]. …”

“…For example, in two studies performed in fixed (or quasi-fixed) objects, visual information about the prescribed LF magnitude and frequency and about the currently exerted LF was presented in oscilloscopes and computer screens [4,10,13]. Moreover, in other studies [6,9], visual information about the prescribed LF magnitude was shown in a computer screen (i.e., visual feedback) while information about LF frequency was given by a metronome (auditory feedback).…”

Finger interaction in a three-dimensional pressing task

“…It is well known that load-resisting (shear) and grip forces are tightly coupled during object manipulation (Johansson and Westling 1984; Jaric et al 2005; De Freitas and Jaric 2009). This coupling has been discussed as a consequence of neural strategies, in particular as reflected in synchronization of motor unit action potentials to different muscles and muscle compartments (Santello and Fuglevand 2004).…”