Control of aperture closure initiation during trunk-assisted reach-to-grasp movements

Abstract

The present study investigated how the involvement and direction of trunk movement during reach-to-grasp movements affect the coordination between the transport and grasping components. Seated young adults made prehensile movements in which the involvement of the trunk was varied; the trunk was not involved, moved forward (flexion), or moved backward (extension) in the sagittal plane during the reach to the object. Each of the trunk movements was combined with an extension or flexion motion of the arm during the reach. Regarding the relation between the trunk and arm motion for arm transport, the onset of wrist motion relative to that of the trunk was delayed to a greater extent for the trunk extension than for the trunk flexion. The variability of the time period from the peak of wrist velocity to the peak of trunk velocity was also significantly greater for trunk extension compared to trunk flexion. These findings indicate that trunk flexion was better integrated into the control of wrist transport than trunk extension. In terms of the temporal relationship between wrist transport and grip aperture, the relation between the time of peak wrist velocity and the time of peak grip aperture did not change or became less steady across conditions. Therefore, the stability of temporal coordination between wrist transport and grip aperture was maintained despite the variation of the pattern of intersegmental coordination between the arm and the trunk during arm transport. The transport-aperture coordination was further assessed in terms of the control law according to which the initiation of aperture closure during the reach occurs when the hand crosses a hand-to-target distance threshold for grasp initiation that is a function of peak aperture, wrist velocity and acceleration, trunk velocity and acceleration, and trunk-to-target distance at the time of aperture closure initiation. The participants increased the hand-to-target distance threshold for grasp initiation in the conditions where the trunk was involved compared to the conditions where the trunk was not involved. An increase also occurred when the trunk was extended compared to when it was flexed. The increased distance threshold implies an increase in the hand-to-target distance-related safety margin for grasping when the trunk is involved, especially when it is extended. These results suggest that the CNS significantly utilizes the parameters of trunk movement together with movement parameters related to the arm and the hand for controlling grasp initiation.

Coordination deficits during trunk-assisted reach-to-grasp movements in Parkinson’s disease

“…2b). Considering that trunk-extension motions were less integrated into the arm motions compared to trunk-flexion motions (Rand et al 2012), the increased amplitude of grip aperture in the older adults in trunk-extension conditions likely reflects the increase in safety margins for grasping to compensate for an uncertainty of the arm transport (Churchill et al 2000; Gentilucci et al 2004; Jackson et al 1995; Rand et al 2004; Wing et al 1986). On the other hand, unchanged peak aperture amplitude in the PD participants reflects a reduced inter-segmental integration between the fingers and trunk.…”

“…The experimental setting and procedure were the same as the previous study conducted with healthy young adults (Rand et al 2012). Participants were comfortably seated at a table.…”

“…Wrist (trunk) velocity during the reach was tangential velocity calculated as the first derivative of wrist (trunk) position. Derivatives were calculated using the sliding window technique, where the data points within the window (the window width was 7 points) were approximated with a quadratic polynomial (Rand et al 2006, 2010, 2012). The polynomial was then used for calculating the analytic derivative at the window’s center (or at other points when the window overlapped the beginning or end of the data array representing the curve).…”

“…Trunk motion is usually initiated prior to arm motion (Rand et al 2012; Saling et al 1996; Wang and Stelmach 2001). However, it is well integrated into arm reaching by the time of peak wrist velocity, thereby preparing for subsequent integration with the grasping component (Rand et al 2012; Yang and Feldman 2010). …”

“…However, previous studies in healthy adults have examined different combinations of motion directions between the trunk and arm during reaching (Ma and Feldman 1995; Pigeon et al 2000) and reach-to-grasping (Rand et al 2012). The latter study demonstrated that trunk extension was less integrated into the control of wrist transport than trunk flexion.…”