Constancy of Preparatory Postural Adjustments for Reaching to Virtual Targets across Different Postural Configurations

Incompatibility Influences Cursor Placement When Pointing to Images of Cups

OBJECTIVE

As images are used within graphical user interfaces to signify menu selection, it is important to understand how image properties can influence cursor placement online.

BACKGROUND

Objects have multiple dimensions that create potential ambiguity and Stroop-like confusions for the operator if a previously habitual response conflicts with the required response.

METHODS

To examine the impact of compatibility and other contextual factors, 41 participants used a computer Mouse or touch screen to place the cursor upon images of full or empty cups that varied in size, and direction of handle.

RESULTS

Cursor placements took longer using the Mouse than touch screen. Although participants were placing the cursor on images, the size of the cup, whether it was empty or full, and the handle orientation interacted in their effects upon response duration and cursor placement. The effects of cup size reversed for empty cups or those with incompatible handles.

CONCLUSION

Context can influence cursor placement. Perceived spillability influenced precision requirements, and the cup handle can serve as target or a flanking distractor.

APPLICATION

Image content can influence screen hotspots. As performance can change with cup spillability, this bodes well for attempts to detect intent from cursor trajectories.

The benefit of knowledge: postural response modulation by foreknowledge of equilibrium perturbation in an upper limb task

For whole-body sway patterns, a compound motor response following an external stimulus may comprise reflexes, postural adjustments (anticipatory or compensatory), and voluntary muscular activity. Responses to equilibrium destabilization may depend on both motor set and a subject`s expectation of the disturbing stimulus. To disentangle these influences on lower limb responses, we studied a model in which subjects (n = 14) were suspended in the air, without foot support, and performed a fast unilateral wrist extension (WE) in response to a passive knee flexion (KF) delivered by a robot. To characterize the responses, electromyographic activity of rectus femoris and reactive leg torque was obtained bilaterally in a series of trials, with or without the requirement of WE (motor set), and/or beforehand information about the upcoming velocity of KF (subject`s expectation). Some fast-velocity trials resulted in StartReact responses, which were used to subclassify leg responses. When subjects were uninformed about the upcoming KF, large rectus femoris responses concurred with a postural reaction in conditions without motor task, and with both postural reaction and postural adjustment when WE was required. WE in response to a low-volume acoustic signal elicited no postural adjustments. When subjects were informed about KF velocity and had to perform WE, large rectus femoris responses corresponded to anticipatory postural adjustment rather than postural reaction. In conclusion, when subjects are suspended in the air and have to respond with WE, the prepared motor set includes anticipatory postural adjustments if KF velocity is known, and additional postural reactions if KF velocity is unknown.

The effects of age and postural constraints on prehension

Older adults adapt the execution of complex motor tasks to use compensatory strategies in the reaching-to-grasping (i.e., prehension) movement. The presence of postural constraints may exacerbate these compensatory strategies. Therefore, we investigated the reach-to-grasp action with different postural constraints (sitting, standing, and walking) in younger and older people and evaluated the postural stability during the reach-to-grasp action. Thirty individuals (15 younger and 15 older adults) performed the prehension under three postural tasks: sitting, standing, and walking. The reaching movement was slower in the walking task than in the other two postural tasks; however, there was no difference between the age groups. For the grasping action, the older adults presented a larger grip aperture, and the peak grip aperture occurred earlier during hand transportation in sitting and standing tasks. In the standing task, the margin of stability was smaller for older adults. In the walking task, there was no difference between the groups for the margin of stability. Also, prehension during sitting and standing tasks were similar, and both differed from walking across age groups. Finally, older adults reduced their margin of stability compared to younger adults, but only in the standing task. The margin of stability was similar between age groups during the walking task. We concluded that age affected grasping (distal component) but not reaching (proximal component), suggesting that healthy older adults have more difficulty controlling distal than proximal body segments.

Evidence for constancy in the modularity of trunk muscle activity preceding reaching: implications for the role of preparatory postural activity

Postural muscle activity precedes voluntary movements of the upper limbs. The traditional view of this activity is that it anticipates perturbations to balance caused by the movement of a limb. However, findings from reach-based paradigms have shown that postural adjustments can initiate center of mass displacement for mobility rather than minimize its displacement for stability. Within this context, altering reaching distance beyond the base of support would place increasing constraints on equilibrium during stance. If the underlying composition of anticipatory postural activity is linked to stability, coordination between muscles (i.e., motor modules) may evolve differently as equilibrium constraints increase. We analyzed the composition of motor modules in functional trunk muscles as participants performed multidirectional reaching movements to targets within and beyond the arm's length. Bilateral trunk and reaching arm muscle activity were recorded. Despite different trunk requirements necessary for successful movement, and the changing biomechanical (i.e., postural) constraints that accompany alterations in reach distance, nonnegative matrix factorization identified functional motor modules derived from preparatory trunk muscle activity that shared common features. Relative similarity in modular weightings (i.e., composition) and spatial activation profiles that reflect movement goals across tasks necessitating differing levels of trunk involvement provides evidence that preparatory postural adjustments are linked to the same task priorities (i.e., movement generation rather than stability).NEW & NOTEWORTHY Reaching within and beyond arm's length places different task constraints upon the required trunk motion necessary for successful movement execution. The identification of constant modular features, including functional muscle weightings and spatial tuning, lend support to the notion that preparatory postural adjustments of the trunk are tied to the same task priorities driving mobility, regardless of the future postural constraints.

Anticipatory weight shift between arms when reaching from a crouched posture

Reaching movements performed from a crouched body posture require a shift of body weight from both arms to one arm. This situation has remained unexamined despite the analogous load requirements during step initiation and the many studies of reaching from a seated or standing posture. To determine whether the body weight shift involves anticipatory or exclusively reactive control, we obtained force plate records, hand kinematics, and arm muscle activity from 11 healthy right-handed participants. They performed reaching movements with their left and right arm in two speed contexts, "comfortable" and "as fast as possible," and two postural contexts, a less stable knees-together posture and a more stable knees-apart posture. Weight-shifts involved anticipatory postural actions (APAs) by the reaching and stance arms that were opposing in the vertical axis and aligned in the side-to-side axis similar to APAs by the legs for step initiation. Weight-shift APAs were correlated in time and magnitude, present in both speed contexts, more vigorous with the knees placed together, and similar when reaching with the dominant and nondominant arm. The initial weight-shift was preceded by bursts of muscle activity in the shoulder and elbow extensors (posterior deltoid and triceps lateral) of the reach arm and shoulder flexor (pectoralis major) of the stance arm, which indicates their causal role; leg muscles may have indirectly contributed but were not recorded. The strong functional similarity of weight-shift APAs during crouched reaching to human stepping and cat reaching suggests that they are a core feature of posture-movement coordination.NEW & NOTEWORTHY This work demonstrates that reaching from a crouched posture is preceded by bimanual anticipatory postural adjustments (APAs) that shift the body weight to the stance limb. Weight-shift APAs are more robust in an unstable body posture (knees together) and involve the shoulder and elbow extensors of the reach arm and shoulder flexor of the stance arm. This pattern mirrors the forelimb coordination of cats reaching and humans initiating a step.