Changes in grasping kinematics due to different start postures of the hand

It was proposed that grasping is a relatively stereotyped movement pattern which can be subdivided into the components of manipulation, transport, and orientation of the hand. However, it is still a matter of debate whether these components are independent of each other. In three experiments we altered the start posture of the hand by either changing the size of the start aperture or the orientation of the hand prior to movement onset. The variation of the aperture size primarily affected the manipulation component of the grip resulting in an overall change of the pre-shaping profile. In contrast, an alteration of the start orientation affected the manipulation and the transport components to a similar extent. These results give further evidence that hand orientation is neither planned nor controlled independently from the other movement components. Moreover, when the grip had to match specific object properties, adjustments were mainly achieved within the first movement part. In contrast, when there were no movement constraints the final finger positions were influenced by the initial start posture of the hand. We found no evidence for a fixed spatial or temporal coupling of the grasp and the transport component in our experiments.     

A wearable vibrotactile biofeedback system improves balance control of healthy young adults following perturbations from quiet stance

Maintaining postural equilibrium requires fast reactions and constant adjustments of the center of mass (CoM) position to prevent falls, especially when there is a sudden perturbation of the support surface. During this study, a newly developed wearable feedback system provided immediate vibrotactile clues to users based on plantar force measurement, in an attempt to reduce reaction time and CoM displacement in response to a perturbation of the floor. Ten healthy young adults participated in this study. They stood on a support surface, which suddenly moved in one of four horizontal directions (forward, backward, left and right), with the biofeedback system turned on or off. The testing sequence of the four perturbation directions and the two system conditions (turned on or off) was randomized. The resulting reaction time and CoM displacement were analysed. Results showed that the vibrotactile feedback system significantly improved balance control during translational perturbations. The positive results of this preliminary study highlight the potential of a plantar force measurement based biofeedback system in improving balance under perturbations of the support surface. Future system optimizations could facilitate its application in fall prevention in real life conditions, such as standing in buses or trains that suddenly decelerate or accelerate.     

Rôle des propriétés inertielles segmentaires sur la perception de l’étendue de l’espace péripersonnel

Previous studies have suggested that the perception of peripersonal space can hardly be achieved using only scene-based visual cues and requires combining visual information with motor representations. Motor representation can be viewed as a component of a predictive system, which includes a neural process that simulates through motor imagery the dynamic behaviour of the body in relation to the environment. In this study, we analysed whether modifying the force required to reach a visual target influences the perception of what is reachable. In a visuomotor task, the experimental group (n = 10) adapted to a 1.5 kg weight attached to the right wrist while performing a series of pointing movements. The control group (n = 10) performed the motor task without inertial perturbation. A perceptual judgement task of what is reachable was performed before and after the motor task. Results showed that inertial perturbation produced initially an undershoot of the target suggesting a lack of motor force to overcome the inertial perturbation, but spatial errors receded progressively through movement rehearsal. Perceptual estimates of what is reachable slightly overestimated action capacities but were not affected by motor adaptation. Thus, modifying motor force required to compensate for inertial perturbation had no direct effect on the perception of peripersonal space. When interpreted in regard to previous experimental work, this result suggests that motor representations may provide information about the sensory or spatial consequences of action rather than the sense of effort associated with motor production.     

Cycling strategies of young and older cyclists

This study concentrates on the cycling strategies of older cyclists (54–62 year olds) in comparison to young cyclists (20–30 year olds). While cycling in a safe laboratory set-up, controlled lateral perturbations are applied to the rear of the bicycle. Three possible strategies to keep balance are analysed for a young and older aged group: steering, lateral trunk movement and outward knee movement. Older subjects appear to rely more on knee movement as a control mechanism than young subjects. Furthermore, the frequency domain analysis revealed that the older adults need more effort to counteract high frequency perturbations. Increased inter-individual variation for the older adults subject group suggests that this group can be seen as a transition group in terms of physical fitness. This explains their increased risk in single-sided bicycle accidents (i.e. accidents involving the cyclist only). Therefore, older cyclists could benefit from improving the stability of cycling at lower speeds.     

Differential effects of a visual illusion on online visual guidance in a stable environment and online adjustments to perturbations

In the reported, experiment participants hit a ball to aim at the vertex of a Müller–Lyer configuration. This configuration either remained stable, changed its shaft length or the orientation of the tails during movement execution. A significant illusion bias was observed in all perturbation conditions, but not in the stationary condition. The illusion bias emerged for perturbations shortly after movement onset and for perturbations during execution, the latter of which allowed only a minimum of time for making adjustments (i.e., approx.170 ms). These findings indicate that allocentric information is exploited for online control when people make rapid adjustments in response to a sudden change in the environment and not when people guide their limb movements to interact with a stable environment.     

Time and direction preparation of the long latency stretch reflex

This study investigated time and direction preparation of motor response to force load while intending to maintain the finger at the initial neutral position. Force load extending or flexing the index finger was given while healthy humans intended to maintain the index finger at the initial neutral position. Electromyographic activity was recorded from the first dorsal interosseous muscle. A precue with or without advanced information regarding the direction of the forthcoming force load was given 1000 ms before force load. Trials without the precue were inserted between the precued trials. A long latency stretch reflex was elicited by force load regardless of its direction, indicating that the long latency stretch reflex is elicited not only by muscle stretch afferents, but also by direction-insensitive sensations. Time preparation of motor response to either direction of force load enhanced the long latency stretch reflex, indicating that time preparation is not mediated by afferent discharge of muscle stretch. Direction preparation enhanced the long latency stretch reflex and increased corticospinal excitability 0–20 ms after force load when force load was given in the direction stretching the muscle. These enhancements must be induced by preset of the afferent pathway mediating segmental stretch reflex.