Influence of perturbation induced by an anticipated load on human motor regulatory systems

The role of anticipatory and reflexive compensatory muscle activation in catching errors under load uncertainty

This experiment investigated the role of anticipatory and reflexive compensatory neuromotor control in catching errors occurring under load uncertainty. Participants performed 64 trials of a one-handed ball catching task using visually identical balls of four different weights without knowing the weight of the ball on each trial. Anticipatory and reflexive compensatory muscle activation were recorded in five muscles (anterior deltoid, biceps brachii, wrist flexors group, triceps brachii, lumbar erector spinae) using the EMG integral. In each muscle, the anticipatory and reflexive compensatory muscle activation were compared between successful catches and catching errors for the lightest ball and the heaviest ball. Anticipatory muscle activation was not implicated in errors made with the lightest ball. However, reflexive compensatory muscle activation in the anterior deltoid, biceps brachii, and wrist flexors were implicated in errors made with the lightest ball. Specifically, catching errors with the lightest ball were characterized by elevated reflexive compensatory muscle activation. In the case of the heaviest ball, both anticipatory (anterior deltoid, wrist flexors) and reflexive compensatory muscle activation (anterior deltoid, biceps brachii, wrist flexors) were implicated in catching errors. That is, catching errors with the heaviest ball were characterized by lower anticipatory and reflexive compensatory muscle activation. Results are considered in the context of the likely influence of limb compliance in catching under load uncertainty.

Adaptability and Prediction of Anticipatory Muscular Activity Parameters to Different Movements in the Sitting Position

Voluntary movement often causes postural perturbation that requires an anticipatory postural adjustment to minimize perturbation and increase the efficiency and coordination during execution. This systematic review focuses specifically on the relationship between the parameters of anticipatory muscular activities and movement finality in sitting position among adults, to study the adaptability and predictability of anticipatory muscular activities parameters to different movements and conditions in sitting position in adults. A systematic literature search was performed using PubMed, Science Direct, Web of Science, Springer-Link, Engineering Village, and EbscoHost. Inclusion and exclusion criteria were applied to retain the most rigorous and specific studies, yielding 76 articles, Seventeen articles were excluded at first reading, and after the application of inclusion and exclusion criteria, 23 were retained. In a sitting position, central nervous system activity precedes movement by diverse anticipatory muscular activities and shows the ability to adapt anticipatory muscular activity parameters to the movement direction, postural stability, or charge weight. In addition, these parameters could be adapted to the speed of execution, as found for the standing position. Parameters of anticipatory muscular activities (duration, order, and amplitude of muscle contractions constituting the anticipatory muscular activity) could be used as a predictive indicator of forthcoming movement. In addition, this systematic review may improve methodology in empirical studies and assistive technology for people with disabilities.

Determinants of awareness, initiation and maintenance of physical activity among the over-fifties: a Delphi study

To develop effective interventions to stimulate physical activity (PA), insight into its underlying variables is needed. The aim of this study was to obtain an overview of the most relevant determinants of awareness, initiation and maintenance of PA among the over-fifties by means of a three-round Delphi study. In the first round, 17 key-experts outlined possible relevant determinants into an open-ended electronic questionnaire. In the second round, 118 experts completed a structured electronic questionnaire that was based on the first round results, in which they scored each determinant on its relevance. In the third round, experts were asked to re-rate the relevance of each determinant, after feedback was given about the group median relevance score. After three rounds, the experts agreed on 30 relevant determinants of the three phases of PA. When compared with longitudinal studies, the Delphi study pointed out new concepts, such as several post-motivational and social and environmental determinants as possible relevant determinants, suggesting that this method has the potential to trace new and promising determinants. The results further showed that next to similarities, much dissimilarity in relevant determinants of awareness, initiation and maintenance of PA was found, suggesting that most determinants could be phase specific.

The contribution of non-digital afferent signals to grip force adjustments evoked by brisk unloading of the arm or the held object

OBJECTIVE

Earlier studies suggest that grip force adjustments evoked by mechanical perturbations result more from cutaneous signals from the fingertips, than from afferent signals from the supporting limb. Generally an increase in tangential load at the fingertips induces an increase in grip force, whereas a decrease in load induces the opposite reaction. Some data suggest that prior knowledge and experience influences the magnitude of grip force adjustments.

METHODS

This study examines the relative contribution of digital and arm afferent signals in the context of brisk involuntary upward flexions obtained either by unloading the arm (ARM) or the held object (OBJECT). Following the perturbation, the tangential load at the fingertips increased in ARM, but decreased in OBJECT. A subsidiary goal was to compare the performance of naive subjects with the performance of trained and informed subjects.

RESULTS

When the perturbation was completely unexpected, grip force increased sharply after OBJECT and ARM unloading. By contrast, when subjects had prior knowledge and experience with the upcoming perturbation, grip responses were clearly differentiated; grip force increased after ARM, but decreased after OBJECT.

CONCLUSIONS

These results challenge the view that cutaneous signals of the fingertips are the driving signals of grip force responses. Instead, afferent signals from the flexed arm would account well for the lack of difference between grip force responses in ARM and OBJECT under unpredictable conditions. These data provide clear evidence that prior knowledge and experience influences reactive grip force control, since subjects became able to repress unnecessary grip force modulation in OBJECT.

SIGNIFICANCE

These data have implications for understanding the initiation and the modulation of grip force adjustments.

TMS-responses during anticipatory postural adjustment in bimanual unloading in humans

Transcranial magnetic brain stimulation (TMS) was used to assess the influence of the corticospinal system on motor output during forearm unloading in humans. Unloading was obtained either "passively" by the experimenter, or "actively" with the subjects' own contralateral arm. Anticipatory postural adjustments consisted of changes in the activity of a forearm flexor muscle prior to active unloading of the limb and acted to stabilize the forearm position. Motor evoked potentials (MEPs) were recorded in the forearm flexor at different times during active and passive unloading, static forearm loading, and during lifting of an equivalent weight by the contralateral arm while the ipsilateral forearm was statically loaded and held stationary. In active unloading, MEP amplitude decreased with the decrease of muscle activity. Passive unloading resulted in a similar decrease of MEP as with active unloading. During stationary forearm loading, the change in MEP corresponded to the degree of loading. If during static loading the contralateral arm has lifted a separate, equivalent weight, the amplitude of MEP decreased. A possible role of direct corticospinal volley and the motor command mediated by subcortical structures in anticipatory postural adjustments is discussed.