This study examined how the central nervous system organizes mediolateral (ML) “anticipatory postural adjustments” (APAs) for stepping initiation (SI) to take into account the postural perturbation induced by voluntary lateral arm raising. Subjects purposely stepped in isolation (“isolated stepping”) or in combination with lateral raising of dominant arm (“motor sequence”). SI was carried out with the leg ipsilateral or controlateral to raising arm. Results showed that APA amplitude increased from “ipsilateral isolated stepping” to “ipsilateral sequence”, but did not change in conditions involving controlateral leg; ML instability increased from “ipsilateral isolated stepping” to “ipsilateral sequence”, but decreased from “controlateral isolated stepping” to “controlateral sequence”. These changes were exacerbated when inertia was added at the hand during raising. These results suggest that APAs for SI are globally scaled as a function of the biomechanical consequences of forthcoming arm movement on ML postural stability.
Eric Yiou, Malha Mezaour and Serge Le Bozec
This study investigated how young healthy subjects control their equilibrium in situations of instability specifically elicited by a reduced capacity of force production in the postural muscle system. Ten subjects displaced a bar forward with both hands at maximal velocity toward a target while standing on the dominant leg (UNID), on the nondominant leg (UNIND), or on both legs. In each stance condition, anticipatory postural adjustments (APAs) were elicited. Along the anteroposterior axis, APAs were two-times longer in UNID and UNIND than in bipedal stance, while the anticipatory inertia forces remained equivalent. The focal performance was maintained without any additive postural perturbation. A small effect of leg dominance could be detected on APAs along the mediolateral axis (i.e., anticipatory inertia forces were higher in UNIND than in UNID). These results stress the adaptability of the central nervous system to the instability specifically elicited by reduced postural muscle system efficiency.
Eric Yiou, Manon Gendre, Thomas Deroche and Serge Le Bozec
This study examined how pleasant and unpleasant emotional states influence the biomechanical organization of both forward and backward step initiation (SI). Participants (N = 31) purposely took a single step toward or away from a screen following the presentation of a pleasant (erotic), unpleasant (mutilation) or neutral (objects and landscapes) image. The main results showed that the reaction time for forward SI was shortened when individuals were exposed to pleasant pictures as compared with unpleasant pictures. The anticipatory whole-body center-ofmass velocity associated with backward SI, as well as the peak of center-of-mass velocity associated with forward SI both reached lower values when individuals were exposed to pleasant pictures as compared with neutral pictures. In contrast, unpleasant pictures did not significantly induce any change in the forward or backward SI parameters. Overall, these results obtained for whole-body approach/avoidance-like behaviors provided mitigated support for the so-called “motivational direction hypothesis.”