Gait initiation is affected during emotional conflict.

The present study examined whether emotion eliciting images affected motor output when a whole body movement was concerned: gait initiation. From a standing posture, 15 young adults were instructed to initiate gait (several steps) towards pleasant or unpleasant images corresponding to a congruent (CO) or an incongruent (IC) task. The movement was performed as soon as possible after the onset of images, using a paradigm of go/nogo. Because an emotional conflict had to be overcome in IC trials, we hypothesized that the longest reaction times and lowest amplitude of early postural adjustments were found during IC trials, as compared to CO trials. We also anticipated that conflict resolution would be improved if the previous trial was IC rather than CO, as previously reported in a typical press button task. Results demonstrated longer reaction times in IC than CO trials revealing that emotional conflict resolution interfered with cognitive resources involved in motor planning. Although the peak of forward velocity reached by the centre of body mass at the end of the first step and length of this step were similar between CO and IC trials, the amplitude of early postural modifications was reduced in IC trials. All together, these findings provided supporting evidence that a defensive response, possibly a freezing-like behaviour, was implemented during IC trials, but in a transient manner. Furthermore, conflict resolution was improved when the previous trial was IC rather than CO in terms of amplitude of early postural response, but not reaction time.

[Brain imaging: towards a more detailed knowledge of the central nervous system]. / Imagerie cérébrale : vers une connaissance plus approfondie du système nerveux central.

Functional imaging allowed the knowledge of the brain to progress significantly. Recent findings about modeling of the activity and the interactivity measured by functional imaging are presented, they allow to apprehend brain functioning and brain plasticity both in normal and pathophysiological conditions. The study of specific circuits in networks should make it possible to define more realistically the dynamic functioning of the central nervous system which underlies numerous brain functions.

Why anticipatory postural adjustments in gait initiation need to be modified when stepping up onto a new level?

The study examined why anticipatory postural adjustments (APA) associated to gait initiation in a stepping up to a new level situation (SU) are reduced as compared to a level walking situation (LW), as previously reported. Five young adults performed gait initiation in both situations at normal and fast speed. Data from a force platform provided gait parameters related to the motion of the body's centre of mass (CM) on the anteroposterior (progression) and vertical axes. The electromyographic activity of the soleus of the stance limb (SOst) and the vastus lateralis of the swing limb (VLsw) were analyzed prior to and after the onset of the double stance phase. The results showed that APA and progression CM velocity at the time of foot contact were smaller in SU, whereas the peak of this velocity was similar in both situations. Thus, the change in progression velocity during the double stance phase had to be greater in SU than in LW. In both velocity conditions, the activity of SOst stopped after the time of foot contact in both situations, but clearly later in SU. So, this ankle plantar flexor muscle would be involved not only in the change of body lift but also in forward CM progression. The latter role of this muscle brought supporting evidence for the reduction of APA in SU, enabling the peak of progression velocity to be similar in both situations. Only in SU, the timing of activation of VLsw and deactivation of SOst strongly co-varied, showing the implementation of a motor synergy to fulfil the new requirements of the task, i.e. body lift.

Initiation of a vertical jump: the human body's upward propulsion depends on control of forward equilibrium.

The aim of the study was to understand: (i), how far anticipatory postural adjustments (APA) participate in the sequences of a complex task; (ii), how, in the sagittal plane, horizontal and vertical progressions of the center of gravity (CG) are managed to achieve the prescribed goal. A posturo-kinetics sequence is identified during which forward CG progression is controlled in terms of a center of pressure (CP)-CG interaction. CP shifts and horizontal CG accelerations are correlated in timing and amplitude. There exists a control of whole body progression during the impulse, implying a braking of forwards movement. Vertical and horizontal CG accelerations are inter-dependent, and backward CP shift duration is related to its forward displacement. During the vertical jump initiation, whole body vertical propulsion and forward fall are linked. To increase impulse amplitude, initial whole body disequilibrium is lengthened and forward CG acceleration is greater. APA programming is probably elaborated on the basis of cognitive evaluation of the antero-posterior disturbance.