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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.

The purpose of this study was to examine whether fatigue of postural muscles might influence the coordination between segmental posture and movement. Seven healthy adults performed series of fifteen fast wrist flexions and extensions while being instructed to keep a dominant upper limb posture as constant as possible. These series of voluntary movements were performed before and after a fatiguing submaximal isometric elbow flexion, and also with or without the help of an elbow support. Surface EMG from muscles Deltoïdeus anterior, Biceps brachii, Triceps brachii, Flexor carpi ulnaris, Extensor carpi radialis were recorded simultaneously with wrist, elbow and shoulder accelerations and wrist and elbow displacements. Fatigue was evidenced by a shift of the elbow and shoulder muscles EMG spectra towards low frequencies. Kinematics of wrist movements and corresponding activations of wrist prime-movers, as well as the background of postural muscle activation before wrist movement were not modified. There were only slight changes in timing of postural muscle activations. These data indicate that postural fatigue induced by a low-level isometric contraction has no effect on voluntary movement and requires no dramatic adaptation in postural control.

This study re-investigates the characteristics of segmental postural adjustments associated with rapid mono-articular movements and analyses their dependence on initial postural conditions. Subjects performed rapid voluntary wrist flexions and extensions while maintaining their upper limb posture as stable as possible, with or without an elbow support. Surface electromyographic activity (EMG) was recorded from Flexor carpi ulnaris, Extensor carpi radialis, Biceps brachii, Triceps brachii and Deltoideus anterior. The kinematics of the three joints and kinetics in the support condition were also recorded. A planar mechanical model was used to determine the muscle torque required to keep the upper limb posture constant while performing wrist movements. All subjects showed anticipatory postural adjustments (APA) which, unlike those described for whole-body postural control, could not counteract in advance the perturbing inter-segmental forces created by the movement. Postural muscles were activated before the wrist movement with a chronology specific to the direction of the wrist movement. Some postural muscular activities anticipated that of the prime-movers in accordance with muscle torque, which had to be applied to the joints to keep the upper limb posture constant. These results reveal that the central nervous system (CNS) uses the same organization of the motor command for the control of both segmental and whole-body posture: APA and corrective postural adjustments (CPA), which are based on well-organized anticipatory postural muscle activities (APMA), except that APA can be non-efficient in segmental postural control. The presence or absence of an elbow support influenced the level of activation of postural muscle but not their chronology. This result suggests that the CNS uses a sequence of APMA: a postural muscle synergy which is predetermined as a function of the intended direction of the movements and modulates the gain towards certain muscles, in accordance with the gravitational effects, and supports reaction changes.

Anticipatory postural adjustments (APA) associated with high vertical jumping were studied in five normal men. The dependence of APA on final equilibrium and task complexity was studied by analyzing the characteristics of single versus repetitive jumps. Subjects performed series of 25 single jumps and 25 repetitive jumps on a force platform, in response to a Go signal. Surface EMG from the Soleus (SO), Tibialis anterior (TA), Vastus lateralis (VL) and Biceps femoris (BF) muscles were recorded simultaneously with the three components of the ground reaction forces. The existence of APA associated with both single and repetitive jumps was evidenced by a backward shift of the center of pressure, an SO deactivation, and a TA activation which preceded the onset of movement by hundreds of milliseconds. In repetitive jumps, SO deactivation occurred later than in single jump, whereas the center of pressure shift started earlier, lasted longer and had a greater amplitude. Moreover, repetitive jumps were initiated later with respect to the Go signal. These data show that the central nervous system anticipates not only the equilibrium disturbance caused by an initial movement but that caused by a whole sequence of movements. They also show that the corresponding APA are based heavily on a single functional ankle synergy.

The aim of this study was to access the adaptability of the gait initiation program by imposing before and during gait a posture that partially prevents the backward shift of the center of foot pressure. Six healthy subjects performed normal gait in the control situation (CS) and gait in the absence of heel ground contact in the test situation (TS) on a force platform at three different speed conditions. It is shown that an increase in the duration of the anticipation phase in TS is necessary to create conditions for progression which allow the subjects to reach a gait velocity similar to the one obtained in CS at the end of the anticipatory movements and also at the end of the first step. Modifications of the gait initiation program occur in order to fulfil the performance in terms of gait velocity.

This study was carried out to determine the existence and the specificity of anticipatory postural adjustments (APA) associated with vertical jump. Single vertical jump is an upward-oriented movement with identical initial and final postures. A consistent backward shift of the center of pressure (CP) clearly demonstrated the existence of APA. This shift mainly resulted from a soleus (SO) deactivation and tibialis anterior (TA) activation. As in gait initiation, the main role of APA associated with vertical jump is to create the necessary disequilibrium to initiate the movement. A comparison of CP and CG (center of gravity) forward shifts between single jump and tip-toe rising allowed us to propose that forward disequilibrium ends when a critical CP position is reached. As in gait initiation, the duration of jump's APA was several hundreds of milliseconds. This showed that when the role of the APA is to create the initial disequilibrium, their duration is greater than when their role is to compensate for the forthcoming postural disturbance due to the movement. Since the goal of the voluntary movement was to jump at a given height, one expected that this parameter could be programmed. APA's timing depended on the amplitude of the vertical jump: the time of onset of SO deactivation and TA activation and that of backward CP shift were dependent on the peak amplitude of the jump. Nevertheless, there was no relationship between the backward CP shift amplitude and the vertical impulse. Thus, it appears that the amplitude of the jump is not fully programmed but must be adjusted after the onset of the movement, i.e., there is a sequential programming of forward and upward movement. It is also of interest to stress that the SO-TA synergy, which is shown here to be at the basis of the APA associated with single vertical jump, has been also found in many forward-oriented movements. Thus, the present data reinforce the idea that SO-TA synergy is part of a functional synergy used by the central nervous system to simplify the postural motor command.

Six sitting healthy subjects were instructed to keep a constant upper limb posture while performing wrist flexions and extensions. Acceleration of the wrist, elbow and shoulder joints, and surface electromyograms (EMGs) of the upper limb's main flexors and extensors were studied. Results indicated the existence of anticipatory (APA) and corrective postural adjustments. The APAs were based on a reproducible directional chronology of postural muscle activations. As shown by a simple mechanical model, this chronology was in accordance with the muscular torque which should be applied to the joints to keep the upper limb posture constant. All these data indicate that APA are involved in segmental posture, where their general organization is similar to those of APA associated with whole-body movements. The use of constant directional postural synergies well agrees with a simplification of the motor control according to Bernstein's theory.

Abstract: Children with unilateral clubfoot (CF) treated conservatively have residual foot deformities and triceps surae m. atrophy. Using surface electromyography of tibialis anterior (TA), gastrocnemius (GA), and peroneus longus muscles, simultaneously with ground reaction forces recordings, the present work assesses the influence of this pathology on the gait initiation process. Ten children with CF and 10 healthy children were investigated. In children with CF, the velocity of the centre of gravity (CG) at the end of gait initiation did not differ from that of healthy children, because of adaptations of anticipation and execution phases. CG velocity at the end of anticipation was lower in children with CF than in healthy children when the swing foot was the affected one, indicating that propulsion was less efficient in this condition. It is shown that this resulted from alterations in anticipation duration, initial centre of pressure position and TA and PL excitations. Execution was shortened when support was provided by the pathological foot: the motor program was adapted to shorten the phase during which equilibrium control might be deficient. Biomechanical characteristics of the execution phase of children with CF did not depend on the swing foot. This indicated that the sound foot cannot be used as a control for accessing residual deficiencies. [Copyright &y& Elsevier]