Christine Assaiante

Ontogenesis of head stabilization in space during locomotion in children : influence of visual cues

The main purpose of this study was to investigate the development of the head stabilization in space strategy (HSSS) during various locomotor tasks in 3- to 8-year-old children and adults. The contribution of visual factors to the HSSS was also examined by applying peripheral visual restriction, stroboscopic visual motion cue restriction, and darkness. The kinematics of the head and trunk rotations (pitch, yaw, and roll) were analyzed by means of an optical TV-image processor (ELITE system). For each of the three angular components, an appropriate “head anchoring index” was defined in order to compare the HSSS with a head stabilization on the trunk strategy. Head-trunk correlation rates were also calculated for each angular component in order to evaluate the head-trunk stiffness. The development of head-trunk coordinations during locomotion under normal vision can be said to involve at least three main periods. The first period occurs from the age of 3 to 6 years, when the HSSS is adopted only while walking on the flat ground. While walking on narrow supports, children in this age-group rather tend to increase the head-trunk stiffness, especially at 6 years of age. The second period includes 7- to 8-year-old children. Children of this age become able to adopt the HSSS while walking on narrow supports. During this period, the HSSS is associated with a large decrease in the head-trunk correlations. Lastly, in adulthood the HSSS is commonly adopted but specifically involves the roll component associated with the lateral body oscillations while walking. Vision was found to have little influence on childrens HSSS while walking, whatever their age. Moreover, darkness induces an increase in the efficiency of the HSSS in adults. This confirms that the HSSS is the most appropriate strategy available for dealing with an increase in the level of equilibrium difficulty and may reflect a “top-down” organization of the postural control while walking. These results also suggest that the HSSS may be mainly of vestibular origin and presumably serves to facilitate the visual input processing, particularly that of the motion and peripheral visual cues which are involved in the control of body equilibrium during locomotion.

Motor control and children with autism: deficit of anticipatory function?

This study aims at investigating how do anticipatory postural adjustments develop in children with autism, during a bimanual load-lifting task that required maintaining the stabilisation of the forearm despite imposed or voluntary unloading. Elbow angle and electromyographic were recorded on the child forearm supporting the load. The forearm stabilisation was as good in children with autism as in the control group. However, in children with autism, the latencies for both kinematics and muscular events indicated an increase of the duration of unloading. These results indicate the use of a feedback rather than a feed-forward mode of control. Impairments in both the building of internal representations and the mastering of timing parameters, could explain the deficient postural anticipation reported in children with autism.

Development of Postural Control in Healthy Children: A Functional Approach

From a set of experimental studies showing how intersegmental coordination develops during childhood in various posturokinetic tasks, we have established a repertoire of equilibrium strategies in the course of ontogenesis. The experimental data demonstrate that the first reference frame used for the organization of balance control during locomotion is the pelvis, especially in young children. Head stabilization during posturokinetic activities, particularly locomotion, constitutes a complex motor skill requiring a long time to develop during childhood. When studying the emergence of postural strategies, it is essential to distinguish between results that can be explained by biomechanical reasons strictly and those reflecting the maturation of the central nervous system (CNS). To address this problem, we have studied our young subjects in situations requiring various types of adaptation. The studies dealing with adaptation of postural strategies aimed at testing short and long-term adaptation capacity of the CNS during imposed transient external biomechanical constraints in healthy children, and during chronic internal constraints in children with skeletal pathologies. In addition to maintenance of balance, another function of posture is to ensure the orientation of a body segment. It appears that the control of orientation and the control of balance both require the trunk as an initial reference frame involving a development from egocentric to exocentric postural control. It is concluded that the first step for children consists in building a repertoire of postural strategies, and the second step consists in learning to select the most appropriate postural strategy, depending on the ability to anticipate the consequence of the movement in order to maintain balance control and the efficiency of the task.

Discrete Visual Samples May Control Locomotor Equilibrium and Foot Positioning in Man

The static or dynamic visual cues required for equilibrium as well as for foot guidance in visually guided locomotion in man were studied using a variety of locomotion supports and illumination and visual conditions. Stroboscopic illumination (brief flashes) and intermittent lighting (longer flashes) were used to control and to vary the visual sampling frequency of static (positional/orientational) visual cues. There were three main findings: First, visual control of foot positioning during locomotion over a narrow support depends mainly upon the availability of high frequency static visual cues (up to about 12 Hz); and third, static visual cues required for equilibrium control are extracted from both the peripheral and the central visual field. Assuming that discrete demands for feedback occur, a simple probabilistic model was proposed, according to which the mean time that elapses following presentation of static visual cues about positions or changes of position accounts for the differences in the difficulty of the various illumination conditions.

Development of postural adjustment during gait initiation: kinematic and EMG analysis.

Abstract The authors studied the development of postural adjustments associated with the initiation of gait in children by using kinematic and electromyographic (EMG) analysis. Participants (N = 28) included infants with 1-4 and 9-17 months of walking experience, children 4-5 years of age, and adults. Anticipatory postural adjustments (APA) were present in the youngest age groups, including a clear anticipatory lateral tilt of the pelvis and the stance leg, which enabled the child to unload the opposite leg shortly before its swing phase. An anticipatory activation of the hip abductor of the leg in stance phase prior to heel-off was found, suggesting pelvis stabilization. APA did not appear consistently until 4-5 years of age. A decrease in segmental oscillations occurred across the ages, indicating better control of intersegmental coordination in the frontal and sagittal planes during the postural phase of gait initiation. Young walkers presented APA involving movements of both the upper and the lower parts of the body, whereas, like adults, 4- to 5-year-olds were able to laterally shift only the pelvis and the stance leg. The oldest children and the adults also showed lower activation levels of hip and knee muscles but higher activation at the ankle level. Those kinematic and EMG results taken together suggest a clear developmental sequence from an en bloc operation of the body through an articulated operation with maturation, walking experience, or both.

Development of Anticipatory Orienting Strategies During Locomotor Tasks in Children

Some basic problems related to the development of goal-directed locomotion in humans are reviewed here. A preliminary study is presented which was aimed at investigating the emergence of anticipatory head orienting strategies during goal-directed locomotion in children. Eight children ranging from 3.5 to 8 years had to walk along a 90 degrees right corner trajectory to reach a goal, both in light and in darkness. The instantaneous orientation in space of the head, trunk, hips and left foot antero/posterior axes was computed by means of an ELITE four-TV camera, 100 Hz system. The results showed that predictive head orienting movements can occur also in the youngest children. The head starts to rotate toward the goal before the corner point of the trajectory is reached. In children, the head peak rotation coincides with the trajectory corner while in adults the peak is attained before. In children, the walking speed is largely decreased in darkness. The results suggest that feedforward control of goal-directed locomotion appears very early in gait development and becomes increasingly important afterwards.

A statistical approach to sensorimotor strategies: conjugate cross-correlations.

A simple method, based on cross-correlation functions (CCFs) between two time series of kinematic or physiological measurements, is proposed for the analysis of multisegmental movements. Special emphasis is placed on measuring accelerations. When the movements of two body segments are coordinated but consistently time lagged, their CCF displays a peak at the corresponding time abscissa. The reproducible positions of the peaks reflect biomechanical or physiological constraints. Several significantly large peaks can be observed in a CCF. It is possible to identify coordinated movements involving more than two segments by applying simple rules of compatibility between the time lags and between the signs of the correlation peaks. With the method proposed, it is possible to determine the signs of relative variation and the time lags of the successive statistically correlated segmental movements. This is particularly useful in the case of both continuous and periodic sensorimotor control, where classical poststimulus methods cannot be applied. Unlike the classical poststimulus methods, this method does not require a time origin, and it is not necessary to monitor the muscles or even to specify exactly which ones are involved. The method is also applicable to experiments involving a time origin (e.g., and applied perturbation), although in this case it is less accurate than the averaging technique. Individual postural strategies can be identified, which suggests some interesting potential applications of the method to clinical studies.

Locomotor skills and balance strategies in adolescents idiopathic scoliosis.

Study Design. Locomotor balance control assessment was performed to study the effect of idiopathic scoliosis on head-trunk coordination in 17 patients with adolescent idiopathic scoliosis (AIS) and 16 control subjects. Objective. The aim of this study was to explore the functional effects of structural spinal deformations like idiopathic scoliosis on the balance strategies used during locomotion. Summary of Background Data. Up to now, the repercussion of the idiopathic scoliosis on head-trunk coordination and balance strategies during locomotion is relatively unknown. Methods. Seventeen patients with AIS (mean age 14 years 3 months, 10° < Cobb angle > 30°) and 16 control subjects (mean age 14 years 1 month) were tested during various locomotor tasks: walking on the ground, walking on a line, and walking on a beam. Balance control was examined in terms of rotation about the vertical axis (yaw) and on a frontal plane (roll). Kinematics of foot, pelvis, trunk, shoulder, and head rotations were measured with an automatic optical TV image processor in order to calculate angular dispersions and segmental stabilizations. Results. Decreasing the walking speed is the main adaptive strategy used in response to balance problems in control subjects as well as patients with AIS. However, patients with AIS performed walking tasks more slowly than normal subjects (around 15%). Moreover, the pelvic stabilization is preserved, despite the structural changes affecting the spine. Lastly, the biomechanical defect resulting from idiopathic scoliosis mainly affects the yaw head stabilization during locomotion. Conclusions. Patients with AIS show substantial similarities with control subjects in adaptive strategies relative to locomotor velocity as well as balance control based on segmental stabilization. In contrast, the loss of the yaw head stabilization strategies, mainly based on the use of vestibular information, probably reflects the presence of vestibular deficits in the patients with AIS.

Voluntary head stabilization in space during oscillatory trunk movements in the frontal plane performed in weightlessness

Abstract The ability voluntarily to stabilize the head in space during lateral rhythmic oscillations (0.59±0.09 Hz) of the trunk has been investigated during microgravity (μG) and normal gravity (nG) conditions (parabolic flights). Five healthy young subjects, who gave informed consent, were examined. The movements were performed with eyes open or eyes closed, during phases of either μG or nG. The main result was that head orientation with respect to vertical may be stabilized about the roll axis under μG with, as well as without vision, despite the reduction in vestibular afferent and muscle proprioceptive inputs. Moreover, the absence of head stabilization about the yaw axis confirms that the degrees of freedom of the neck can be independently controlled, as was previously reported. These results seem to indicate that voluntary head stabilization does not depend crucially upon static vestibular afferents. Head stabilization in space may in fact be organized on the basis of either dynamic vestibular afferents or a short-term memorized postural body schema.

Impairment of a cortical event-related desynchronisation during a bimanual load-lifting task in children with autistic disorder

In autism, the abilities of communication are affected, associated with abnormalities of cognitive, sensorial and motor development. In a previous study based on a load-lifting task, we showed impairment of anticipation in children with autism as evidenced by kinematics and eletromyographic recordings [Neurosci. Lett. 348 (2003) 17]. In the present study, we assessed the cortical counterparts of the use of anticipatory postural adjustments in a group of control children and in a group of children with autism. The tasks required maintaining a stable forearm position despite imposed or voluntary lifting of an object placed either on the controlateral forearm or on a support. We investigated the differences between the two groups of children on the Event-Related Desynchronisation (ERD) which precedes movement onset in adults [Electroencephalogr. Clin. Neurophysiol. 46 (1979) 138]. Electroencephalogram (EEG) power evolution of a 6-8-Hz frequency band was averaged before and after imposed or voluntary movement onset. EEG reactivity of control and autistic children did not differ during the imposed unloading condition, but significant differences appeared in the voluntary unloading situations. Before lifting the object, control children showed an ERD above the left motor areas. An ERD also occurred above the right motor areas when the object was placed on their forearm. This indicates that the ERD can also translate the use of anticipatory postural adjustments. By contrast, children with autism did not show an ERD in the two voluntary situations. This suggests a central deficit of anticipation in both postural and motor control in children with autism.