Postural control system.

The postural control system has two main functions: first, to build up posture against gravity and ensure that balance is maintained; and second, to fix the orientation and position of the segments that serve as a reference frame for perception and action with respect to the external world. This dual function of postural control is based on four components: reference values, such as orientation of body segments and position of the center of gravity (an internal representation of the body or postural body scheme); multisensory inputs regulating orientation and stabilization of body segments; and flexible postural reactions or anticipations for balance recovery after disturbance, or postural stabilization during voluntary movement. The recent data related to the organization of this system will be discussed in normal subjects (during ontogenesis), the elderly and in patients with relevant deficits.

Axial rotation in Parkinson's disease.

AIMS: To investigate the ability of patients with Parkinson's disease to perform a rotation around the longitudinal axis of the body. Three questions were raised. Is body rotation impaired in Parkinson's disease? Is there a level of the kinematic chain from the head to the foot at which the impairment is more severe? Is the deficit related to the general slowness of movement in Parkinson's disease? METHODS: Kinematic data were recorded. The temporal organisation of body rotation during gait initiation was analysed in 10 patients with Parkinson's disease, who were all at an advanced stage of the disease and had all experienced falls and freezing during their daily life, and in five controls. The latency of the onset of the rotation of each segment was measured by taking the onset of the postural phase of step initiation as reference value. Locomotor variables were also analysed. RESULTS: Body rotation was found to be impaired in patients with Parkinson's disease, as the delay in the onset of the rotation of each segment is greater than that in controls. Moreover, a specific uncoupling in the onset of shoulder and pelvis segment rotation was seen in patients. This impairment of rotation is not related only to the general slowness of movements. CONCLUSION: Patients with Parkinson's disease were found to have an impairment of posturo-kinetic coordination and impaired capacity to exert appropriate ground reaction forces to orient the pelvis in space.

Behavioral outcomes following below-knee amputation in the coordination between balance and leg movement.

Lateral leg movement is accompanied by opposite movements of the supporting leg and trunk segments. This kinematic synergy shifts the center of mass (CM) towards the supporting foot and stabilizes its final position, while the leg movement is being performed. The aim of the present study was to provide insight in the behavioral substitution process responsible for the performance of this kinematic synergy. The kinematic synergy was assessed by the principal component analysis (PCA) applied to both hip joints and supporting ankle joint. Patients after unilateral below-knee amputation and control subjects were asked to perform a lateral leg raising. The first principal component (PC(1)) accounted for more than 99% of the total angular variance for all subjects (amputees and controls). PC(1) thus well represents the possibility to describe this complex multi-joint movement as a one degree of freedom movement with fixed ratios between joint angular time course. In control subjects, the time covariation between joints changes holds during all phases of the leg movement (postural phase, ascending and braking phases). In amputees, PC(1) score decreased during the ascending phase of the movement (i.e. when the body weight transfer is completed, while the movement is initiated). We conclude that a feedback mechanism is involved and discuss the hypothesis that this inter-joint coordination in amputees results from a failure in the pre-setting of the inter-joint coupling.

Goal directed locomotion and balance control in autistic children.

This article focuses on postural anticipation and multi-joint coordination during locomotion in healthy and autistic children. Three questions were addressed. (1) Are gait parameters modified in autistic children? (2) Is equilibrium control affected in autistic children? (3) Is locomotion adjusted to the experimenter-imposed goal? Six healthy children and nine autistic children were instructed to walk to a location (a child-sized playhouse) inside the psychomotor room of the pedopsychiatric centre located approximately 5 m in front of them. A kinematic analysis of gait (ELITE system) indicates that, rather than gait parameters or balance control, the main components affected in autistic children during locomotion are the goal of the action, the orientation towards this goal and the definition of the trajectory due probably to an impairment of movement planning.

Postural control systems in developmental perspective.

How can the adult postural organisation be elucidated using an ontogenetic approach, and what questions can be raised about ontogenesis starting from the organisation of adult posture? These questions will be addressed taking three aspects of postural organisation. The first is the internal representation of erect posture, including the role played by the various sensory inputs in this representation. The second aspect relates to the variables which are controlled during erect posture: is it the body orientation with respect to the vertical or the localisation of the centre of gravity with respect to the feet which is controlled? The third aspect concerns the coordination between posture, equilibrium and movement, focusing on the role played by an internal representation of the external world and its interactions with the body segments in organising the anticipatory postural adjustments. The central organisation of coordinated control will also be considered. Each of these aspects will be discussed in relation to ontogenetic considerations.

Comparative study of the posterior red nucleus in baboons and gibbons.

The posterior red nucleus (PRN) was studied in two species of primates by the technique of retrograde degeneration of rubrospinal cells following transection of the spinal cord at different levels. The form of the PRN was reconstructed for both a quadruped monkey (baboon) and an anthropoid with erect posture (gibbon). The PRN contains polymorphic cells characterized by their very chromophilic and granular Nissl substance. These neurons vary in diameter from 25 micrometer to 70 micrometer. Some of them give rise to the rubrospinal tract. Baboon: The approximately 1,300 rubrospinal cells in this species are divided into two equal groups, one related to the contralateral forelimb, with axons ending between the second cervical and third thoracic segment, and the other related to the contralateral hindlimb, projecting caudally beyond T3. Following a high cervical lesion, nondegenerated cells of similar description remain throughout the nucleus. A significantly large group of these cells occurs medially and may be the source of fibers ending in the brain stem or cerebellum. Gibbon: In this species, the number of rubrospinal cells controlling the hindlimb is less than half that found in the baboon. This reduction in the gibbon is much greater for medium-sized cells, but is also significant for the giant cells. These results obtained from primates are compared with those reported for the cat. A possible function for the PRN in the control of limb movements is discussed from the viewpoint of phylogeny.

Balance control during lateral arm raising in humans.

The aim of the present study was to determine (i) whether the centre of gravity (CG) shift resulting from the mechanical effect of arm raising in the frontal plane is minimized in standing subjects and (ii) whether this putative minimization is affected by the support conditions (unipedal vs bipedal) and/or by adding a load (3.5 kg) to the moving hand. Our results indicate first that a CG control does exist during bipedal stance (compensating for 31% of the mechanical shift), which increases markedly in unipedal stance (73%) and secondly, that an additional load is compensated for solely in the case of unipedal stance (81%).

Is the trunk a reference frame for calculating leg position?

Naive subjects and dancers were instructed to raise a leg laterally toward 45 degrees. The final position reached by the leg by each group of subjects was quite different: 48 degrees in dancers, i.e. close to the required value, and 56 degrees in the naive subjects. The reason for this difference was investigated. During the body weight transfer toward the supporting side prior to the leg movement, naive subjects inclined both leg and trunk laterally, whereas the dancers' trunk remained vertical. It was observed that in naive subjects the trunk inclination and the overestimation of the final leg position were closely correlated. The results suggest that in both naive subjects and dancers, the trunk axis serves as a reference value for calculating the leg position.

Static and dynamic postural control in long-term microgravity: evidence of a dual adaptation.

The adaptation of dynamic movement-posture coordination during forward trunk bending was investigated in long-term weightlessness. Three-dimensional movement analysis was carried out in two astronauts during a 4-mo microgravity exposure. The principal component analysis was applied to joint-angle kinematics for the assessment of angular synergies. The anteroposterior center of mass (CM) displacement accompanying trunk flexion was also quantified. The results reveal that subjects kept typically terrestrial strategies of movement-posture coordination. The temporary disruption of joint-angular synergies observed at subjects' first in-flight session was promptly recovered when repetitive sessions in flight were analyzed. The CM anteroposterior shift was consistently <3-4 cm, suggesting that subjects could dynamically control the CM position throughout the whole flight. This is in contrast to the observed profound microgravity-induced disruption of the quasi-static body orientation and initial CM positioning. Although this study was based on only two subjects, evidence is provided that static and dynamic postural control might be under two separate mechanisms, adapting with their specific time course to the constraints of microgravity.

Red nucleus: past and future.

The red nucleus has greatly interested scientists for almost a century. This can be explained by the fact that problems of general interest are encountered when studying this nucleus. Some of them are outlined in this paper, such as the phylogenetic evolution of the rubrospinal tract, the respective roles of the rubrospinal and pyramidal tracts in the execution of various types of movements, and the respective roles of these two tracts in movement automatization.

Unitary acitvity of ventrolateral nucleus during placing movement and associated postural adjustment.

The activity of neurons in the ventrolateral nucleus of the thalamus (VL) was analyzed in the cat during placing movements of both the right and left forelimbs. The purpose was to determine if thalamic neuron discharge patterns could be related to movement and/or to postural changes. Placing tests were conducted on animals maintained in a standing position and partially restrained by a hammock. Each of the forelimbs rested on a flat surface containing a strain gauge which allowed the stance forces to be measured. Two mobile plates, one on the left and one on the right, were used to elicit a contact placing reaction. The response was composed of an isometric phase, during which the body weight was shifted from the stimulated limb to the opposite forelimb while the stimulated limb was gently pushed backwards, and a movement phase during which the stimulated paw actually accomplished the placing reaction. (1) About half the recorded neurons (47/86) in the VL region demonstrated a change in activity, generally an increase of discharge frequency, during placing of the right of left forelimb. (2) Almost all the reactive units (45/47) responded to contralateral placing. Two units changed their activity with ipsilateral placing only, whereas 18 units were active during placing of both forelimbs. The discharge pattern of cells activated during ipsilateral placing was considered as related to the isometric postural adjustment of the contralateral limbs. (3) Cells reactive during contralateral tests were located preferentially in the ventrolateral part of VL. Units reactive during ipsilateral tests (postural units) were also clustered in the ventrolateral half of VL which is the zone controlling limb musculature. Some of the 'postural units' were identified as receiving afferents from cerebellar nuclei and projecting to motor cortex. (4) Changes in discharge frequency were observed during either the isometric phase, the movement phase, or both. The same type of patterns were observed during contralateral and ipsilateral placing. About half of the cells responding during the contralateral placing movement did so throughout the entire duration of the movement, and were not specifically related to either the flexion or the extension phase of the placing. (5) The timing of the discharge of VL units with respect to the isometric phase and to the movement phase of the motor sequence varied from cell to cell. No topographic arrangement of neurons with the same pattern of discharge could be found within the nucleus.

Arm raising in humans under loaded vs. unloaded and bipedal vs. unipedal conditions.

The aim of the present experiment was to study the central organization of equilibrium control during arm raising in the frontal plane. Nine adult subjects (five seniors and four young adults) were asked to raise their right arm to a horizontal position in the frontal plane in two support conditions (bipedal vs. unipedal) and two load conditions (unloaded vs. a 3.5-kg load added on the moving hand). No instructions were given concerning the movement speed. The movements were performed at about half the maximum speed achievable under reaction time conditions. The final lateral center of mass (CM) position 1 s after the movement offset, and the time course of the CM shift during the movement were studied in the four experimental conditions, using a CM compensation index. The electromyographic (EMG) pattern of the main muscles involved in the movement performance and in the postural control were studied in three out of nine subjects during movements performed at two velocities (at the preferred speed and as fast as possible). The results indicate that (1) the CM shift remains minimized in the frontal plane during the time course of the arm movement and during the final stabilization of the arm regardless of the stance and load conditions; (2) the time course of the CM compensation index remains stable during the first 400 ms after the movement onset, decreasing late in the movement and increasing again at the end of the stabilization stage. A modelisation suggests that the time course is the result of the interaction of two controls: a first one, putative feedforward, starting early and decreasing with time and a second one, putative feedback, starting late in the movement and increasing with time; (3) both early and late index values are influenced by the support and load conditions, the highest index values being observed during unipedal stance and load conditions; (4) activation of quadratus lomborum (QL) contralateral to the raising arm is time locked with the deltoidus activation of the raising arm in both fast and slow movements: this contralateral QL activation corresponds to an anticipatory postural adjustment (APA) aimed at minimizing the CM shift.

Is the center of gravity controlled during upper trunk movements?

The question was addressed in this study as to whether the kinematic synergy responsible for equilibrium control during upper trunk movements may involve an actual evaluation of the weight of the body segments. Five adult subjects were asked to bend the upper trunk forward or backward to an angle of about 35 degrees, first without any load and then with a 10 kg load fixed to their shoulders. The center of gravity (CG) shift in the sagittal direction which occurred at the end of the movement was compared versus without a load. Two patterns of CG shift were identified. In the first pattern shown by three subjects in forward and three in backward trunk bending, the CG shift remained the same, while in the other pattern (two subjects for forward and two for backward trunk bending), the CG shift increased by an amount corresponding to the purely mechanical effect of the load. These results indicate that the actual weight of the segments may be evaluated on the basis of sensory inputs and may be responsible for a change in the kinematic synergy which preserves the CG control during the upper trunk movement.

The postural reaction to the drop of a hindlimb support in the standing cat remains following sensorimotor cortical ablation.

The postural response to an unexpected drop of either hindlimb platform was studied in the freely standing cat. The vertical forces, the forelimb electromyographic (EMG) activities and the movement of the trunk were analysed. A stereotyped diagonal pattern of support was observed. The imposed unloading of one hindlimb was followed by unloading of the diagonally opposite forelimb and loading of the two other limbs. In the ipsilateral loaded forelimb, the force increase is preceded by an activation of the biceps and is concomitant with a triceps coactivation. In the contralateral unloaded forelimb, reciprocal changes of biceps (activation) and triceps (inhibition) were observed and preceded the force decrease. Lateral or vertical displacement or rotation of the vertebral column at the high thoracic level, as evidenced by movie recording, began no earlier than 100 ms after the hindlimb drop and thus followed the EMG changes. Unilateral sensorimotor cortex ablation had no effect on this pattern or on the latencies of forelimb myographic responses. These results suggest that the hindlimb drop triggers stereotyped forelimb EMG responses which are organized centrally and that these responses are not mediated via a sensorimotor cortical loop.

Preparatory postural adjustments in parkinsonian patients with postural instability.

BACKGROUND: Postural instability is a common problem in patients with Parkinson's disease. This paper reports results of a study undertaken to investigate some of the possible mechanisms responsible for this instability. METHODS: Preparatory postural adjustments associated with a lateral leg raising task were studied in five parkinsonian patients and four age-matched controls. Recordings included ground reaction forces, kinematics, and surface EMG activity from multiple leg muscles. RESULTS: In normal subjects there was a well-defined sequence of events preceding the onset of leg elevation, beginning with a transfer of centre of foot pressure (CP), initially toward the moving leg and then back to the support side, followed by displacement of the trunk toward the support side. In the more severely affected parkinsonian patients, the amplitude of the initial displacement of CP was markedly reduced. The interval between the earliest force changes and the onset of leg elevation was prolonged and the relative timing of the kenematic adjustments during this interval was disrupted. In addition the alternating burst and periods of inhibition observed in the EMG recordings from the normal subjects were replaced by continuous tonic EMG activity. CONCLUSIONS: These observations suggest that abnormalities in programming preparatory postural adjustments may contribute to postural instability in some patients with advanced Parkinson's disease.

Coordination of axial rotation and step execution: deficits in Parkinson's disease.

To determine why parkinsonian patients (PP) present some difficulties to initiate locomotion, a diagonal step has been investigated in two tasks in five control subjects (CS) and in ten PP. In the first task, the subjects had to perform one diagonal step without change in their orientation (WR); in the second task, they had to perform one diagonal step with a body rotation in the step direction (RO). The defended hypothesis is that the gait initiation deficits in Parkinson disease are a consequence of their difficulties to coordinate al the component of a complex movement. The analysed parameters were the duration of the postural and movement phases, the step length and velocity, and the amplitude of the horizontal ground reaction forces during each phase. Compared to CS, the PP showed a lengthening of the postural phase, a decrease in the step length and velocity and a reduction of the horizontal forces. The comparisons between the performances obtained in the WR versus those obtained the RO show in CS that the performances remained unchanged, whereas in PP the performances were significantly more altered in the RO. It illustrates the specific deficit occurring in PP while performing complex tasks where coordination between several components has to be achieved simultaneously.

Reorganization of equilibrium and movement control strategies after total knee arthroplasty.

This work was aimed at identifying changes in posturomotor control strategies in patients with unilateral total knee arthroplasty. Using kinetic and kinematic data, a previous study had revealed that, during a side step, patients with unilateral knee arthritis showed a shortened monopodal phase and a lengthened postural phase when the affected leg was the supporting one. It was expected that these strategies would be modified after undergoing total knee arthroplasty. Postoperatively the durations of the monopodal phase and of the postural phase became similar when the operated limb was supporting and when the sound limb was supporting. Concerning the upper body movements, the same asymmetrical results as before surgery were observed. Hence, patients with total knee arthroplasty exhibit posturomotor strategies which, although they become close to normal, remain asymmetrical. The durations of the monopodal and of the postural phases could be considered to assess the results of total knee arthroplasty.

The postural support of movement in cat and dog.

The postural adjustment which accompanies single limb movement in the standing cat and dog was analyzed. Four trays equipped with strain gauges were used for measuring the vertical forces exerted by each limb before and during movement performance. Three types of movements were analyzed: flexion movements elicited by motor cortex stimulation, placing movements, conditioned movements of either forelimb or hindlimb (lift-off in cat, flexion with maintained final position in dog). In both cats and dogs the postural adjustment during movement consists of a bipedal stance on two diagonally opposite limbs. Large quantitative differences were observed depending on the type of movement. Cortical stimulation elicited an adjustment where changes of forces exerted by the forelimb a hindlimb were nearly equal. During conditioned fore- and hindlimb lift-off in the cat there was a tendency to use only forelimbs for the postural adjustment associated with forelimb movement and hindlimbs for the adjustments associated with hindlimb movement. For placing in the cat and conditioned movement in the dog, the adjustment was intermediate, that is a predominant contribution of forelimb support with forelimb movements but nevertheless an associated contribution from hindlimbs. The general significance of the results with respect to the mechanism of postural adjustment associated with movement is analyzed.

Body orientation and regulation of the center of gravity during movement under water.

Professional divers were instructed to adopt a vertical posture under water with their feet fixed to the ground and to perform a fast forward or backward upper trunk bending movement in response to a tone. Kinematic and EMG analyses were performed. It was first noted that the divers adopted a forward inclined, erect posture, suggesting that the verticality was misevaluated, although the effects of gravity were still exerted on the otoliths. Second, the upper trunk movements were still accompanied by opposite movements of lower segments and, as a result, the center of gravity displacement was still minimized, although not so accurately as on the ground. The EMG pattern consisting of early activation of a set of trunk, thigh, and shank muscles continued to occur under water. These results suggest that "axial synergies" associated with upper trunk movements are learned motor habits that regulate the center of gravity position regardless of the equilibrium constraints.

Axial synergies under microgravity conditions.

Fast forward and backward upper trunk movements were analyzed and compared under microgravity and under preflight and postflight conditions. The kinematic analysis showed that the upper trunk movements were accompanied by hip and knee movements in the opposite direction both under microgravity and under normal gravitational conditions. This suggests that the center of mass position with respect to the feet is still regulated under microgravity when the feet are attached to the floor. The EMG analysis during backward movements shows that under preflight conditions a set of muscles (ErSp, BF, Sol) in the back of the body are activated early on. Under microgravity, the early Sol activation was replaced by an early TA activation, which was still present at the first postflight recording and was then replaced by the early Sol activation observed under preflight conditions. This finding shows that the EMG pattern underlying the axial synergies is flexible and that adaptive changes take place both under microgravity and after return to Earth.