Effect of head position and visual condition on balance control in inverted stance.

This study analyzed the effect of head position and visual condition on the control of balance in handstand, a gymnastics posture that necessitates adaptation of sensory information processing. Five expert gymnasts participated. Centre of pressure trajectories and kinematics of different body segments were recorded. The gymnasts were instructed to maintain three handstands as long as possible in four head positions, with and without vision. Performances and postural stability was much better in the standard and dorsiflexion positions than in the aligned and ventroflexion positions under the two conditions of vision. Performances were lower without vision in the standard and dorsiflexion position. If vision clearly plays an important role, yet the tonic neck reflexes also seem to contribute greatly to control body sways during inverted posture.

Behavioural changes induced by early and long-term gravito-inertial force modification in the rat.

The study concerns rats conceived, born and raised in a hypergravity environment (HG: 2 g) for 3 months using a centrifuge. They were then exposed to terrestrial gravity (1 g) and submitted to behavioural tests investigating their spontaneous locomotor activity (open-field), their posture (support surface), and their vestibular function (air-righting reflex). Performances were compared to age-matched control rats housed at 1 g for the same time period. Results showed static and dynamic behavioural deficits as early as the rats were exposed to normal gravity. They exhibited strongly increased motor activity in open-field, with longer travelled distances and more scattered trajectories; in addition, the HG rats displayed more numerous rearings than controls did. They showed postural changes characterized by an enlarged support surface and they did not succeed in the air-righting reflex, due to increased time-delay for head righting. None of these changes were permanent. Indeed, for all tests, the HG rats tested after 3 weeks spent in normal terrestrial gravity exhibited behaviours similar to those of the controls. HG-induced changes in the functional properties of the vestibular system may explain the deficits showed by the HG rats once exposed to normal gravity. The adaptation process to 1 g leading to the appearance of normal behaviour takes about 3 weeks. It likely implicates a central re-evaluation of the sensory inputs and an updating of the motor commands.

Kinematics of treadmill locomotion in rats conceived, born, and reared in a hypergravity field (2 g). Adaptation to 1 g.

The kinematics of treadmill locomotion in rats conceived, born, and raised in a hypergravity environment (HG: 2g) until the age of 3 months was investigated for 5 weeks after their exposition to earth's gravity. The locomotor performance of the HG rats (N=7) was compared to that of age-matched control rats (N=8) housed at 1g for the same period. Kinematic analysis of treadmill locomotion was performed up to 35 days of terrestrial life by an optoelectronic motion analyzer (ELITE system). Results showed that the HG rats exhibited a faster locomotor rhythm (increased number of steps/s), walked closer to the ground, and had a more dorsiflexed foot position. Also, HG rats had shorter steps. The data also highlight a fast adaptation to normal gravity since all the locomotor parameters returned to normal values within 3 weeks. The locomotor modifications may be seen as the persistence of a hypergravity-induced posturo-locomotor adaptation in the centrifuge and/or to more functional changes of sensorimotor systems. Because locomotor performance of HG rats is not severely affected, it is concluded that early development of locomotion processes is highly resistant to gravito-inertial changes.

Postural and kinetic coordination following cortical stimuli which induce flexion movements in the cat's limbs.

The relationship between movement and the postural adjustment associated with it has been studied following flexion movements of a limb induced by cortical stimulation in chronically implanted cats. Variations in the vertical force exerted by each limb, as well as the displacement of the centre of gravity (projected onto the plane of stance), were investigated. It was established that: The pattern of postural adjustment observed in these conditions is diagonal; that is to say, flexion of one limb is accompanied by diminution in the vertical force exerted by the diagonally opposite limb and an increase in the force exerted by the other two. The displacement of the projected centre of gravity is sterotyped during the first phase of variation in forces. Its extent may be considered as an index of the efficiency of postural adjustment. The study of the latencies of the changes in force indicates, at least in the case of hindlimb movements, that postural adjustment is an active phenomenon and does not arise passively from the interplay of purely mechanical forces. In the same conditions, an analysis of the time course and latency of recorded changes in force indicates that postural adjustment is not triggered reflexly by the disequilibrium brought about by movement but is a result of a central nervous command.

Influence of pyramidotomy on limb flexion movements induced by cortical stimulation and on associated postural adjustment in the cat.

Flexion movements induced by cortical stimulation and the associated postural adjustments in bilaterally pyramidotomised cats have been studied by means of an apparatus which measures separately the changes of force under each limb in the upright position of the animal. The results show: (1) The general motor behaviour of the animal is not affected by the lesion. The principal deficit is loss of tactile placing reaction in the forelimbs; there also appears a state akin to a vestibular syndrome when a light restraining harness is placed around the back and chest. (2) Stimulation of the motor cortex continues to evoke flexion movements of the contralateral limbs together with associated postural adjustment. Coordination between movement and postural adjustment is generally similar to that observed before pyramidal section, and response thresholds are unchanged. (3) Measurements reveal great quantitative modification both of the movement and the postural adjustment after the lesion. Weight shift latencies are increased and more dispersed in time, while the weight shifts themselves are reduced in amplitude and speed. (4) All these changes are more marked in the case of forelimb, rather than hindlimb, flexion, emphasising the greater degree of pyramidal influence on forelimb activity in the normal animal. (5) The results as a whole underline the ability of non-pyramidal systems, under the control of the motor cortex, to bring about both limb flexion and the associated postural adjustments in the absence of the corticospinal pathway.

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.

Posturo-kinetic effects on kicking movements of a lack of initial ground support under the moving leg.

The posturo-kinetic effects of having no initial ground support under the movement leg were studied during kicking movements. Considerable effects were observed on the efficiency of the voluntary movement, which was noticeably reduced, as well as on the organisation of the postural adjustments. The fact that the efficiency of the movement was lower than under normal conditions shows that the posturo-kinetic reorganisation does not succeed in compensating for the lack of initial thrust of the foot against the floor. These results underline the important contribution of the postural adjustments to the performance of the movement itself, besides their role in maintaining equilibrium and posture.

Relationships between the force of voluntary leg movements and the associated postural adjustments.

Two types of kicking movements were studied: "touch mode" (minimal impact force) and "strike mode" (maximal impact force). Notable differences have been found to exist between the two kicking conditions. In the "strike mode", the onset of the voluntary movement occurred later and the movements of the postural segments preceding the voluntary movement were significantly longer and larger. These results confirm that the early postural adjustments contribute importantly to the efficiency of these movements.

Muscular exercise improves knee position sense in humans.

To determine how position sense depends on the functional state of the receptors involved, we assessed the accuracy of knee position sense before and after a moderate exercise on a cycle ergometer. Measurements were done on 32 healthy subjects with four protocols combining two tasks (intramodal: using the contralateral leg, and crossmodal: using a scheme of a leg on a screen) and two ways of positioning (active and passive). Results showed an improved position sense after exercise with the intramodal protocol combined with active positioning of the reference leg. Whatever the mechanisms involved, enhanced motor performances after exercise can be due not only to improved mechanical properties of the muscles but also to better kinesthetic sensibility.

Injection of a cholinergic agonist in the dorsolateral pontine tegmentum of cats affects the posturokinetic responses to cortical stimulation.

Microinjection into the dorsolateral pontine tegmentum of the muscarinic agonist bethanechol, leading to activation of cholinoceptive pontine reticular formation (pRF) neurons and the related medullary inhibitory reticulospinal system, did not modify the threshold, latency and amplitude of the forelimb flexion elicited by unilateral stimulation of the corresponding motor cortex. However, the remaining limbs which displayed a diagonal pattern of postural adjustments showed a dissociation of their postural responses in 2 components: the early component of central origin greatly decreased in amplitude, while the late component attributed to reflex mechanism triggered by the unbalance brought about by the flexion movement increased. Further evidence indicated that the pRF system intervenes in the gain regulation of the early postural responses during the cortically induced limb movement.

Gait initiation and impairments of ground reaction forces as illustrated in old age by 'La marche à petits pas'.

Ground reaction forces during gait initiation and kinematics of the first step were recorded in 11 elderly patients with idiopathic 'marche à petits pas' and 18 age-matched normal adults. Smaller values of vertical forces, and impaired amplitudes and directions of anteroposterior forces might explain start difficulties of the patients. Higher vertical displacement of the foot, and laborious establishment of the rhythm in the patients could also be related to the perturbations observed in the reaction forces. These perturbations are likely to reflect impairments of muscular synergies of the lower limbs and the lack of limb co-ordination.

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.

Biomechanical study of the mechanisms of postural adjustment accompanying learned and induced limb movements in cats and dogs.

Trajectories of the center of pressure and center of gravity projections were studied in dynamic conditions during learned movement and movement evoked by the motor cortex stimulation (induced movement) in dogs and cats. The learned movement began with initial displacement of the center of pressure towards the limb performing movement. It corresponded often with initial increasing of the performing limb pressure on support and it was a reason of initial acceleration of the center of gravity in the opposite direction. Induced movement began with decrease of the performing limb pressure on support. It could result in the initial displacement of the center of pressure in the wrong direction but usually it was corrected quickly and the general trajectories of the center of pressure and the center of gravity were similar to ones observed during learned movement. Results suggest different programs for movement and appropriate postural adjustment.

Analysis of vertical displacements of the center of gravity in the cat during limb flexion induced by cortical stimulation.

A limb movement and the associated postural adjustment result in a displacement of the center of gravity of the body. The vertical component of this displacement has been calculated from the variations in the sum of vertical forces at each limb. Through these variations, it is possible to measure the vertical acceleration of the center of gravity. Velocity and displacement are then obtained by two successive integrations of acceleration values. The magnitude of displacement of the center of gravity depends on the magnitude of limb displacement, and hindlimb flexion induces larger displacement than forelimb flexion. It is also directly linked to the time course of the vertical force variations recorded at each limb. The feed-forward character of the postural adjustment appears to minimize this amplitude, which leads us to discuss the functional significance of this postural adjustment.

Somatotopical effects of local microinjection of GABAergic agents in Deiters nucleus on the posturokinetic responses to cortical stimulation.

The postural adjustments that accompany the limb flexion following stimulation of the motor cortex in the cat are in part at least mediated by the lateral vestibular nucleus (LVN). In fact, inactivation of vestibulospinal (VS) neurons by unilateral injection of GABA agonists into the LVN decreased the early component of the responses in all of the postural limbs without modifying the threshold, latency, or amplitude of the cortically induced flexion movement. Just the opposite result was obtained after injection into the same structure of GABA antagonists. Experiments were performed to find out whether these VS influences were somatotopically organized. Microinjection of 0.25 microL of the GABA-A agonist muscimol or the GABA-B agonist baclofen (at 1.5 to 4 micrograms/microL saline) into the rostroventral part of the LVN (rvLVN) of one side produced hypotonia in the ipsilateral and hypertonia in the contralateral forelimb. The same injection also reduced the early component of the postural responses to cortical stimulation in both forelimbs, but not in the hindlimbs. On the other hand, unilateral injection of the same agents into the dorsocaudal part of the LVN (dcLVN) produced similar effects, but they were limited to the hindlimbs. The results obtained, which lasted for about 2 to 3 h, were not only site-specific but were also dose-dependent. Injection into the rvLVN or dcLVN of 0.25 microL of the GABA-A antagonist bicuculline or the GABA-B antagonist phaclofen (at 8 or 5 micrograms/microL saline) produced localized changes in posture as well as in the post-urokinetic responses to cortical stimulation. These were opposite in sign to those elicited by the corresponding agonists. In conclusion, it appears that: 1) the motor cortex utilizes descending volleys passing through the LVN in order to elicit the early postural adjustments during a cortically induced limb movement; 2) the resulting VS influences are somatotopically organized; 3) the amplitude of these postural responses can be affected by inhibitory influences probably exerted by Purkinje cells of the cerebellar vermis on the related VS neurons through both GABA-A and GABA-B receptors.

Associated movements, postural adjustments and synergies: some comments about the history and significance of three motor concepts.

Associated movements, postural adjustments and synergies are three motor concepts directly related to each other, although their historical developments have followed relatively independent paths. We show in this study how some notions, often very old, are evolving and become more precise as the number of observations and experimental results increases. We show also how the relationships between these notions and the terms associated with them may vary depending on the historical period and on the authors. The various kinds of muscular activity related to a movement, which are the object of this review, present several aspects, each of them being often described by different terms, which are analyzed. On the other hand we study the various meanings of some terms or expressions, which may correspond to very different aspects of the motor system. The three concepts analyzed have in common a definition from final manifestations (essentially muscular and mechanical) of motor activity. This characteristic is probably due to the lack of enough experimental data to use more refined criteria for definition, like those based upon nervous structures implicated, mechanisms involved or functional significance. The situation may change in the near future, as this field of investigation is now entering a phase of rapid development.

Postural adjustments to head movements in the cat.

Head movements induced by motor cortex stimulation in the cat are accompanied by variations in the vertical force exerted by each limb. These postural responses were found to show stereotyped patterns: with head dorsiflexions an increase was observed in the force exerted by the anterior limbs and a decrease at the posterior limb level. From comparison between the latencies of the force variations, the beginning of head acceleration, and EMG activity in the limb extensor muscles, it was concluded that triggering of these postural responses is not reflex, but depends on the same command as the movement itself. This early response might be a means of avoiding the downward movement of the trunk which would otherwise result from the reaction force corresponding to the upward head movement.

Effects of pressure stimulation of the body surface on placing reactions.

The effects of pressure stimulation of the body surface on placing reactions were investigated in intact unanesthetized cats. A slight contact applied to the dorsum of the fore- and hindpaws produced the typical placing reactions in these preparations. These reflexes, however, became more prominent if, in addition to the exteroceptive input, a proprioceptive input was elicited after plantar flexion of the paws. Slight pressure applied to the upper part of the body greatly depressed not only tactile placing reactions, but also to a lesser extent proprioceptive placing reactions. Moreover, these reflexes were less prominent and more sluggish and fatiguable than in the normal cats. The depression of the placing reactions elicited by the cutaneous input during pressure applied to the body surface mainly affected the forelimbs. However, as soon as the pressure stimulation was removed, the placing reactions reappeared or became as prominent as usual. We postulate that the exteroceptive input resulting from body pressure decreased the placing reactions by exciting the Purkinje cells of the intermediate cortex of the cerebellum, which are in turn inhibitory on the interpositus nucleus, thus reducing the neuronal discharge of the interpositorubrospinal pathway acting on flexor motoneurons.

Cholinoceptive pontine reticular structures modify the postural adjustments during the limb movements induced by cortical stimulation.

1. Activation of the pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system decreases the postural activity. This effect can be achieved either by local injection into the dorsal pontine tegmentum of cholinergic agonists which excite cholinoceptive pRF neurons, or by injection of noradrenergic agents which block the inhibitory influence exerted by the locus coeruleus (LC) neurons on the pRF. The main aim on the present study was to analyze the effects of tonic activation of these pRF neurons on the postural adjustments accompanying limb movements induced by motor cortex stimulation. In particular, electrodes were implanted chronically in the motor cortex of cats and stainless steel guide tubes of small size, later used for drug injection, were set bilaterally into sites just above the responsive regions. 2. Limb flexion elicited by stimulation of the motor cortex was accompanied by a diagonal pattern of postural adjustment, characterized by a decreased force exerted by the limb diagonally opposite to the moving one and an increased force exerted by the other two. 3. Microinjection into the pRF of both sides of 0.25 microliter of the muscarinic agonist bethanechol at the concentration of 8 or 16 micrograms/microliters in buffered artificial cerebrospinal fluid produced a short-lasting episode of postural atonia followed by a period of reduced postural activity, during which the cats were still able to stand on the measurement platform. Under this condition no changes in threshold, latency and amplitude of the flexion response were observed in the performing limb; however, the postural responses were considerably affected. In particular, when the performing limb was a forelimb, the other anterior limb showed a dissociation of the postural response in two distinct components. The first anticipatory component, which had a short latency (12-15 msec) and was considered to be centrally triggered, decreased in amplitude after injection of bethanechol and sometimes disappeared; on the other hand the second component, which had a long latency (50-60 msec) and was thus considered to be of reflex origin, increased in amplitude, due to the instability resulting from the depression of the early postural response. Similar results also affected to a lesser extent the hindlimbs. Moreover, body oscillations were observed and monitored from the force platforms following the late component of the postural responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in posturo-kinetic limb responses to cortical stimulation following unilateral neck deafferentation in the cat.

Unilateral neck deafferentation produces in cats a postural asymmetry, characterized by an increase in the extensor activity of the ipsilateral limbs and a decreased activity in the contralateral limbs; moreover, the placing reactions are severely depressed in the ipsilateral limbs. The present experiments were performed to investigate the influence of unilateral section of the cervical dorsal roots C1-C2 or C1-C3 on the postural adjustments occurring during flexion limb movements induced by cortical stimulation in chronically implanted cats. Only weak stimulations were used and motor activity was mainly isometric, to facilitate quantification of responses analyzed by measuring changes in vertical force exerted at the level of each limb in the up-right position of the animal. The results obtained were the following. The general motor behaviour of the animal was not affected by the lesion. In particular, stimulation of the motor cortex evoked not only flexion movement of a contralateral limb but also an initial postural adjustment consisting of a diagonal force pattern. The response threshold of the moving limb increased on the hypertonic side and decreased on the hypotonic side. Moreover, the changes in threshold of the performing limb following unilateral neck deafferentation were coupled with parallel changes in threshold of the limbs involved in the postural adjustment. There were great quantitative modifications both of the movement and the postural adjustment after the neck deafferentation. In fact weight shift latencies increased when the performing limb was ipsilateral to the hypertonic side, while the weight shifts themselves decreased in amplitude and speed. The opposite occurred when the performing limb was ipsilateral to the hypotonic side. Parallel with these changes, the slope of the response curve of the moving limb as well as of the limbs involved in the postural adjustment decreased following stimulation of the motor cortex at different stimulus intensities if the performing limb was ipsilateral to the hypertonic side, but increased if the performing limb was ipsilateral to the hypotonic side. These changes compensated after chronic neck deafferentation. In summary, the results indicate that well coordinated posturo-kinetic responses were still elicited by cortical stimulation in spite of the prominent changes in posture and reflex movements induced by unilateral neck deafferentation. In particular, changes in threshold, latency, amplitude and speed of the force changes under the limb making a movement were coupled with parallel alterations of the force changes under the limbs undergoing postural adjustment.(ABSTRACT TRUNCATED AT 400 WORDS)