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.

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).

Effect of asymmetrical limb loading on early postural adjustments associated with gait initiation in young healthy adults.

The aim of this work was to show how postural adjustments are modified in healthy subjects when gait initiation is made more difficult by an increase of the supported body weight under the starting limb. We show that the impulse due to the push of the foot against the ground is enhanced by an increase of the amplitude and duration of the initial vertical force variation. The corresponding lateral excursion of the global center of pressure (CPg) is determined mainly by this force variation, but also by lateral displacements of the centers of pressure under each foot.

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.

Early postural adjustments associated with gait initiation and age-related walking difficulties.

To better understand the functional role of early postural adjustments associated with gait initiation, we studied ground reaction forces before the first step in a group of 10 young and 18 elderly normal subjects as well as in a group of 11 patients with marche à petits pas, an age-related gait disorder in which difficulties in executing the first step are encountered. Kinematic data and ground reaction forces were recorded synchronously and processed by an ELITE motion analysis system. Start difficulties in the patients might be explained by impaired amplitudes and directions of anteroposterior forces as well as smaller values of vertical forces than in the controls walking with comparable step lengths.

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.

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.

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.

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.

Effect of optic nerve stimulation on neurons in pericruciate cortex of cats.

The distribution of optic chiasm input to different types of neurons in pericruciate cortex of cats agreed with previous work using light flashes. Neuron response times served to differentiate the input pathways to pericruciate cortex, and the types of neurons they influence. Input from the optic chiasm arrived in three distinct surges: the first via the superior colliculus, the second via an unidentified pathway, and the third via the visual cortex.

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.

GABA receptors in Deiters nucleus modulate posturokinetic responses to cortical stimulation in the cat.

The early component of the postural responses which accompany the limb flexion during unilateral stimulation of the motor cortex in the cat is not of reflex origin, but results from a central command. These postural adjustments are characterized by a decreased force under the limb diagonally opposite to the moving one and an increased force under the other two. Since the lateral vestibular nucleus (LVN) exerts an excitatory influence on ipsilateral limb extensor motoneurons, experiments were performed in cats to establish whether the cortical-induced postural changes were mediated through the LVN.

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.

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.

Ambulatory computer system for recording and controlling locomotion.

Description of an original miniature computer system capable of recording biomechanical parameters during locomotion, providing auditory signals which can be used in biofeedback locomotor learning, and triggering functional electrical stimulation of nerves and muscles at preselected moments in the locomotor cycle. The system, which is worn on the patient's back, is connected to appropriate gauges with which it is possible to measure knee angle and to record the time of heel contact with the ground. The relevant data are stored in the system memory and can be transferred to a microcassette and later displayed as a function of time on a video screen or printed on paper.

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.

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.

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.

Effects of pressure stimulation of the body surface on posture and vestibulospinal reflexes.

The effects of pressure stimulation of the body surface on postural activities as well as on the response gain of limb extensors to natural stimulation of labyrinth receptors were investigated in intact, as well as in decerebrate cats. In intact, unanesthetized cats, slight pressure applied symmetrically to the body surface at the chest level decreased the tonic activity of the axial (neck) and limb extensor musculature, as well as the proprioceptive reflexes induced by passive flexion of the limbs. The positive supporting reaction caused by pressure applied to the pad of the foot was also depressed.

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.

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.

[Vertical displacements of the center of gravity in the cat during movement of limb flexion induced by motor cortex stimulation].

A limb movement and the corresponding postural adjustment results in a displacement of the body's center of gravity. The vertical component of this displacement has been calculated by measuring the variations of the sum of vertical forces at each limb. The displacements are upwards and are larger in the case of hindlimb flexion than forelimb flexion.

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.

Description and applications of a programmable stimulation system for electrophysiology, used as a computer peripheral.

An original programmable stimulation and data acquisition system of general interest for electrophysiology is presented. It can be used as a peripheral of any 12 (or more) bits mini- or microcomputer. With the software described it is possible to determine stimulation parameters automatically as a function of results obtained during the experiment or from the keyboard.