Vincent Nougier

Perturbation of the postural control system induced by muscular fatigue

In this experiment, we induced muscular fatigue of ankle plantar-flexors to examine how it deteriorates the regulation of bipedal quiet upright standing. Postural stability was assessed in conditions with and without vision over 60 s period to examine not only classical postural variables (time- and frequency-domain analyses), but also structural variables (stabilogram-diffusion analysis). Muscular fatigue was induced with repeated plantar-flexion of both legs. With muscular fatigue, subjects exhibited an increased postural sway (faster center of pressure (CP) velocity, and greater CP mean and median frequency) and a decreased long-term scaling exponent compared with the control conditions. The fatigue conditions, however, did not modify the range of oscillations and the variability of the postural oscillations around the mean position of CP. The effects of muscular fatigue were similar with eyes open and eyes closed. These results suggest that fatigue did induce some changes in the control mode of postural stability, but the detection/action capabilities of the sensorimotor system remained partly efficient when the ankle plantar-flexors were fatigued. Furthermore, the decreased long-term scaling exponent observed with fatigue suggests that the control of upright stance operates in a less stochastic and more antipersistent manner when fatigue is present (i.e. past and future behaviors were more negatively correlated and thus more tightly regulated). Altogether, the present results suggest that, compared with the no-fatigue conditions, fatigue places higher demands on the postural control system by increasing the frequency of actions needed to regulate the upright stance.

The effect of expertise in gymnastics on postural control

The goal of this paper was (1) to investigate if gymnasts have a more stable standing posture than experts in other sports, and (2) to determine how much gymnasts are affected by the removal of vision in different postural tasks. Six expert gymnasts and six experts in other non-gymnastic sports were asked to maintain balance in three standing postures of increasing difficulty: bipedal, unipedal, and unipedal + unstable support (i.e. 7 cm thick foam surface). Each posture was tested successively with and without vision. Based on the displacement of the center of pressure (range and mean average speed), the results showed that when visual cues were available, postural sway increased with the difficulty of the task, but both groups had comparable performance in all the tasks. When vision was removed, although both groups demonstrated larger postural sway in the unipedal tasks, this effect was less accentuated for the gymnasts. We concluded that gymnasts are able to use the remaining sensory modalities to compensate for the lack of vision in unstable postures.

Postural sway under muscle vibration and muscle fatigue in humans

Separate studies have demonstrated that vibration and fatigue of ankle muscles alter postural control. The purpose of the present experiment was to investigate the effect of ankle muscle vibration on the regulation of postural sway in bipedal stance following ankle muscle fatigue. Center of foot pressure displacements were recorded using a force platform. Results showed a similar increase in postural sway under muscle fatigue as well as under muscle vibration. Interestingly, under muscle fatigue muscle vibration did not induce a further increase in postural sway. Two hypotheses could, at least, account for this observation: (1). fatigued muscles are less sensitive to muscle vibration and (2). the central nervous system relies less upon proprioceptive information originating from fatigued muscles for regulating postural sway.

Can vision compensate for a lower limbs muscular fatigue for controlling posture in humans

The purpose of the present experiment was to investigate the effect of vision on postural control following a lower limbs muscular fatigue. Ten adult males were asked to maintain a single leg upright posture as immobile as possible in two conditions of no-fatigue and fatigue of the calf muscles. This muscles fatigue was achieved by standing on the toes until exhaustion. Within a trial, vision was suppressed or reinserted by eyes closure or opening. Center of foot pressure displacements were recorded using a force platform. Results showed that the availability of vision allowed the subjects to immediately cope with the destabilizing effect induced by muscular fatigue.

Movement control in golf putting

The purpose of the study was to understand how force is controlled for impact movements such as golf putting. Expert players (10) and control subjects (10) executed a putt as accurately as possible, in order to reach a target distance of 1, 2, 3, or 4 m. Movements of the club were recorded at 200 Hz via a SELSPOT system. Overall, the results showed that, in order to increase club velocity at the moment of contact with the ball with increasing distance of the target, subjects increased the downswing (DS) amplitude maintaining DS movement time constant. The change in force required to reach the different distances seemed to rely on an adjustment of the magnitude of the motor command within the same time period. Furthermore, our results showed that the movement of putting consists primarily in specifying the amplitude of the Backswing (BS) as a function of the distance of the target. This gives rise to a motor impulse originating the force-time function required for an adequate DS movement.

Attentional demand for regulating postural sway: the effect of expertise in gymnastics

A dual-task paradigm was used to investigate whether the expertise in motor skills requiring a fine postural control can modify the attentional demand necessary for regulating postural sway. Seven expert gymnasts and seven experts in other non-gymnastic sports were asked to respond as rapidly as possible to an unpredictable auditory stimulus while maintaining stable seated and in three upright postures of increasing difficulty: bipedal, unipedal, and unipedal on an unstable support (i.e. a 7 cm thick foam surface). RT values were used as an index of the attentional demand necessary for performing the postural tasks. Results showed that the attentional demand necessary for regulating postural sway increased as the postural task increased in difficulty. Interestingly, this effect was smaller for the gymnasts during unipedal stance. These findings suggest a decreased dependency on attentional processes for regulating postural sway during unipedal stance in gymnasts with respect to non-gymnasts.

The effect of expertise in gymnastics on proprioceptive sensory integration in human subjects

The ability of experts in motor skills requiring a fine postural control to keep a stable upright posture while facing the task of reinserting proprioceptive information was investigated. Seven expert gymnasts and seven experts in other non-gymnastics sports were asked to stand as immobile as possible in two conditions of vision and no-vision. Within a trial, ankle proprioceptive input was modified by means of tendon vibration of both antagonistic ankle muscles. Center of foot pressure (COP) displacements were recorded using a force platform. Contrasting with the non-gymnasts, the gymnasts were able to rapidly take advantage of the reinsertion of proprioceptive information to decrease their COP displacements. These results suggest that the efficiency of the integration process leading to the reweighting of sensory information can be significantly improved through a specific training.

Effects of a reaction time task on postural control in humans

The purpose of the present experiments was to investigate the effects of a reaction time (RT) task on postural control, during and following the execution. Three different RT tasks consisting in verbal responses to visual and auditory stimuli were supposed to require different demands on cognitive resources. There was also a control condition in which no concurrent task was required. Postural control was assessed using a force platform. Results showed that (1) center of foot pressure (COP) displacements significantly decreased while concurrently performing the RT tasks, (2) this effect lasted for at least 10 s following the secondary task performance and (3) COP displacements decreased similarly, whatever the attentional requirements of the secondary task.

Manual asymmetries in reaching movement control. I: Study of right-handers.

Two experiments investigated manual asymmetries in the control of rapid reaching movements according to the movement parameters to be controlled. Single- and double-step reaching movements were performed by right-handed subjects with both hands. Pro and retroactive processes involved in rapid movement control were investigated. Manual performances and kinematic properties of hand movements showed that various forms of hemispheric specialization were involved in sensori-motor information processing. It was shown that the effects of hemispheric specialization were specific to the task constraints, that is, to the various operations involved in movement control.

Aging and Postural Control: Postural Perturbations Caused by Changing the Visual Anchor

OBJECTIVE: To determine the effect of modifying the stable visual anchor on the postural stability of older individuals. The visual anchor was changed by opening doors similar to those found in an elevator cage. Lighting intensities inside and outside the cage were varied to create increasing or decreasing luminosity conditions. The effect of adding a cognitive load (counting backwards) was also tested.