Cognitive and motor aspects of a coincidence-timing task in Cerebral Palsy children.

Cerebral Palsy (CP) is a clinical syndrome involving postural and motor deficits. CP children are less accurate than healthy ones when trying to reach a target. Thus, it is difficult for CP children to perform anticipation-coincidence tasks requiring temporal and/or spatial accuracy to reach the target at the good place in the right time. The purpose of the present experiment was to further investigate CP children's ability to perform anticipation-coincidence tasks, by dissociating the cognitive from the motor aspects of the task. 11 CP children aged 6-14 years, 51 healthy children aged 6-13 years, and 13 healthy adults performed, as accurately as possible, a coincidence-timing in response to a specific sound of a musical sequence. Two experimental conditions were manipulated: In the verbal condition, temporal estimation occurred through a simple verbal response whereas in the motor condition, temporal estimation was performed by reaching a target at a self-paced velocity. In the verbal condition, CP children made similar temporal errors than their healthy counterpart. However, even though all participants underestimated stimulus occurrence, CP children also exhibited greater and more variable temporal errors when they provided a motor response for estimating stimulus occurrence. These data suggested that CP children were able to anticipate stimulus occurrence and to partially take into account their sensory-motor deficits to reach the target at this time occurrence.

Achilles tendon vibration shifts the center of pressure backward in standing and forward in sitting in young subjects.

OBJECTIVE: An Achilles tendon (AT) vibration is known to disrupt the postural control in standing ("vibration-induced falling", VIF) and to backward tilt the postural vertical in restrained sitting position, suggesting a link between AT vibration and internal representations involved in postural control. A recalibration of some troubles in body orientation by oriented sensory manipulations could be of great clinical interest. In order to use the VIF paradigm in a procedure suitable for a rehabilitation context, AT vibration deserves to be more investigated in sitting, for security reasons, and first in young participants. METHOD: In 12 healthy participants (6 men/6 women; 23.3±1.9 years), posturographic data to AT vibrations (85 Hz) were recorded over 30 s standing and 40 s sitting trials. RESULTS: Surprisingly, four types of differences were found between standing and sitting AT vibrations: presence/absence of VIF and kinesthetic illusion, opposite directions of the centre of foot pressure (CoP) displacements, different temporal profiles. In standing: VIF without kinesthetic illusion, rapid backward shift of CoP with a peak of -54.6±11.3 mm (with respect to baseline P<0.001) 2.8±0.2 s after vibration onset. In sitting: surprising systematic forward shift of the CoP throughout the duration of AT vibration with a maximum of 27.9±18.9 mm (P<0.05 with respect to the baseline) 19.6±0.3 s after vibration onset, associated with a kinesthetic illusion in most subjects. CONCLUSION: The present study invalidates our idea to extend in sitting the VIF paradigm but the unexpected results open a new window about the basic mechanisms underlying muscle vibration effects.

Does the new Helix 3D hip joint improve walking of hip disarticulated amputees?

PURPOSE: Testing the new hip joint Helix3D efficiency through clinical data and walking parameters. METHOD: Three young hip-disarticulated patients (P1, P2 and P3) were assessed both with their previous prosthesis at first day, then four days and six months after being trained with a new prosthesis equipped with the Helix3D hip joint. Assessments comprised a satisfaction questionnaire, a two-minute walk test and a recording of main spatiotemporal gait parameters RESULTS: After four days with the Helix3D, the satisfaction for the prosthesis was improved for P1, unchanged for P2 and reduced for P3. Distance walked during two minutes increased for P1, unchanged for P2 and slightly improved for P3. Gait pattern was improved in P1, only. P1 abandoned the Helix3D at six months due to an ischiatic wound. P2 and P3 chose not to use the Helix at the end of the four days training period because they could not adapt to the Helix3D characteristics (hydraulic control of hip extension and assistance to hip flexion) and because they did not gain enough benefits. Despite much effort to adjust the prosthesis, the three patients definitively abandoned the Helix3D because of comfort problems, and decided to walk with their previous prosthesis equipped with a monocentric hip joint or even with crutches only. CONCLUSION: The Helix3D hip joint may need further developments to get clinically relevant for hip-disarticulated amputees who may also need a long training period to adapt to its technical characteristics.

Superimposed electrical stimulation comfortably improves the endurance of maximal voluntary contractions.

AIM: Electrical stimulation has shown to improve muscle endurance in sub-maximal contractions but sessions were painful due to the electric stimuli parameters. Therefore, the present study tested the effects of the superimposed electrical stimulation technique using comfortable current on endurance in repetitions of maximal voluntary contraction. METHODS: Seventeen young healthy subjects performed fifty maximal voluntary contractions of the triceps brachii in two conditions of contraction (voluntary vs. voluntary + superimposed electrical stimulation). RESULTS: Peak force and force-time integral were consistently decreased in the voluntary muscular contraction condition after the 20th - 30th trials whereas they were maintained in the superimposed electrical stimulation condition (P<0.05) until the end of the fifty trials. CONCLUSION: The superimposition of neuromuscular electrical stimulation extends the muscle ability to repeat maximal voluntary contractions. The present results also evidenced the ability of the superimposed electrical stimulation technique to make the mechanisms of muscle central fatigue inefficient.

Vertical frames of reference and control of body orientation.

The present paper aims at critically reviewing the most outstanding and recent studies regarding the control of body orientation in the vertical space. A first part defines the general concepts used throughout this manuscript. The second part investigates the vertical perception and the main factors which affect it, while trying to overcome the five areas of theoretical and experimental controversies we have identified in the literature. The third part of this review presents the different theoretical models of the vertical perception and body orientation in space. Finally, the last part focuses on the functional coupling between perception of the vertical and orientation of the body in space. It considers more particularly how these two dimensions interact for explaining the observed behaviors.

Effects of external feedback about body tilt: Influence on the Subjective Proprioceptive Horizon.

The present study investigated a cognitive aspect upon spatial perception, namely the impact of a true or false verbal feedback (FB) about the magnitude of body tilt on Subjective Proprioceptive Horizon (SPH) estimates. Subjects were asked to set their extended arm normal to gravity for different pitch body tilts up to 9 degrees . True FB were provided at all body tilt angles, whereas false FB were provided only at 6 degrees backward and 6 degrees forward body tilts for half of the trials. Our data confirmed previous results about the egocentric influence of body tilt itself upon SPH: estimates were linearly lowered with forward tilts and elevated with backward tilts. In addition, results showed a significant effect of the nature of the external FB provided to the subjects. When subjects received a false FB inducing a 3 degrees forward bias relative to physical body tilt, they set their SPH consequently higher than when they received a false FB inducing a 3 degrees backward bias. These findings clearly indicated that false cognitive information about body tilt might significantly modify the judgement of a geocentric direction of space, such as the SPH. This may have deleterious repercussions in aeronautics when pilots have to localize external objects relative to earth-based directions in darkened environments.

[Prediction of falls with performance on Timed "Up-and-Go" and one-leg-balance tests and additional cognitive tasks]. / Modification des performances au Timed "Up and Go" test et à l'appui monopodal par l'addition d'une charge cognitive: valeur discriminative des résultats.

OBJECTIVE: This study aimed to determine whether ordinary clinical performance on the timed "up-and-go" and one-leg-balance tests varied with additional cognitive tasks and the predictive value of this combination for future falls. METHODOLOGY: The sample comprised 95 women with osteoporosis who lived independently in the community and were older than 70 years (mean 73.4+/-1.7 years) who were randomized to perform the timed "up-and-go" (TUG) and one-leg-balance (OLB) tests. The tests were performed with or without an additional cognitive task (math task involving subtraction by 2 s or 5 s or addition by 3 s). RESULTS: For both TUG and OLB, performance decreased in the dual-task condition (P<0.05) but did not differ in older women with and without a history of falling, whatever the test, and with or without an additional cognitive load. CONCLUSION: Performance on the TUG and OLB tests was less efficient when patients simultaneously performed a cognitive task. Performance on these tests, without or with cognitive tasks, did not predict falls in a sample of women who lived independently.

Effects of lower limbs muscular fatigue on anticipatory postural adjustments during arm motions in humans.

BACKGROUND: The purpose of the present experiment was to examine the effects of lower limbs muscular fatigue on the anticipatory postural adjustments during a voluntary arm raising movement. METHODS: Eight adult males (mean age 29+/-6 years) voluntarily participated to the experiment. Subjects' task consisted in performing an unilateral ante-flexion of the right arm over 90 deg, holding a 500 g weight, once having stabilized their upright posture. A similar movement execution was required. Measurements were made in a no-fatigue situation and under a high level of fatigue of lower limbs. Surface EMG of four postural muscles (the ipsilateral and contralateral Gastrocnemius and the ipsilateral and contralateral Semitendinosus) and of one muscle involved in the arm raising (the Anterior Deltoïdeus of the right arm) was recorded. Body sway was also monitored using a force platform. RESULTS: With fatigue, quantitative EMG analysis showed a decrease of the ipsilateral Semitendinosus activity whereas the temporal EMG analysis showed an increase of its latency with respect to movement onset. CONCLUSIONS: These data suggested a functional adaptation resulting in an invariance of global anticipatory postural adjustments for the two conditions of no-fatigue and fatigue and ensuring an adequate execution of the movement.

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.

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.

The gap effect for eye and hand movements in double-step pointing.

The existence of a temporal gap between the offset of a fixation target and the onset of a peripheral target generally reduces the saccadic and manual reaction time in response to the peripheral target. Using a double-step paradigm, the present experiment investigated whether a temporal gap between the extinction of the first target and the presentation of the second target can help in reducing the time to trigger the corrective eye movements and to correct the arm trajectory towards the final target position. A gap was introduced between the presentation of the initial target and a new unexpected goal-target during the movement. The results replicated the gap effect for the corrective saccade to the second target, but revealed an opposite effect for the correction of the reaching movements as the arm correction occurred later in the Gap than in the No-Gap conditions. These results suggest that the information available for the arm motor system to correct the trajectory in relation to the second target was different in the Gap and No-Gap conditions. In the No-Gap condition, the correction of reaching movements would be based on retinal errors between the first and the second targets whereas, in the Gap condition, the correction would be based on information derived from the corrective saccade-related signals to the second target.

Accuracy level of pointing movements performed during slow passive whole-body rotations.

Seated observers requested to detect low-velocity passive rotations show a high motion-detection threshold. However, when standing on a slowly rotating platform, their equilibrium is preserved, suggesting that cognitive sensing and sensorimotor reactions do not share the same central processes. The present experiments investigated the ability of observers seated on a slowly rotating chair in total darkness to indicate with their hand the position of briefly flashed targets (Experiment 1) and to indicate the subjective horizon with an outstretched arm (Experiment 2) or with a target driven by a joystick (Experiment 3). The overall hypothesis stated that egocentric coding of the position of a target should not be affected by sensing or not-sensing body rotation (Experiment 1), while geocentric positioning may (Experiments 2 and 3). Our data partially supported the hypothesis. Subjects pointed accurately to the memorized targets (Experiment 1), whereas misperception of body orientation was a source of inaccuracy for actions referred to a geocentric frame (Experiments 2 and 3). More interestingly, subjects' perceptions changed as a single, smooth, and monotonic function of tilt, independent of whether the perception of body orientation was present or not.

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.

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.

Manual asymmetries in reaching movement control. II: Study of left-handers.

Two experiments performed with left-handed subjects investigated how the manual asymmetries and hemispheric specialization involved in visuo-manual coordination are associated with handedness. Pro and retroactive processes involved in rapid movement control were analyzed according to the different movement parameters to be controlled, similar to studies performed with right-handers (Boulinguez, Nougier and Velay, 2001). Manual performances and kinematic properties of reaching movements showed that the left and right hands of left-handers behaved in the same way as the left and right hands of right-handers. Results are discussed in the light of the independence of handedness and other forms of cerebral dominance in sensori-motor information processing involved in hand movements.

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.

Effects of gymnastics expertise on the perception of body orientation in the pitch dimension.

The purpose of this study was to investigate how experts in motor skills requiring a fine postural control perceive their body orientation with few gravity based sensory cues. In Experiment 1, expert gymnasts and controls had to detect their body tilt when pitching at a velocity of 0.05 deg.s(-1), in two conditions of body restriction (strapped and body cast altering the somatosensory cues). Contrary to the experts, the controls exhibited a larger body tilt when totally restrained in the body cast. In Experiment 2, subjects had to estimate their Subjective Postural Vertical (SPV) starting from different angles of pitch tilt. The controls exhibited significant errors of SPV judgement whereas the experts were very precise. These results suggest that 1) somatosensory cues are more informative than otolithic cues for the perception of body orientation, and 2) the efficiency of otolithic and/or interoceptive inputs can be improved through a specific training to compensate for the lack of somatosensory cues.

Control of goal-directed movements: the contribution of orienting of visual attention and motor preparation.

Three experiments investigated the role of attention and motor preparation for the control of goal-directed movements. In Experiment 1 (double step paradigm), a movement correction was required on 25% of the trials towards the left or right of the initial target. Within these 25% of trials, the probability of location of the second target was manipulated. The efficiency of movement control increased when increasing the probability of correcting the movement in a given direction. In Experiment 2, attentional processes were isolated by asking the subjects to verbally detect the more or less probable target displacement, without correcting their movement. Subjects were able to orient visual attention during movement execution, thus improving the processing of visual feedbacks from target displacement. In Experiment 3, motor preparation processes were isolated by asking the subjects to correct their movement towards a fixed target in response to a more or less probable mechanical perturbation. It was shown that motor preparation not only specifies the initial movement parameters but may also include some parameters of the most probable movement modulations. Overall, these results highlight the role of both attentional and motor preparation processes in the control of goal-directed movements and suggest that the feedback-based corrections of the movement are modulated by a feedforward control.

Emotions, heart rate and performance in archery. A case study.

BACKGROUND: A case study of an elite female archer was conducted to gain insight into individual psychophysical reactions accompanying an athletic event, and to test predictions of pre-performance emotions effects upon performance. Good performance was expected when the actual pre-performance emotions resembled the recalled optimal emotion pattern. Conversely, poor performance was expected when the actual pre-performance emotions paralleled the recalled ineffective emotion pattern. EXPERIMENTAL DESIGN: the investigation comprised individual emotion profiling, emotions and heart rate monitoring, final interview and performance evaluation. SETTING: The research was accomplished during the 1996 European Archery Championships, one of the most important international archery competitions. PARTICIPANT: An 18-year-old female athlete of the Italian archery national team. INTERVENTIONS: Because of the exploratory nature of the study, no intervention was implemented. MEASURES: Emotion profiling was carried out using an idiographic approach based on recalled optimal and poor performances, according to the Individual Zones of Optimal Functioning (IZOF) model. Emotions, heart rate, and performance were monitored across the five days of practice and competition. RESULTS: Individual pre-performance optimal emotion pattern, heart rate deceleration during shooting, consistent shooting scores were revealed throughout practice and competition. CONCLUSIONS: The good performance predicted on the basis of pre-performance emotion assessments was met and was confirmed by the archer's interpretation.

Contribution of ankle, knee, and hip joints to the perception threshold for support surface rotation.

The purpose of the present experiment was to investigate the extent to which subjects can perceive, at very slow velocities, an angular rotation of the support surface about the medio-lateral axis of the ankle, knee, hip, or neck joint when visual cues are not available. Subjects were passively displaced on a slowly rotating platform at .01, .03, and .05 deg/sec. The subjects' task was to detect movements of the platform in four different postural conditions allowing body oscillations about the ankle, knee, hip, or neck joint. In Experiment 1, subjects had to detect backward and forward rotation (pitching). In Experiment 2, they had to detect left and right rotations of the platform (rolling). In Experiment 3, subjects had to detect both backward/forward and left/right rotations of the platform, with the body fixed and the head either fixed or free to move. Overall, when the body was free to oscillate about the ankle, knee, or hip joints, a similar threshold for movement perception was observed. This threshold was lower for rolling than for pitching. Interestingly, in these postural conditions, an unconscious compensation in the direction opposite to the platform rotation was observed on most trials. The threshold for movement perception was much higher when the head was the only segment free to oscillate about the neck joint. These results suggest that, in static conditions, the otoliths are poor detectors of the direction of gravity forces. They also suggest that accurate perception of body orientation is improved when proprioceptive information can be dynamically integrated.