Delayed postural control during self-generated perturbations in the frail older adults

International audiencePurpose: The aim of this study was to investigate the coordination between posture and movement in pathological aging (frailty) in comparison with normal aging, with the hypothesis that in pathological aging, postural control evolves towards a more reactive mode for which the perturbation induced by the movement is not anticipated and leads to delayed and late postural adjustments. Methods: Elderly subjects performed rapid focal arm-raising movements towards a target, from an upright standing position in two stimuli conditions: simple reaction time and choice reaction time (CRT). Hand and center of pressure (CoP) kinematics were compared between a control group and a frail group of the same age. Results: In frail individuals, the entire movement was impaired and slowed down. In addition, postural adjustments that classically precede and accompany the focal arm movement were delayed and reduced, especially in the CRT condition in which the motor prediction is more limited. Finally, a correlation between the time to CoP maximal velocity and the timed up-and-go score was observed. Conclusion: In these patients, it was concluded that the control of the CoP displacement evolved from a proactive mode in which the perturbation associated with the arm movement is anticipated toward a more reactive mode in which the perturbation is compensated by late and delayed adjustments

Voluntary imitation in alzheimer's disease patients

Although Alzheimer's disease (AD) primarily manifests as cognitive deficits, the implicit sensorimotor processes that underlie social interactions, such as automatic imitation, seem to be preserved in mild and moderate stages of the disease, as is the ability to communicate with other persons. Nevertheless, when AD patients face more challenging tasks, which do not rely on automatic processes but on explicit voluntary mechanisms and require the patient to pay attention to external events, the cognitive deficits resulting from the disease might negatively affect patients' behavior. The aim of the present study was to investigate whether voluntary motor imitation, i.e., a volitional mechanism that involves observing another person's action and translating this perception into one's own action, was affected in patients with AD. Further, we tested whether this ability was modulated by the nature of the observed stimulus by comparing the ability to reproduce the kinematic features of a human demonstrator with that of a computerized-stimulus. AD patients showed an intact ability to reproduce the velocity of the observed movements, particularly when the stimulus was a human agent. This result suggests that high-level cognitive processes involved in voluntary imitation might be preserved in mild and moderate stages of AD and that voluntary imitation abilities might benefit from the implicit interpersonal communication established between the patient and the human demonstrator

Functional Electrical Stimulation Alters the Postural Component of Locomotor Activity in Healthy Humans

International audienceKnowledge of the effects of Functional Electrical Stimulation (FES) of different intensity on postural stability during walking in healthy subjects is necessary before these relationships in patients with postural disorders can be assessed and understood. We examined healthy subjects in Control group walking on a treadmill for 40 min and in FES group provided with 30 min of stimulation, which intensity increased every 10 min. The main difference between Control and FES group was the progressive increase of trunk oscillations in sagittal, frontal, and horizontal planes and an increase of relative stance duration in parallel with FES intensity increase. Both Control and FES groups exhibited shank elevation angle increase as an after-effect. It is concluded, that high intensity FES significantly changes the postural component of locomotor activity, but the fatigue signs afterwards were not FES specific

Pointing To Double-step Visual Stimuli From A Standing Position: Motor Corrections When the Speed-accuracy Trade-off Is Unexpectedly Modified In-flight. A Breakdown of the Perception-action Coupling

Import JabRef | WosArea Neurosciences and NeurologyInternational audienceThe time required to complete a fast and accurate movement is a function of its amplitude and the target size. This phenomenon refers to the well known speed-accuracy trade-off. Some interpretations have suggested that the speed-accuracy trade-off is already integrated into the movement planning phase. More specifically, pointing movements may be planned to minimize the variance of the final hand position. However, goal-directed movements can be altered at any time, if for instance, the target location is changed during execution. Thus, one possible limitation of these interpretations may be that they underestimate feedback processes. To further investigate this hypothesis we designed an experiment in which the speed-accuracy trade-off was unexpectedly varied at the hand movement onset by modifying separately the target distance or size, or by modifying both of them simultaneously. These pointing movements were executed from an upright standing position. Our main results showed that the movement time increased when there was a change to the size or location of the target. In addition, the terminal variability of finger position did not change. In other words, it showed that the movement velocity is modulated according to the target size and distance during motor programming or during the final approach, independently of the final variability of the hand position. It suggests that when the speed-accuracy trade-off is unexpectedly modified, terminal feedbacks based on intermediate representations of the endpoint velocity are used to monitor and control the hand displacement. There is clearly no obvious perception-action coupling in this case but rather intermediate processing that may be involved. (C) 2011 IBRO. Published by Elsevier Ltd. All rights reserved

Inertial properties of the arm are accurately predicted during motor imagery

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Interhemispheric inhibition is dynamically regulated during action observation

It is now well established that the motor system plays a pivotal role in action observation and that the neurophysiological processes underlying perception and action overlaps. However, while various experiments have shown a specific facilitation of the contralateral motor cortex during action observation, no information is available concerning the dynamics of interhemispheric interactions. The aim of the present study was, therefore, to assess interhemispheric inhibition during the observation of others' actions. We designed a transcranial magnetic stimulation (TMS) experiment in which we measured both corticospinal excitability and interhemispheric inhibition, this latter by means of the ipsilateral silent period (iSP), while participants observed two motor tasks (tapping or grasping). We show that the iSP is enhanced during movement observation and that this modulation is tuned to the kinematics of the observed movements. An additional experiment was performed in which the TMS intensity was adjusted to match corticospinal excitability between rest and action observation. This resulted in a relative decrease of iSP. Overall, our data strongly suggest that action observation, as action execution, involves interhemispheric inhibitory mechanisms between the two motor cortices, and that this neural activity appears to be firmly shaped by the ongoing observed movement and its intrinsic dynamics

Effets d’une séance d’exercices excentriques sur la récupération à court terme de la contractilité musculaire

Les effets à court terme de 5 séries de 10 contractions maximales excentriques des fléchisseurs du coude, réalisées sur un ergomètre isocinétique, sont étudiés. Le moment maximal excentrique, isométrique, concentrique, l’activité myoélectrique des biceps et triceps brachii, le niveau d’activation volontaire, l’amplitude de la réponse M, l’amplitude de la secousse, les vitesses maximales de contraction et de relaxation sont mesurés avant (Témoin), 2 minutes après (Post), 24 heures (Post24h) et 48 heures (Post48h) après la séance d’exercice. Le moment est significativement diminué de Témoin à Post48h, quel que soit le mode de contraction, excepté le moment concentrique mesuré à 240°.s-l, qui récupère sa valeur Témoin à Post48h. Le niveau d’activation est significativement réduit à Post (p < 0,05), puis retrouve sa valeur Témoin à Post24h. L’amplitude de la secousse, ainsi que les vitesses maximales de contraction et de relaxation sont significativement diminuées pendant toute la période de mesure (p < 0,01). L’amplitude de la réponse M n’a pas été modifiée à la suite de la séance. Ces résultats montrent que, pendant une période de récupération de 48 heures, la diminution du moment est principalement due à une atteinte périphérique du système neuro-musculaire, et que la récupération de la force doit être étroitement liée à la valeur de force développée

Mechanisms contributing to knee extensor strength loss after prolonged running exercise

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Activation of human quadriceps femoris during isometric, concentric, and eccentric contractions

Maximal and submaximal activation level of the right knee-extensor muscle group were studied during isometric and slow isokinetic muscular contractions in eight male subjects. The activation level was quantified by means of the twitch interpolation technique. A single electrical impulse was delivered, whatever the contraction mode, on the femoral nerve at a constant 50° knee flexion (0° = full extension). Concentric, eccentric (both at 20°/s velocity), and isometric voluntary activation levels were then calculated. The mean activation levels during maximal eccentric and maximal concentric contractions were 88.3 and 89.7%, respectively, and were significantly lower ( P < 0.05) with respect to maximal isometric contractions (95.2%). The relationship between voluntary activation levels and submaximal torques was linearly fitted ( P < 0.01): comparison of slopes indicated lower activation levels during submaximal eccentric compared with isometric or concentric contractions. It is concluded that reduced neural drive is present during 20°/s maximal concentric and both maximal and submaximal eccentric contractions. These results indicate a voluntary activation dependency on both tension levels and type of muscular actions in the human knee-extensor muscle group