Inaccurate estimation of center of mass state from kinematics during human running

The 9.5th international symposium on Adaptive Motion of Animals and Machines. Ottawa，Canada (Virtual Platform). 2021-06-22/25. Adaptive Motion of Animals and Machines Organizing Committee

Le rôle des ajustements posturaux dans la coordination motrice

My thesis is that efficient motor coordination relies on the adjustment of the body mechanical properties to the task at hand, by the postural system. I first considered tasks which require immobility, such as standing still and precise and accurate tool manipulation. I developed a generic model of stabilization with neural delays. According to this model, body mechanical properties such as stiffness and inertia play a critical role for maintaining immobility despite external perturbations, since they determine the relative speed at which perturbations will be amplified during the response delay. I then considered the task of initiating movement. The main purpose of postural control is considered to be to counteract the destabilizing force of gravity. However, from the consideration of the mechanical requirements for movement, I arrived at a different interpretation of the experimental findings on stance and movement initiation. I therefore developed the mobility theory, which states that during skilled movement the postural system adjusts the position of the centre of mass (CoM) in view of providing torque for the movement. I then tested to what extent this ability to adjust the CoM position to the task at hand was affected by aging. I performed an experimental analysis of the postural adjustments of elderly fallers and non-fallers. I showed that both elderly fallers and non-fallers are able to adjust their posture to the task, given advance warning, but elderly fallers are less able to rapidly adjust the CoM position during the task. Thus, motor coordination relies on the rapid adjustment of the body mechanical properties to changing task requirements.Ma thèse est que la coordination motrice s’appuie sur un ajustement des propriétés mécaniques de leurs corps à la tâche en cours, par le système postural. Certaines tâches, comme se tenir debout, ou manipuler un outil avec précision, nécessitent de rester immobile. J’ai développé un modèle générique de stabilité aux perturbations, malgré le délai de réponse neurale. D’après ce modèle, les propriétés mécaniques du corps, telles que la raideur et l’inertie, jouent un rôle fondamental dans le maintien de l’immobilité face aux perturbations, puisqu’elles déterminent la vitesse relative à laquelle ces perturbations sont amplifiées pendant le délai de réponse neurale. Lors de l’initiation du mouvement, la fonction fondamentale communément attribuée au système postural est celle de contrecarrer la déstabilisation due à la gravité. Cependant, en considérant les conditions mécaniques requises pour l’initiation du mouvement, je suis arrivée à une interprétation différente des résultats expérimentaux sur la posture debout et l’initiation du mouvement. J’ai développé la théorie de la mobilité, selon laquelle le système postural ajuste la position du centre de masse (CdM) afin de donner de l’élan au mouvement. J’ai ensuite testé si cette capacité à ajuster la position du CdM à la tâche était affectée lors du vieillissement. J’ai analysé des données expérimentales sur les ajustements posturaux de sujets âgés chuteurs et non chuteurs. J’ai montré que, s’ils sont prévenus en avance, aussi bien les chuteurs que les non chuteurs ajustent la position de leur CdM à la tâche. Cependant, les chuteurs ont une capacité réduite à ajuster rapidement la position de leur CdM au cours de la tâche

Anticipatory coadaptation of ankle stiffness and sensorimotor gain for standing balance

International audienceExternal perturbation forces may compromise standing balance. The nervous system can intervene only after a delay greater than 100 ms, during which the body falls freely. With ageing, sensorimotor delays are prolonged, posing a critical threat to balance. We study a generic model of stabilisation with neural delays to understand how the organism should adapt to challenging balance conditions. The model suggests that ankle stiffness should be increased in anticipation of perturbations, for example by muscle co-contraction, so as to slow down body fall during the neural response delay. Increased ankle muscle co-contraction is indeed observed in young adults when standing in challenging balance conditions, and in older relative to young adults during normal stance. In parallel, the analysis of the model shows that increases in either stiffness or neural delay must be coordinated with decreases in spinal sensorimotor gains, otherwise the feedback itself becomes destabilizing. Accordingly, a decrease in spinal feedback is observed in challenging conditions, and with age-related increases in neural delay. These observations have been previously interpreted as indicating an increased reliance on cortical rather than spinal control of balance, despite the fact that cortical responses have a longer latency. Our analysis challenges this interpretation by showing that these observations are consistent with a functional coadaptation of spinal feedback gains to functional changes in stiffness and neural delay

Mobility as the Purpose of Postural Control

Counteracting the destabilizing force of gravity is usually considered to be the main purpose of postural control. However, from the consideration of the mechanical requirements for movement, we argue that posture is adjusted in view of providing impetus for movement. Thus, we show that the posture that is usually adopted in quiet standing in fact allows torque for potential movement. Moreover, when performing a movement—either voluntarily or in response to an external perturbation—we show that the postural adjustments are organized both spatially and temporally so as to provide the required torque for the movement. Thus, when movement is performed skillfully, the force of gravity is not counteracted but actually used to provide impetus to movement. This ability to move one's weight so as to exploit the torque of gravity seems to be dependent on development and skill learning, and is impaired in aging

Postural adjustments in anticipation of predictable perturbations allow elderly fallers to achieve a balance recovery performance equivalent to elderly non-fallers

Background In numerous laboratory-based perturbation experiments, differences in the balance recovery performance of elderly fallers and non-fallers are moderate or absent. This performance may be affected by the subjects adjusting their initial posture in anticipation of the perturbation. Research questions: Do elderly fallers and non-fallers adjust their posture in anticipation of externally-imposed perturbations in a laboratory setting? How does this impact their balance recovery performance? Methods 21 elderly non-fallers, 18 age-matched elderly fallers and 11 young adults performed both a forward waist-pull perturbation task and a Choice Stepping Reaction Time (CSRT) task. Whole-body kinematics and ground reaction forces were recorded. For each group, we evaluated the balance recovery performance in the perturbation task, change in initial center of mass (CoM) position between the CSRT and the perturbation task, and the influence of initial CoM position on task performance. Results The balance recovery performance of elderly fallers was equivalent to elderly non-fallers (p?>?0.5 Kolmogorov-Smirnov test). All subject groups anticipated forward perturbations by shifting their CoM backward compared to the CSRT task (young: 2.1% of lower limb length, elderly non-fallers: 2.7%, elderly fallers: 2.2%, Hodges-Lehmann estimator, p

Profound and sustained response with next-generation ALK inhibitors in patients with relapsed or progressive ALK-positive anaplastic large cell lymphoma with central nervous system involvement

International audienc