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International audience; Purpose: In the face of empiricism prevailing to set an ideal position in cycling, the present study aims to find whether interlimb coordination and muscular activation patterns would provide relevant parameters to characterise an optimal saddle height.Methods: Eighteen experimented bikers were asked to pedal at a rate of 90 rpm under three randomized experimental conditions: Usual Saddle Height (USH), Saddle Height plus (SHP) (leg fully extended, pedal down, ankle in a neutral position) and Saddle Height minus (SHM) (same saddle height difference applied inversely). From the 3D coordinates, the kinematics of ankle, knee and hip were computed and the relative phase between the joints periodic motion was assessed through a Continuous Relative Phase (CRP) algorithm. Muscular activity was recorded through a surface EMG system.Results: While the hip, the knee, and the ankle range of motion were affected by saddle height changes, CRP highlights modifications in interjoint coordination, particularly between ankle and knee and ankle and hip, indicating significant tendency to an in-phase coordination for the experimental conditions compared to the USH condition.Conclusions: Whereas saddle height changes affect joint kinematics and muscular patterns, the freeing of neuro-muscular degrees of freedom in the USH condition allows motor output to adapt dynamically during its very execution. Results suggest that interjoint coordination is an integrative variable providing a relevant parameter to define a correct saddle height in cycling. To reach an ideal cycling position, it may help avoiding the deleterious and painful consequences of a cyclist's inadequate position.

International audience

International audience; À partir d’un rapprochement entre deux expériences menées indépendamment sur un même paradigme, le présent article s’intéresse aux contraintes qui influencent la graphomotricité. Une étude princeps menée par Athènes et al. (2004)sur la reproduction de formes graphiques simples montre que cinq formes sont préférentielles lorsque le bras effecteur n’est pas contraint. Lorsque l’avant-bras est fixé, nos résultats suggèrent l’existence de seulement quatre formes préférentielles. Nos résultats suggèrent que l’ajout d’une contrainte liée sur l’effecteur n’a pas forcément un effet délétère sur le paysage graphomoteur. Si cette hypothèse était confirmée par des études ultérieures, il serait possible d’exploiter les résultats dans le but de faire apparaître de nouvelles formes préférentielles qui ne sont pas spontanément reproduites avec précision sans contrainte ajoutée

International audience; This study aimed to investigate the effects of practice on bimanual coordination dynamics and attentional demands. Participants were asked to perform a dual-task associating a cyclic antiphase bimanual pattern and a discrete reaction time task. A pretest determined each individual critical transition frequency. In the training session, participants practised 120 trials. They were instructed to maintain the antiphase coordination pattern at the critical transition frequency. The training session was interrupted and followed by an intermediate test (after 60 trials) and a post-test (30 min after 120 trials), respectively. A retention test was performed 7 days after the end of the training session. Results showed that: (i) the number of transitions decreased as a consequence of practice; and (ii), subjects were able to maintain the antiphase pattern at a higher frequency than in the pretest. Analysis of the trade-off between relative phase variability and reaction time showed that participants were able to maintain a higher level of stability at the same (intermediate and post-test) or a lower attentional cost (retention test). These findings show that phase transition dynamics and pattern stability can be significantly modified as a result of practice. Changes in the trade-off between pattern stability and cost with learning confirm that the attentional cost incurred by the central nervous system to maintain pattern stability decreased with practice. In line with recent neurobiological studies, the present study provides new insights regarding relationships between brain processes, attentional demands and coordinated behaviour in learning bimanual patterns.

Handwriting is considered as a sequence of cyclic movement and results from the coordination of two orthogonal coupled oscillators. We bear on the assumption that handwriting results from the dynamics of non-linear coupled oscillators. Our main concern was to draw dynamical phenomena that govern the formation of graphic patterns, their co-articulation as well as their destabilization under diverse constraints. In this way, three experiments had been conduced. In order to identify the underlying dynamics, right-handers produced 26 shapes corresponding to fixed values of relative phase and amplitude between the two orthogonal oscillators. Results revealed that among all shapes, only a few were spontaneously stable for each task (0°, 45°/53°, 135°/127° and 180°), which corresponded to ellipses and strokes in various orientations. Those preferred coordination patterns were characterized by attraction of nearby shapes, and by a higher accuracy and stability: hallmark of attractors. Moreover, there was a differential degree of stability among the preferred patterns. Based on this differential stability, the second experiment confirmed that under a high movement velocity or with the non dominant hand, the least stable pattern degraded steeper whereas the most stable preferred patterns kept a highly accurate and stable performance. Finally, our last experiment confirmed that the rules of transition between those preferred patterns come from their properties of stability: the time to switch from a most stable pattern to a least stable pattern took more time than on the other way. Alike all periodic motion, graphic skills exhibit preferred coordination patterns, nonlinear transition and predictable deterioration which corroborate the existence of dynamics underlying handwriting. This underlying dynamics comes from the dynamics of non linear coupled oscillator and might be described by the dynamic of a few variable(s). Our results open a new window to understand the production of handwriting and get robust predictions about its acquisition and degradation that had never explained by the traditional models of handwriting., L'écriture est un acte de coordination motrice entre les mouvements rythmiques des doigts et du poignet qui sont assimilables à des oscillateurs approximativement orthogonaux. L'hypothèse défendue dans cette thèse est que l'écriture, comme l'ensemble de la motricité rythmique, peut être étudiée comme un processus auto-organisé et qu'elle répond aux modèles d'oscillateurs couplés non linéaires. Cette approche dynamique de l'écriture a pour objectif d'identifier la dynamique de coordination spontanée de l'écriture sur laquelle repose la production de formes graphiques plus ou moins précises et stables ainsi que leur dégradation et leur changement en fonction des conditions dans lesquelles un individu écrit. Trois expériences ont été conduites afin de répondre à cet objectif. Afin d'identifier la dynamique spontanée, les participants ont produit 26 formes graphiques apparaissant sur une tablette digitale, qui correspondaient à des valeurs fixes de phase relative et d'amplitude relative entre les deux oscillateurs orthogonaux. Les résultats ont révélé que les participants adoptaient spontanément quatre patrons de coordination stables pour chaque tâche (i.e., 0°, 45°, 135°, 180°) correspondant à des traits et des ellipses dans différentes orientations. Ces patrons étaient caractérisés par de plus grande précision et stabilité temporelle ainsi que par une attraction des patrons adjacents : tous sont les signes identificateurs d'attracteurs. En se basant sur la stabilité différentielle de ces attracteurs, notre 2ième expérience a confirmé que sous une vitesse de mouvement élevée ou avec la main non dominante, les patrons les moins stables se déstabilisaient substantiellement, alors que les patrons les plus stables maintenaient un haut niveau de stabilité et de précision. Enfin, la 3ième étude a montré que les règles de passage entre les patrons stables étaient également sous l'égide de la dynamique de coordination spontanée. Comme tous les mouvements périodiques, la formation de patrons graphiques stables, leur co-articulation, ainsi que leur dégradation sont tributaires d'une dynamique de coordination sous jacente. Cette dynamique est celle d'oscillateurs couplés non linéaires, si bien que l'écriture pourrait être décrite par un nombre limité de variable(s) collective(s), en particulier la phase relative. Ces résultats ouvrent une porte pour comprendre la production d'écriture et faire des prédictions robustes concernant son acquisition et sa dégradation, un projet difficilement envisageable dans des modèles traditionnels de l'écriture.