36 research outputs found
Restitution d'Ă©nergie Ă©lastique et locomotion (REEL) : une approche adimensionnelle
L'objectif de ce travail est de développer une approche adimensionnelle de la locomotion humaine, et plus précisément de la
marche et de la course. En d'autres termes, le principal enjeu de cette thĂšse est d'induire des similitudes locomotrices
entre des hommes de tailles différentes. Ces similitudes locomotrices attendues entre des individus de différentes tailles
sont les mĂȘmes que celles que les physiciens recherchent lors de l'Ă©laboration de prototype Ă partir de maquette. Dans
l'approche que nous tentons de développer tout au long de ce document, nous considérons qu'un individu petit est le modÚle
réduit d'un plus grand. Notre approche est au croisement des champs de la physique, de la modélisation et de la
biomécanique.
L'application de l'analyse dimensionnelle aux modĂšles simples de locomotion permet de mettre en avant l'intĂ©rĂȘt des nombres
adimensionnels de Froude (vitesse adimensionnelle) et Strouhal (fréquence adimensionnelle) pour étudier la locomotion
humaine. Ces modÚles simples de locomotion simplifient le corps humain à la masse du corps concentrée au centre de gravité
oscillant à l'extrémité d'un ressort. Ils prennent en compte une composante élastique et mettent en avant des transferts se
réalisant au centre de gravité entre les énergies cinétique, potentielle de pesanteur et potentielle élastique. Le rapport
de ces énergies est appelé Modela. Modela possÚde deux variantes, une pour la marche et l'autre pour la course, et est
dépendant de Froude et Strouhal.
Dans un premier temps, les conditions expérimentales de vitesse de déplacement (à partir de Froude) et de fréquence de pas
(à partir de Strouhal), toutes deux relatives à l'anthropométrie des individus, ont permis d'engendrer des similitudes
locomotrices pour la marche et la course chez des individus de tailles diffĂ©rentes. Ces rĂ©sultats rĂ©vĂšlent tout l'intĂ©rĂȘt
d'une approche adimensionnelle de la locomotion en montrant qu'exprimés indépendamment de l'anthropométrie des individus,
leurs comportements adimensionnels est le mĂȘme. Utiliser cette approche pour comparer des locomotions au sein mĂȘme de
l'espĂšce humaine a un grand intĂ©rĂȘt pour Ă©tudier des comportements dĂ©viants d'un comportement standard. Aussi, cette
approche peut ĂȘtre un moyen de mettre en avant des organisations du mouvement communes Ă diffĂ©rentes espĂšces.
Dans un second temps, l'accent est mis sur la comparaison entre le modĂšle simple et le modĂšle complexe du corps humain. D'un
coté, le modÚle simple du corps humain prend en compte une composante élastique et ne s'intéresse qu'au centre de gravité.
De l'autre cotĂ©, le corps humain peut ĂȘtre modĂ©lisĂ© comme un ensemble de segments corporels articulĂ©s entre eux. Ici, un
lien est fait entre le mouvement global du centre de gravité et les coordinations des segments poly-articulés, lors du
mouvement, et tout ce que cela engendre en termes de transfert d'Ă©nergie. Le rapprochement des deux modĂšles explique comment
un individu peut se comporter comme une masse bondissante lors de la marche et la course ou comment les expérimentations futures pourront investir le champ de l'élasticité humaine et de l'économie d'énergie.The aim of this paper is to develop a dimensionless approach of the human locomotion, and more specifically of walking and
running gaits. In other terms, the main goal of this PhD thesis is to induce locomotor similarity between different-sized
humans. These similarities are the same that the physicians look for when they design a prototype from a scale model.
Throughout the thesis paper, this approach allows the consideration that a small human is a reduced model of a tall one. Our
approach is cross-fielded like Physics, Modelization, and Biomechanics.
The dimensional analysis application to the common locomotion models allows to highlight the interest of using the
dimensionless numbers of Froude (dimensionless speed) and Strouhal (dimensionless frequency) to study human locomotion.
These locomotion models are reduced to the body mass represented at its center of mass oscillating at the end of a massless
spring. They take into account an elastic component and enlighten transfers occurring at the center of mass between the
kinetic, potential and elastic energies. The ratio of these energies is called Modela. A Modela corresponds to both walking
and running, and depends on Froude and Strouhal.
First, the experimental conditions such as speed displacement relative to anthropometry (from Froude) and step frequency
relative to anthropometry (from Strouhal) allow us to generate locomotor similarity between different-sized subjects for
walking and running. These results reveal the interest of the dimensionless approach of the locomotion by showing that the
dimensionless behaviors are the same when they are expressed independently of the subject anthropometry. The use of this
approach to compare human locomotions is interesting to study behavior different to the gold dimensionless standard. Also,
this approach may be a means to highlight a global organization of the movement which is common to many species.
Then, the comparison between the simple model and the complex model is investigated. In one hand, the model takes into
account an elastic component and only describe the center of mass movement. In the other hand, the human body is represented
as a whole of body segment poly-articulated. A link is done between the global movement of the center of mass and the
movement of the poly-articular model, and especially regarding for the energy transfers. The link between the models explain
how a subject has the same behavior of a spring mass, and how the future works will be able to investigate the fields of the human elasticity and the saving energy mechanisms
A global database for metacommunity ecology, integrating species, traits, environment and space
The use of functional information in the form of species traits plays an important role in explaining biodiversity patterns and responses to environmental changes. Although relationships between species composition, their traits, and the environment have been extensively studied on a case-by-case basis, results are variable, and it remains unclear how generalizable these relationships are across ecosystems, taxa and spatial scales. To address this gap, we collated 80 datasets from trait-based studies into a global database for metaCommunity Ecology: Species, Traits, Environment and Space; âCESTESâ. Each dataset includes four matrices: species community abundances or presences/absences across multiple sites, species trait information, environmental variables and spatial coordinates of the sampling sites. The CESTES database is a live database: it will be maintained and expanded in the future as new datasets become available. By its harmonized structure, and the diversity of ecosystem types, taxonomic groups, and spatial scales it covers, the CESTES database provides an important opportunity for synthetic trait-based research in community ecology
Functional Structure of Biological Communities Predicts Ecosystem Multifunctionality
The accelerating rate of change in biodiversity patterns, mediated by ever increasing human pressures and global warming, demands a better understanding of the relationship between the structure of biological communities and ecosystem functioning (BEF). Recent investigations suggest that the functional structure of communities, i.e. the composition and diversity of functional traits, is the main driver of ecological processes. However, the predictive power of BEF research is still low, the integration of all components of functional community structure as predictors is still lacking, and the multifunctionality of ecosystems (i.e. rates of multiple processes) must be considered. Here, using a multiple-processes framework from grassland biodiversity experiments, we show that functional identity of species and functional divergence among species, rather than species diversity per se, together promote the level of ecosystem multifunctionality with a predictive power of 80%. Our results suggest that primary productivity and decomposition rates, two key ecosystem processes upon which the global carbon cycle depends, are primarily sustained by specialist species, i.e. those that hold specialized combinations of traits and perform particular functions. Contrary to studies focusing on single ecosystem functions and considering species richness as the sole measure of biodiversity, we found a linear and non-saturating effect of the functional structure of communities on ecosystem multifunctionality. Thus, sustaining multiple ecological processes would require focusing on trait dominance and on the degree of community specialization, even in species-rich assemblages
Analyse dynamique tridimensionnelle du pĂ©nalty au handball selon le niveau dâexpertise et la zone ciblĂ©e
Le but de cette Ă©tude est de mettre en Ă©vidence des diffĂ©rences dâorganisation du
mouvement lors du pĂ©nalty au handball en fonction du niveau dâexpertise et de la zone de
tir. Six personnes expertes et huit personnes novices ont effectué un enchaßnement de tirs
dans diffĂ©rentes zones de lâembut. Les donnĂ©es cinĂ©matiques des segments des joueurs ont
Ă©tĂ© enregistrĂ©es Ă lâaide dâun systĂšme dâanalyse du mouvement tridimensionnelle (VICON).
Les résultats ont dévoilé une vitesse de balle plus grande pour les tirs ciblant la partie
basse de lâembut (18,6 ± 0,7 et 18,4 ± 0,8 m/s contre 17 ± 0,6 et 17,5 ± 0,7 m/s). Les
moments résultants obtenus par dynamique inverse au niveau du poignet (1,1 ± 0,6 Nm contre
 â0,4 ± 0,3 Nm) et du coude (7,9 ± 5,9 Nm contre  â10,5 ± 7,3 Nm) ont permis de
diffĂ©rencier les experts et les novices. Lâorganisation du mouvement est dĂ©pendante de la
zone ciblĂ©e et du niveau dâexpertise
A preliminary study suggests that walk-to-run transition is consistent with mechanical optimization
The aim of this study was to compute moment cost function (MCF) at different walking and running speeds. We hypothesized that MCF would be optimal at low speeds during walking and at high speeds during running
Modela-r as a Froude and Strouhal dimensionless numbers combination for dynamic similarity in running
International audienceThe aim of this study was to test the hypothesis t hat running at fixed fractions of Froude (Nfr) and Strouhal (Str) dimensionless numbe rs combinations induce dynamic similarity between humans of different sizes. Ninet een subjects ran in three experimental conditions, i) constant speed, ii) similar speed (N fr) and iii) similar speed and similar step frequency (Nfr and Str combination). In addition to anthropometric data, temporal, kinematic and kinetic parameters were assessed at each stage to measure dynamic similarity informed by dimensional scale factors and by the decrease of dimensionless mechanical parameter variability. Over a total of 54 dynamic parameters, dynamic similarity from scale factors was met for 16 (mean r = 0.51), 32 (mean r = 0.49) and 52 (mean r = 0.60) parameters in the first, the second and the third experimental conditions, r espectively. The variability of the dimensionless preceding parameters was lower in the third condition than in the others. This study shows that the combination of Nfr and Str, computed from the dimensionless energy ratio at the center of gravity (Modela-r) ensures d ynamic similarity between different-sized subjects. The relevance of using similar experiment al conditions to compare mechanical dimensionless parameters is also proved and will hi ghlight the study of running techniques, or equipment, and will allow the identification of abnormal and pathogenic running patterns. Modela-r may be adapted to study other abilities requiring b ounces in human or animal locomotion or to conduct investigations in comparat ive biomechanics
Walking dynamic similarity induced by a combination of Froude and Strouhal dimensionless numbers: Modela-w
International audienceThe aim of this study was to assess the accuracy of a new dimensionless number associating Froude (Nfr) and Strouhal (Str) called Modela-w to induce walking dynamic similarity among humans of different sizes. Nineteen subjects walked in three experimental conditions: (i) constant speed, (ii) similar speed (Nfr) and (iii) similar speed and similar step frequency (Modela-w). The dynamic similarity was evaluated from scale factors computed with anthropometric, temporal, kinematic and kinetic data and from the decrease of the variability of the parameters expressed in their dimensionless form. Over a total of 36 dynamic parameters, dynamic similarity from scale factors was met for 11 (mean r = 0.51), 22 (mean r = 0.52) and 30 (mean r = 0.69) parameters in the first, the second and the third experimental conditions, respectively. Modela-w also reduced the variability of the dimensionless preceding parameters compared to the other experimental conditions. This study shows that the combination of Nfr and Str called Modela-w ensures dynamic similarity between different-sized subjects and allows scientists to impose similar experimental conditions removing all anthropometric effects
Transferability between Isolated Joint Torques and a Maximum Polyarticular Task: A Preliminary Study
International audienceThe aims of this study were to determine if isolated maximum joint torques and joint torques during a maximum polyarticular task (i.e. cycling at maximum power) are correlated despite joint angle and velocity discrepancies, and to assess if an isolated joint-specific torque production capability at slow angular velocity is related to cycling power. Nine cyclists completed two different evaluations of their lower limb maximum joint torques. Maximum Isolated Torques were assessed on isolated joint movements using an isokinetic ergometer and Maximum Pedalling Torques were calculated at the ankle, knee and hip for flexion and extension by inverse dynamics during cycling at maximum power. A correlation analysis was made between Maximum Isolated Torques and respective Maximum Pedalling Torques [3 joints x (flexion + extension)], showing no significant relationship. Only one significant relationship was found between cycling maximum power and knee extension Maximum Isolated Torque (r=0.68, p<0.05). Lack of correlations between isolated joint torques measured at slow angular velocity and the same joint torques involved in a polyarticular task shows that transfers between both are not direct due to differences in joint angular velocities and in mono-articular versus poly articular joint torque production capabilities. However, this study confirms that maximum power in cycling is correlated with slow angular velocity mono-articular maximum knee extension torque
An algorithm to decompose ground reaction forces and moments from a single force platform in walking gait
International audienceIn walking experimental conditions, subjects are sometimes unable to perform two steps on two different forceplates. This leads the authors to develop methods for discerning right and left ground reaction data while they are summed during the double support in walking. The aim of this study is to propose an adaptive transition function that considers the walking speed and ground reaction forces (GRF). A transition function is used to estimate left and right side GRF signals in double support. It includes a shape coefficient adjusted using single support GRF parameters. This shape coefficient is optimized by a non-linear least-square curve-fitting procedure to match the estimated signals with real GRF. A multiple regression is then performed to identify GRF parameters of major importance selected to compute the right and left GRF of the double support. Relative RMSE (RMSER), maximum GRF differences normalized to body mass and differences of center of pressure (CoP) are computed between real and decomposed signals. During double support, RMSER are 6%, 18%, 3.8%, 4.3%, 3%, and 12.3% for anterior force, lateral force, vertical force, frontal moment, sagittal moment and transverse moment, respectively. Maximum GRF differences normalized to body mass are lower than 1 N/kg and mean CoP difference is 0.0135 m, when comparing real to decomposed signals during double support. This work shows the accuracy of an adaptive transition function to decompose GRF and moment of right and left sides. This method is especially useful to accurately discern right and left GRF data in single force platform configurations
Nutrient recycling by coastal macrofauna: intra- versus interspecific differences
In the context of global change, improving our understanding of how species communities shape ecosystem functioning and stability is a key issue. Therefore, we have to adopt a functional approach by considering the role of organisms in ecosystem processes. Nutrient recycling is important for sustaining primary productivity in aquatic systems but has been largely overlooked for macro-organisms. In a Mediterranean coastal ecosystem we found that per capita ammonium and phosphate excretion rates for the 9 dominant nektonic macrofauna species exceeded those of 3 benthic bivalves by factor 100. Body mass and species identity together significantly explained these interspecific differences in excretion rates. Significant differences in the effect of body mass on nutrient excretion rates were also found among the 7 fish species. More studies are needed to further explore the biological determinants of the intra-and interspecific variability of excretion rates as well as their consequences on marine ecosystem functioning