Section: New Results

Biomechanics and Motion Analysis

Interaction strategies between two walkers to avoid collision

Participants : Armel Crétual, Julien Pettré, Anne-Hélène Olivier, Antoine Marin.

Walkers are extremely efficient in avoiding collisions, even in relatively condition of density. We experimentally addressed two questions. What are the conditions for walkers to perform adaptations to their trajectory, and second, how avoidance performed in time. We checked several hypothesis, that led to two contributions, as presented in [15] . First, human are able to anticipate the future conditions of an interactions and the distance they would meet. They react accordingly, i.e., if and only if a future risk of collision can be predicted. Second, we demonstrated that the avoidance is performed with anticipation, i.e., avoidance maneuvers are over before walkers get at closest distance.

Quantification of pathological motion

Participant : Armel Crétual.

In clinical routine, precise quantification of patients' gesture remains a challenge. Several simple means are daily used by practicians in physical medecine. Their main drawback is often a large inter-operator variability and even sometimes an intra-operator one. To overcome this, we have developed and validated still simple to remain usable) but much more objective tools in two different fields: gait and shoulder laxity.

First, we have proposed a new index of gait quantification based on EMG profiles called KeR-EGI (for Kerpape-Rennes EMG-based Gait Index). Our recent works allowed us to demonstrate its reproducibility even in patients with severe troubles. Moreover, we have also demonstrated the complementarity of this index based on muscular activation and an index based on kinematics, the Edinburgh Visual Gait Score (EVGS) that can be computed easily from a simple video recordings of the patient's gait. Indeed, we have shown that the relationship between these indices depends on the fact that pathology is congenital or acquired. Using both indices at the time, allows to evaluate the potential kinematics compensation the patient does to improve his/her gait despite a damaged motor control.

Secondly, in shoulder surgery, the surgeon has to choose between different protocols depending on whether the patient is hyperlax or not. Until now, shoulder laxity is very roughly evaluated without actual measurement and above all mobilizing only one axis (external rotation) of this complex joint. By measuring precisely the whole Range Of Motion of 28 subjects recruited to ensure a large spectrum of laxity (from hypo to hyper-laxity), we have shown that the usual clinical indices fail to actually classify subjects, as they do focus on only one dimension of mobility. From, that result, we have then proposed a new method to evaluate laxity that remains simple and usable in daily routine but that takes into account all dimensions of shoulder's mobility.

Modeling gesture in sports: fin swimming

Participants : Nicolas Bideau, Guillaume Nicolas, Benoit Bideau, Richard Kulpa.

In swimming, experimental approaches are commonly used to analyze performance. However, due to obvious limitations in experimental approaches (impoossibility to standardize any situations etc.), it is difficult to characterize surrounding fluid. To overcome this limitation, we currently develop analysis, modeling and simulation of aquatic locomotion, using CFD computer simulation and new methods based on animation of virtual characters.

• A first application of this topic enables to evaluate the influence of swim fin flexibility on efficiency during swimming based on a CFD structure interaction model. Finite elements simulations are carried out for various material properties and various prescribed kinematics. Besides the significant effect of flexibility on propulsive forces, the results indicate that the propulsive efficiency is greatly influenced by the stroke frequency and the initial angle of attack. For the selected material properties, the results show that efficiency increases from 3.6 percents to 11.9 percents when the stroke frequency is increased from 0 to 1.7 Hz. Moreover efficiency is clearly increased from 5.0 percents to 24.2 percents when increasing the angle of attack from 0 to 45 degrees. Therefore, an interesting prospect of the present work could be an enhancement of the design of better performing swim fins.

• A second application of this topic related to aquatic propulsion deals with a new method to evaluate cross-sectional area based on computer animation of swimming. Indeed, reducing cross sectional area (CSA) during starts and turns is a key part of performance optimisation. Different methods have been used to obtain this parameter without any standard: total human body volume to the power 2/3, wetted area or frontal area based on planimetry technique (PT). These different methods can lead to discrepancies in drag values. Recently, we used two synchronized camcorders to evaluate drag parameters during the different phases of an undulatory stroke cycle. However, such a technique needs accurate synchronization and calibration of the different camcorders views. The aim of this study is to provide a new method based on animation of virtual characters to obtain instantaneous cross-sectional area in an undulatory stroke cycle. Its main advantage is to obtain cross-sectional area as well as biomechanical analysis with a single camcorder in a sagittal plan and without space calibration. A camcorder placed side-on to the swimmer recorded the undulatory movements in the sagittal plane of eight swimmers. This information provided the angles between limbs. These data were then used by our animation engine to animate a virtual swimmer whose anthropometric data came from the real swimmer. A specific algorithm has been developed to automatically obtain the CSA using body outlines. In order to validate our method, we also calculated the CSA using PT with a frontal camcorder view of the same undulatory movements. Our results show similar values of maximum CSA using PT and the frontal camcorder view and our algorithm based on 3D animation. The mean coefficient of variation between the results obtained from the two methods is 7.3 percents. This difference could be related to the level of details of the mesh used to model the avatar. One prospect to this work is to take resistive and propulsive body segments into account in CSA calculation. From this method, we intend to better understand swimming hydrodynamics and the way CSA influences active drag. More generally, this approach has been designed to provide new practical insights into swimming analysis protocols.