Devoted to the study of articular kinematics - movements, speeds and accelerations of body segments - the optical systems are based mainly on optoelectronic and video techniques. In the first case markers are placed on the skin of the subject near the joint. “The position of these markers has been codified by the International Society of Biomechanics according to the activity studied. Therefore in the case of walking they will be placed at precise spots on the lower limbs. However, during a recent experiment relating to the service of the top tennis players of the Francophone Tennis Association - Olivier Rochus, Steve Darcis, etc. -, we positioned the markers over the entire body of the athletes, as well as on their racket”, explains Bénedicte Forthomme. When the sportsman moves, photosensitive cameras detect the movement of the markers in real time. Thanks to digital software and data processing, the biomechanics scientist obtains a reproduction of the movements of the body segments in three-dimensional space, as well as the calculation of speeds and accelerations. It is therefore not a traditional video of movement.

In fact there are two main types of optoelectronic systems, passive, where the markers reflect the infrared light emitted by the camera, and active, where each marker emits its own infrared light. “The main drawback of the passive system is that, if two markers are relatively close, the system can be seen to be incapable of distinguishing them from each other. The active system avoids this pitfall. There are two disadvantages however: a limitation of the number of markers being able to be used simultaneously and the need to connect electric cables to the markers,’ explains Cédric Schwartz, a post-doctoral engineer at the Human Movement Analysis Laboratory.

Microscopic approach to music

Globally the active electronic systems offer greater simplicity of use and better flexibility. For this reason LAMH opted for one of them: the Codamotion. The laboratory in Liège was equipped with four cameras in order to cover a large field of view taking and to be able to follow complex movements in this way. “Even for just walking, one three-dimensional unit would not suffice if we were simultaneously interested in the left and right foot of the walker”, stresses Cédric Schwartz.

The maximal frequency of data acquisition from the system depends on the number of markers. If there are 56, for example, it is limited to 100 hertz; if there are 6, to 800 hertz. Yet, to describe the position of a body segment in 3D requires a minimum of three markers. Consequently, only two segments are studied if we work at 800 hertz.