Analysis on the plié and grand plié in classical ballet with magneto-inertial measurement units

Ballet dancers look for the perfection of movements not only for the elegance of the performance, but, above all, to prevent from injuries. The plié and the grand plié are movements of paramount importance in ballet dancing and they are extensively performed by beginners. An improper exercise execution can eventually lead to severe injuries. Thus, the design of a wearable system able to provide a quantitative estimation of the movement quality can help determining and correcting the different types of mistakes. At the same time, the system must be easy to use and must not hinder the movements of the dancer. In this work, we propose a system based on magneto-inertial measurement units (MIMU) that measures the orientation of the dancer femur and shinbone, during the plié and the grand plié performed by a professional ballet dancer. The dancer was asked to perform the two classical positions in a series of correct and incorrect executions. When the plié is performed incorrectly, the average femur inclination angle increases from 36.3±0.7° to 39.7±1.4° and the average femur direction angle decreases from 94.5±4.0° to 70.5±3.4°. When the grand plié is performed incorrectly, the average femur inclination angle increases from 67.2±0.5° to 79.0±3.0°, the average femur direction angle increases from 39.0±2.3° to 51.6±6.3° and the average shinbone direction angle increases from 116.3±2.6° to 142.8±4.0°. These results show that the proposed wearable system is able to identify and estimate the improper execution of the tasks, without hindering or alterating the movement of the dancer.