New frontiers of markerless motion capture: application to swim biomechanics and gait analysis

During the last decade markerless motion capture techniques have gained an increasing interest in the biomechanics community. These techniques are only based on synchronized video sequences of the subject and allow noninvasive measurements of body segments kinematics. The main advantage over commercial marker-based motion capture systems is that they do not rely on skin based markers for tracking, capturing the subject's motion in a natural and easier way. Furthermore, markerless analysis can be performed in non-controlled environments where markers cannot be employed. An exemplary case is the kinematic analysis of swimmers, which requires underwater human motion tracking, therefore it cannot rely on retro reflective marker based motion capture systems. The major drawbacks of the state of the art of swim biomechanics are the manual digitization of feature points on video sequences of the subject, which is a time consuming task, and that it has often been limited to the sagittal plane. Since the motion in front-crawl swimming occurs on different planes, for a descriptive analysis of three-dimensional motion a multi-camera setup is needed, which requires not only calibration, but also synchronization of the cameras. These procedures are hindered by the underwater experimental environment. In this context markerless systems, without requiring to draw or attach markers to the athlete skin, are suitable for underwater quantitative analysis during training or even competition. One of the aims of the present thesis is the development of a video-based, markerless system for the analysis of arm movements during front crawl swimming, On the other hand, in the clinical field, the application of markerless techniques is still debated. This is mainly due to a limited number of papers dedicated to the comparison with the state of the art of marker based motion capture, especially in term of repeatability and accuracy in the estimation of the three dimensional joint rotations. So far in the present thesis the application of markerless techniques to data acquired with a marker-based system has been investigated. The latter was employed to directly compare the joint kinematics estimated with the markerless technique with the one determined with traditional marker based systems, while excluding differences in calibration and synchronization of the cameras. In this context for marker placement the CAST protocol was adopted, which could be considered the most reliable in term of accuracy for joints rotation estimation. Indeed, a combined markerless and markerbased system could be considered a versatile solution for gait laboratories, which could perform different type of analyses based on the desired degree of accuracy and unencumbrance of the subject. The present thesis is organized as follows: the first chapter contains an introduction to markerless motion capture systems and the fields in which they have been applied. The second chapter presents the state-of-the-art of markerless techniques reported in literature, and a detailed description of the actual approach that has been employed and developed. The third and fourth chapters focus respectively on the application of the markerless technique to front-crawl swimming employing underwater cameras, and to gait analysis employing video sequences acquired with a marker-based system. Some background information is given first, presenting the open issues that are addressed in this thesis. A description of the experimental setup is provided, along with the explanation of the data processing method that has been developed to address the related issues. Finally, experimental results are presented and discussed. Conclusions and future developments are the object of the fifth chapter.