A 3D multibody approach to the study of human motion: jumps and gait analyses

Motion analysis of the human body can be supported by several experimental techniques developed in recent years. The use of three-dimensional stereophotogrammetric motion capture systems has become a fundamental technique for the kinematic analysis of complex motions. Such systems are usually combined with other instruments exploited for the dynamic measurements: ground reaction forces can be measured by single or multiaxial force platforms. Even if a great amount of experimental data can be obtained from experimental investigations it is sometimes difficult to establish cause-effect relationships in complex dynamic systems directly from measurements. Therefore, computer modeling and simulation, based on experimental data, can provide useful insights for human biomechanics. This work describes the analyses of two common gestures, jumps [1] and gait, which represent two test cases where the proposed methodology is applied. The motions have been executed in laboratory where a 3D motion capture system has been used to acquire the coordinates of a set of markers placed at anatomical landmarks, while a six-axis force plate has been used to measure the ground reaction forces. Simulations of the acquired motions have been developed by means of the multibody software OpenSim [2], an open-source platform for modeling, simulating, and analyzing the human body motion. After a 3D multibody model of the human body has been set up and scaled on the basis of the specific anthropometric characteristics, the kinematic and dynamic experimental data have been provided as input for the inverse kinematics and inverse dynamics analyses. Such operations, combined with a residual reduction algorithm which minimizes the error between the measured reaction forces and the reactions forces obtained form the inverse dynamics analysis, allows to perform a simulation of the human body motions with dynamically consistent kinematic data. Results of simulations can be processed in order to obtain the joint angles, moments and powers. Furthermore, a series of auxiliary data can be estimated, including the joint reactions and the motion of the center of gravity of the single limbs or of the whole body.