Fig. 1: Virtual Sensei mocap suit

Our task required the tracking of fast and complex 3D motions; hence, the motion capture instrument had to assure an accurate high-frequency data capture. Furthermore, the data should be captured without interfering with the athlete’s freedom of movement. A type of equipment that satisfies those requirements is the optical motion capture system.

The chosen system is composed of eight high-speed infrared cameras, that can track human movements at 100 fps. A kit of reflective markers is also necessary. These, once applied in a particular configuration on the athlete’s body through a special mocap suit (Fig. 1), allow the optical recognition of the motion.

Kinetic energy

Considering that the purpose is the objective evaluation of sport motions efficacy, particularly important is the choice of the performance criteria. Taking into consideration the variability of the anthropometry, the influence not only of the kinematic parameters of a gesture (trajectories, velocities, accelerations), but also of the inertial ones (masses, geometries and moments of inertia) is evident. The kinetic energy takes both velocity and inertial parameters into account.

Therefore, velocities and accelerations (linear and angular) of the limbs and of the entire body are computed. Masses and moments of inertia of each body part are estimated knowing the athlete’s gender, body weight and height.

One of the purposes of this research is to point out the subdivision and transfer of energy among the limbs during the execution of techniques. Since kinetic energy is a function of mass, geometry and velocity, it is possible to evaluate a sports motion through a single parameter that takes into account not only the kinematic characteristics, but also the athlete’s anthropometric dimensions.