D21 2D/3D Kinematics

Topic

This kinematic analysis describes the final geometry of the athlete's movement—specifically their position, velocity, acceleration, and joint angles—without considering the forces causing the movement. Consequently, this analysis forms the basis for any subsequent biomechanical diagnosis of the athlete, including the identification of the dynamic mode established by another model in this set.

A 3D motion capture system—whether using an infrared camera tracking a reflective marker on the athlete or an inertial system placed directly on their body—records the spatial position of each marker over time at a high sampling rate; this is the same 3D motion analysis system used in another model within this set.

Numerical differentiation of these measured positions yields the velocity and acceleration of each marker over time, although this process introduces additional noise that must be filtered out before proceeding with the analysis. A Butterworth low-pass filter removes this high-frequency noise without significantly distorting the underlying movement signal; the cutoff frequency used is much lower for typical human movement than for a brief, sudden impact on the athlete. A lower cutoff frequency allows the filter to remove a greater proportion of high-frequency noise, albeit at the cost of smoothing the actual movement signal to some extent.

The final joint angle for each of the athlete's joints is calculated based on the segmental vectors of the body segments adjacent to that joint. Consequently, this same inverse kinematics approach allows for the estimation of the final position of each of the athlete's internal joints based solely on the positions of external markers placed on the skin, eliminating the need for direct measurement of the internal joints.

Thus, given the athlete's movement geometry upon which this model is based, the final three-dimensional position of each marker ultimately determines the final joint angle, angular velocity, linear and angular acceleration, step length, step frequency, and the final trajectory of the center of mass—the very same center-of-mass variable established by another model in this set. Recording the markers' three-dimensional positions at an even higher sampling frequency results in even greater precision for the final values ​​derived from them, thereby enhancing the biomechanical diagnosis for which the model was originally designed.

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