Theia Model Description

There are currently three available models in Theia3D:

1. Default Model

   • Consists of two kinematic chains: lower body (pelvis and legs) and upper body (torso, arms, and head).

   • No abdomen or neck segments.

   • Shoulder joints and head segment are allowed 6 degrees of freedom.

2. Full Body Model

   1. Consists of one, whole-body kinematic chain.

   2. Abdomen and neck segments included.

   3. Shoulder joints are allowed 6 degrees of freedom.

   4. Head is allowed 3 degrees of freedom.

3. Separate Arm and Head Model

   • Consists of five kinematic chains: lower body (pelvis and legs), torso, left arm, right arm, and head.

   • No abdomen or neck segments.

   • Shoulder joints and head segment are allowed 6 degrees of freedom.

Pose

Model pose can be exported to .c3d, .fbx, and .json files.

The .c3d files contain the 4x4 pose matrices for each model segment and the local coordinates of the anatomical landmarks of the distal segments of the model (feet, hands, head). These files can be processed using Visual3D.

The .fbx files contain the hierarchical skeleton, pose, and bone meshes of the animation model. The first frame of the file contains the model in a “T-Pose”. The skeleton must be solved using the Full Body Model in order to save pose files in FBX format.

The .json files contain information about how the trial was processed (Theia3D version, model, preferences, etc.) and the 4x4 pose matrices for each body segment for every frame of the trial.

The Theia3D kinematic model, shown as 3D segments in the Theia3D viewer.

Inertial Properties

With each model, inertial properties are estimated using established regression equations similar to Dempster’s, which provide values for segment masses, centers of mass, and moments of inertia based on subject-specific anthropometric estimates and validated population data. Theia3D combines deep learning with inverse kinematics to estimate 3D body movement, using calculated segment positions, orientations to support further biomechanical analysis.

Coordinate System

The 4x4 pose matrices for each body segment convey the position and orientation of the segment’s local coordinate system, which are visualized with the x-axis as red, y-axis as green, and z-axis as blue. Local coordinate system origins are visualized as white spheres