Plask to AnyBody: Motion Analysis

This time, let's explore the method of conducting musculoskeletal mechanics analysis using Plask Motion's AI Motion Capture and Anybody. Anybody allows for ergonomic analysis using BVH Files extracted from Plask. Dive deeper into this topic through this article!

This article cites information from Terrabyte Co., Ltd. Terrabyte Co., Ltd is a consulting firm based in Tokyo, Japan, specializing in the sale of CAE (Computer Aided Engineering) software.

Workflow

1. Prepare a video (in mp4 format) recorded with a standard camera.
2. Import the video into Plask Motion and then export it in BVH format.
3. Apply the BVH format file in AnyBody's motion data usage workflow: AnyBody Workflow→AnyMocap

1. Prepare a Video Recorded with a Standard Camera (in mp4 format).

2. Import Video into Plask Motion and Export in BVH Format.

In Plask Motion, the extracted animation can be exported in BVH format.

3. Apply the BVH Format File in AnyBody's Motion Data Usage Workflow

【Example of Lumbar Load Analysis, Red Line is 3400N】 This motion was also recorded using optical motion capture with reflective markers simultaneously with video camera filming. Comparing the two, generally valid results (similar to optical MOCAP) were obtained.

MoCap vs Plask

We have now reached a point where the entire series of tasks can be completed in just a matter of minutes. Notably, where previously motion data usable in AnyBody (C3D format, or electrocardiogram format) required measurement, editing, and output processes using specific motion measurement devices, it's now possible to conduct musculoskeletal analysis from just a single video data file.

Furthermore, with the 'Plask' service, we were able to obtain 3D motion data in BVH file format. Typically, common AI posture estimation tools only identify the 3D coordinates of keypoints (joint centers), often resulting in significant length variations between keypoints, which can be problematic when applied to human characters. This requires appropriate correction when applying the data, which usually takes time. However, 3D motion data via BVH, with invariant skeletal lengths (Offsets), is very convenient for rigid link models like AnyBody.

Nevertheless, there are still limitations like the following, which were encountered this time and will need to be addressed in future iterations of such services:

• Since body parts are represented as a single 'rod', movements like internal rotation of shoulder joints or pronation/supination (rotation along the bone's axis) of elbow joints may not be well estimated. The same applies to left and right rotation of the neck. (While challenging by default, it seems accuracy can be improved by customizing the translation and rotation of the 24 keypoints via API in Plask).
• It's impossible to determine the specific body size of the test subject (Plask already has its own human manikin prepared. The size of certain body parts is preset, but it seems customizable via API).
• Drifts under the feet and in the pelvis are not completely resolved.
• In cases of videos shot from oblique angles, an unnatural direction of gravity (vertical direction) for the body may be estimated. (It seems that videos from the front, parallel to the ground, are required).
• Naturally, the external environment other than the human body (floor, walls, chairs, heavy objects, operating devices, etc.) does not exist. For mechanical analysis (force analysis), it is essential to properly reflect the interaction of forces from these elements, just like in reality.

About Anybody

Anybody is a musculoskeletal mechanics analysis software developed at Aalborg University in Denmark and is widely used globally. It calculates the forces acting on each part of the human body (muscle activity, muscle and antagonist muscle forces, elastic energy of tendons, joint forces and moments, etc.) when a motion is applied to a human musculoskeletal model using inverse dynamics analysis. Detailed human models based on specimen anatomy are provided.

In addition to model expansion/reduction scaling, analysis can be performed using bone geometry measured by MRI/CT. CAD data can be incorporated into models to evaluate the effects of industrial products, exercise equipment, prosthetics, and implants on the human body.

This software is highly flexible, supporting joint angle input and motion capture measurement data, and allows for the customization of output data.

Reference

• BVH Export in Plask Motion
• Anybody with Plask by Terrabyte Co., Ltd.
• Using BVH Files in Anybody
• Introduction to Anybody by Terrabyte Co., Ltd.