Table of Contents
Motion Capture
Motion capture (informally “mo-cap” or “mocap”) involves the recording of movement from the real world. In a biomechanical context this recording is usually performed using a validated and calibrated measurement system. The movement in question is often human movement, but easily extends to animal movement (including horses, dogs, cats, and goats) and robots.
Types of Motion Capture
There are several different data collection modalities for recording and measuring human movement, each with their own strengths and weaknesses. This section provides a brief overview for these approaches but is in no way comprehensive. Users are highly recommended to conduct their own research to determine which motion capture approach is best suited to the questions they are trying to answer.
Marker-based Motion Capture
Marker-based motion capture systems can be further subdivided into passive and active systems.
Passive marker-based systems generally use retroflective markers placed on the body in accordance with a particular marker set. Cameras are set up to record the near infrared light reflected by these markers and estimate the 3-dimensional position of these markers throughout the movement. A model, essentially a mapping from markers to the skeleton, providing these marker positions with biomechanical meaning is defined by the user and this model is applied to the recorded 3D marker positions during dynamic movements of interest to produce pose estimates. There are a number of considerations to keep in mind when selecting a marker set and placing markers.
Active marker-based systems generally use markers that emit light, which are again placed on the body. Cameras record the light emitted by these markers and estimate their 3-dimensional position throughout the movement.
Markerless Motion Capture
Markerless motion capture systems generally apply deep learning techniques to estimate pose directly from video frames. The output of these systems is therefore generally rotation matrices that fully represent the pose (3D position and orientation) of the associated segment.
Some markerless motion capture systems incorporate a generative approach whereby a model is used to help infer pose from the system's measurements while other markerless motion capture systems rely solely on the measurements received.
Inertial Measurement Units
Inertial measurement units are strapped to participants or otherwise worn and combine accelerometers, gyroscopes and sometimes magnetometers. These signals are fused to estimate the pose of the associated segment.
Visual3D is able to process data from IMU systems that write out an MVNX file.
Force Platforms
Force platforms provide critical data for biomechanical analysis because they measure the interaction between the participant and the environment. Reaction force data measured by force platforms gives the calculated joint forces, moments and powers physical meaning.
Force platforms report ANALOG signals that are NOT the force signals of interest. These force signals must be computed from the reported analog signals using several parameters that specify the type of computations required. Different force platforms require different computations, which make this a non-trivial task.
Electromyography
Electromyography (EMG) signals are electrical signals generated by the activity of skeletal muscles when they respond to inputs from the central nervous system. When a muscle contracts, it generates electrical activity that can be measured using electrodes. The electrodes can be placed on the surface of the skin or they can be intramuscular (inserted under the skin with a narrow needle). EMG data is collected as a voltage difference between a recording site and a reference site. Typically the recording site will be the fleshy part of a muscle and the reference site would be near a bony part of the body.
Vendors
There are many different motion capture vendors who manufacture and sell the hardware necessary for the scientific measurement of movement. This section provides a brief overview of those vendors with whom HAS-Motion has established compatibility, it is not an exhaustive list. Users are highly recommended to conduct their own research to determine which motion capture system is appropriate for their intended uses.
As a general observation, Visual3D can open file that is stored in the C3D file format, which is the biomechanical community's open-source, common standard for storing 3-dimensional movement data. The C3D file format (along with a list of vendors which support the C3D file format) is described at C3D.org.
Below is a list of some commercial motion capture vendors that Visual3D can read data from. Some vendors can also stream real-time data into Visual3D for certain systems and configurations (designated RT).