Motion Capture Software and Systems for Gait Analysis and Biomedicine
- Sunday, 01 April 2007
Nowadays, there are many technologies available that are used to quantify the motion of humans and animals. However, recent innovations in optical motion capture systems have added unprecedented capabilities, allowing the same program to be used by clinicians to assess walking deficiencies, i.e., gait analysis, of their patients; by movie animators to quantify the subtle motions of groups of actors; by engineers to evaluate how workers interact with machinery; and by researchers to study the locomotion of animals and better design walking and flying robots.
Vicon’s motion capture systems use specialized software and proprietary high-speed and high-resolution imaging cameras to determine the three-dimensional coordinates of moving markers. These markers can be discs, hemispheres or spheres that are covered with retro-reflective coatings. Infrared LEDs ringing the cameras cause these markers to contrast with the background, thus permitting the system to detect their positions in real time. Typical applications encircle four or more cameras around the space where the action is to take place. From one to hundreds of retro-reflective markers are attached onto the subject at the locations of interest to be tracked, and their positions are computed to subpixel accuracy within each camera. The cameras then transmit data to the host computer via gigabit-Ethernet delivering real-time data access to data from all cameras. With these coordinates, triangulation algorithms compute the 3-D positions of every visible marker. Sophisticated template matching routines are then used to create frame-to-frame marker paths over time and solve occlusion problems.
Vicon’s Nexus software can work in real time, permitting immediate visualization of data, or in offline mode, recording the movement for later analysis. It has been designed to display data as they are collected: 3-D marker data, analog data, and reference video images, as well as the current system status. The operator can view reconstructed data in a 3-D workspace as the motion occurs, or view stored motions for review purposes. With Nexus, the operator configures separate windows to view video, Cartesian graphs, and animated figures in real time. Within this single software platform, slow or fast motions like head vibration, ingress-egress, airbag deployment, and seated operator task analysis can be examined live.
Nexus is designed to be straightforward to integrate into both large and small environments. The modular design of the components gives it a completely scaleable architecture, permitting more capabilities to be added as needed. For example, an optical capture system with three cameras has been used to track the limb movements of fetal rats to study the development of their neural connections and reflexes under the influence of various drugs. The rat pups are roughly the size of a small jellybean and the markers used are 0.5 mm in diameter. Conversely, engineers developing artificial limbs typically use eight cameras when analyzing walking gait, but will use up to 20 cameras for assessing the performance of their prostheses during cutting maneuvers or walking over irregular and inclined surfaces.