Motion capture recording process - by Barbara Malbrook
Coventry University Motion Laboratory
The laboratory has a performance area measuring 4m x 4m and includes a fully-sprung dance floor. A 12 camera Vicon MX40 3-dimensional optical tracking system was used for the motion data capture and analysis of the actors’ performance, together with twin AMTI forceplates to measure their ground reaction forces. The objective data obtained on the performers’ movements included positions of segments of the body, position of centre of mass, joint angles, weight distribution, linear and angular velocities and linear and angular accelerations. The motion data can be manipulated to be viewed from any angle and at any speed.
Prior to each data capture session the optical tracking system is calibrated and therefore cameras at known locations in the laboratory are used to triangulate to markers placed at anatomical landmarks on the body of the actor e.g. sternum, knees, elbows, head etc. The actor wears a special motion capture suit to which typically 41 markers are attached. The markers are ‘passive’ and therefore require no power supply which means the actors are not hindered by wires or other electronic paraphernalia. Each camera operates in the near-infrared part of the spectrum and only ‘sees’ the 9mm retro-reflective markers, not the actor. The capture rate of all markers is usually 250 times per second with an accuracy of less than 1mm within the capture volume. Once the motion has been captured each marker has to be identified to be able to post-process and reconstruct the whole trial.
Although the actors can see and feel their posture and movements, they cannot see the forces affecting their muscles. To understand these forces it is necessary to analyse the kinematics of movement, which provides a comprehensive description of the movement, and the kinetics which gives information on the forces acting on the body. Muscle modelling software has been developed in-house as an analytical tool to calculate internal muscle forces. The software can illustrate various properties of the muscle strength, force or activity throughout the performance and these are shown as muscle thickening (bulging) or colour change ranging from blue, through to green, yellow and red as the maximal. The quantitative muscle data can also output in graph format.
The muscle model is a whole body database of muscles and includes 666 muscle segments together with their individual force data. The database also includes the anatomical origins, insertion and wrapping points of each muscle. The software calculates the forces required throughout the whole body to generate specific movement. Further research is being undertaken to refine the layering of each muscle, especially over segmental joints.
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The Coventry lab |
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Actor and mocap suit |
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Calibration process |
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Mocap software |
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Muscle analysis graph |
Motion capture output examples
Skeletal body movement animation (wmv 12.6mb)
Muscle analysis animation (wmv 3.2mb)
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