Page 95 - Handbook of Biomechatronics

90 Jeff Christenson

6.6 IR Body Markers and Camera Tracking Three-Dimensional
Motion Capture

When discussing EMG and muscle movements, the focus was on capturing
one muscle group. However, the body is composed of many muscle groups
functioning in coordination. In order to capture entire body movements,
three-dimensional (3D) motion capture is used.
A motion capture system consists of a minimum of four IR cameras (gen-
erally six) mounted such that they view the laboratory observation area from
multiple angles (Fig. 32). The subject is instrumented with IR reflection
balls, generally about 25in. in diameter. These balls are placed on the subject
over critical joints or limbs such as the base of the neck, shoulder blades,
elbow, wrist, hand, hip, knee, and ankle (Lupu and Ungureanu, 2013).
When the subject is instrumented, a calibration routine is performed to
map the joint space and create a model in the software. After calibration, the
combined data from the IR cameras give precise positioning the body.
Using joint-space algorithms, Newtonian physical effects can be calculated,
such as joint accelerations and torques (https://www.vicon.com/what-is-
motion-capture, Accessed 23 August 2017).
Motion capture systems are often combined with force plates to measure
ground reaction forces or treadmills to observe motions within the 3D
motion capture space.
An alternative to the IR cameras and indicators is to use a single IR cam-
era with no joint markers, such as is used with the Microsoft Kinect motion
capture system. This method is not as precise since the joint positions are

Fig. 32 Graphical representation of a motion sensor lab. The four infrared cameras are
positioned to capture the motion in the center of the lab. Infrared markers are placed on
the subject for the cameras to track.