Measurement of instantaneous center of rotation and its application to the analysis of joint motion

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1 Development of an algorithm for ICR calculation The instantaneous center of rotation (ICR) can be calculated using following quantities : position of one point on a moving segment and angle of the segment. So far only quantities at two consecutive times were used to calculate ICR. Therefore ICR data have been less reliable. In this investigation, we proposed new algorithm for ICR calculation based on smoothing technique. That is, to determine velocity and angular velocity of a segment, we adopted numerical low-pass differentiation. Then we analyzed error of this method and discussed optimal sampling interval of position data. We compared this method with other methods by computer simulation and verified high accuracy of our method.2 ICR measurement system We made an ICR measurement system composed of a position sensor and a personal computer. The error of this system was within +3mm.3 Application to the tempolomandibular joint motion We applied this measurement system to temporomandibular joint motion. Loci of ICR at habitual opening and closing were measured. These loci were located under the condyle. According to the relationship between location of ICR and sliding at joint surface, it is shown that sliding is inevitable at the tempolomandibular joint. Moment arms of masticatory muscles around the ICR increased as two times of those around a fixed point, the condyle.