THE MOTION ANALYSIS SYSTEM AND THE MOTION AREA Normal values in young adults FINGERTIP H-Y. CHIU, F. C. SU and S-T. WANG From the Section of Plastic Surgery, Department of Surgery, Institute of Biomedical Engineering and the Department of Public Health, National Cheng-Kung University, Tainan, Taiwan, Republic of China The reliability of the motion analysis system and the normal value of the fingertip motion area have been studied in young adults. The results indicate that the motion analysis system is a reliable tool for the evaluation of fingertip motion. It was found that the fingertip motion area in young adults has a linear correlation with the square of the finger length. Therefore, the normal value of the fingertip motion area can be calculated from the finger length. Journal of Hand Surgery (British and European Volume, 1998) 23B." 1:53-56 Computer-aided motion analysis instrumentation has been designed to provide objective spatial and temporal assessment of the movement of body segments. It has been primarily used for gait analysis. A video-based motion analysis system has been used to evaluate the upper-extremity performance in athletes and musicians (An and Bejjani, 1990). Harding et al (1993) have also used optoelectric motion analysis to measure finger movement during piano playing. Recently, the motion analysis system has been used to evaluate fingertip motion in hand injury patients (Chiu and Su, 1996). By using this technique, the finger function of patients can be assessed along with coexisting dysfunctions and deformities. The curve derived from the motion analysis system and the area calculated from it can be compared in serial examinations. However, the fingertip motion area in normal individuals has not yet been described. Therefore, the present study was done to assess the normal value of the fingertip motion area measured by the motion analysis system in young adults. SUBJECTS AND METHODS Subjects who had congenital anomalies or scars caused by previous injury in the hand were excluded. Thirty-two young volunteers (18 women and 14 men) were involved in this study. Their ages ranged from 18 to 26 years, with a mean of 22.3 years. Ten volunteers (four women and six men) were involved in the first part of the study to assess the relia- bility of the measurement of the fingertip motion area by the motion analysis system. In this part of the study, the fingertip motion area of the left long finger of each volunteer was studied three times with a 1-week interval between each study. The remaining 22 volunteers (14 women and eight men) were studied to assess the normal value of fingertip motion area in young adults. Since the length of the finger influences the fingertip motion area, the length of each finger studied was measured before the motion analysis study. The finger length and the fingertip motion area were measured in all the fingers of both hands in these volunteers. The dynamic length of the finger should be measured to the base of the axis of rotation of the metacarpo- phalangeal joint. This measurement is difficult to do. In this study, the finger length was measured from a surface landmark. We marked the highest point of the knuckle of each finger when the hand was in the intrinsic-plus position with the fingers in the horizontal plane. The dis- tance between this point and the corresponding fingertip was measured to give the length of each finger. The motion areas of the fingertips were evaluated by the ExpertVision motion analysis system (Motion Analysis Corporation, California, USA). This system consists of six CCD video cameras, two VP 320 video processors, a Sun 4/110 workstation and an IBM com- patible personal computer which is capable of tracking the motion of reflecting markers in three dimensions with an accuracy within 0.1% error of the field of view. The calibration of the volume space in which the hand motion tasks were performed and the locations of the reflective markers on the hand were similar to those reported previously (Chiu and Su; )996). The subject was asked to adopt five postures when filmed by the video cameras (Chiu, 1995). Starting with all the finger joints in full extension, the subject was then instructed to make an intrinsic-plus posture, followed by flexion of the PIP joints while maintaining extension of the DIP joints. The subject was asked to make a fist, and then to extend the MP joints while maintaining flexion of the PIP and DIP joints (hook grip), and finally to return to the origi- nal posture. It took less than 5 seconds to complete such a cycle. The video system worked at 60 Hz during record- ing of the motion cycle and we recorded continuously for 10 seconds during each examination. The three-dimensional coordinates of the markers were registered and reconstructed using the motion analysis system software. Three markers on the dorsum of the hand were used to construct a local hand coordi- nate system. Through transformation between the laboratory coordinate system and the hand coordinate system, the marker on the fingertip was expressed in the hand coordinate system. Finally, a computer software system was developed for numerical integration to corn- 53