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MOTION ANALYSIS OF THE UPPER-LIMB BASED ON INERTIAL SENSORS: PART 3 - ASSESSMENT OF INSTRUMENTAL ACCURACY FOR A NEW PROTOCOL
Poster Session 1/Human Motion. 14:45-15:45, Room 103 & Alley Area,
Poster 66
S545
MOTION ANALYSIS OF THE UPPER-LIMB BASED ON INERTIAL SENSORS: PART 3 – ASSESSMENT OF INSTRUMENTAL ACCURACY FOR A NEW PROTOCOL A.G. Cutti1, A. Giovanardi1,2, P. Garofalo1,2, L. Rocchi2, A. Davalli1 INAIL Prosthesis Centre, Vigorso di Budrio, Italy; email: [email protected] 2 DEIS, University of Bologna, Italy
1
INTRODUCTION Inertial Sensors Systems (ISS) are a new generation of low cost, portable, fully wearable motion analysis systems recently become available on the market [1]. ISS sensing units (SU) are lightweight boxes integrating accelerometers, gyroscopes and magnetometers. For each SU a local system of reference (SoR) is defined, and its relative orientation with respect to a global, earth-based SoR is computed by the ISS in real-time. Based on this property, in [2] a protocol has been presented which allows the estimation of elbow, humero-thoracic and scapulo-thoracic 3D kinematics. In [3] it was proved that the protocol, once used in combination with the Xsens MT9B ISS (Xsens Technologies, NL), is based on bone-embedded SoRs representative of the joints functional anatomy. As a further step, the aim of this third abstract was to assess the limits of the Xsens dynamic accuracy during the execution of the protocol [2]. It was proved in fact that the Xsens accuracy is sensitive to direction and velocity of rotation [4] and that it generally depends on the type of motion (Xsens Technical Manual). METHODS For the assessment, we compared the joint kinematics measured by the MT9B system for a healthy subject with that measured by an optoelectronic system (Vicon 460, Oxford Metrics, UK), assumed as the gold-standard. To make the comparison possible, the subject was acquired at the same time with both systems, applying for both systems the protocol described in [2]: in the Vicon system, each MT9B SU was simulated by a cluster of 4 markers, rigidly attached to the SU itself. In order to restrict the source of differences between correspondent joint angles to instrumental errors of the MT9B, a hand-eye calibration [5] was applied, before the experimental session, between each cluster and the correspondent SU. The hand-eye calibration allowed, in fact, to relate the technical SoR of the clusters to the SU local SoR. The subject was asked to perform 5 times all the 7 single-joint, single-degree-of-freedom tasks described in [3], a shoulder internal-external rotation with the arm abducted 90°, a hand-to-nape task in the sagittal plane and a hand-to-top-of-head task in the frontal plane. In each task, the movement was cyclically repeated 5 times. After synchronization, correspondent joint angles patterns measured by MT9B and Vicon were compared by computing the RMS error, the correlation coefficient (r) and the angular coefficient of the regression line (m). Since RMS, r and m do not have a normal distribution, their median values and quartiles were computed for the entire set of data. RESULTS AND DISCUSSION From the experimental session, 450 couples of correspondent angles patterns (as measured by MT9B and Vicon) were obtained. After excluding 24 of them due to recording problems, 426 RMS, r and m values were obtained (Figure 1). For the 99% of patterns couples the RMS ranged between XXI ISB Congress, Poster Sessions, Wednesday 4 July 2007
Figure 1: Box-plots of RMS, r and m, for the whole set of data and for the data in the low-RoM and high-RoM groups. 0.2° and 3.2°; 98% of the couples had r above 0.85 and for 97% of couples m ranged between 0.77 and 1.16. These results prove the considerable accuracy of the system as well as its ability to generate angles patterns highly correlated and of close magnitude with the gold-standard. A visual analysis of r data showed that angles patterns featuring a limited range of motion (RoM) were characterized by a lower correlation. For this reason a threshold was defined to discriminate between low-RoM and high-RoM patterns. For elbow and shoulder the threshold was fixed to 24° while for the scapula it was fixed to 12° to take into account the joints different maximum RoM. Based on the threshold, RMS, r and m data were divided into the low-RoM (247 samples) and the high-RoM groups (179 samples), as shown in Figure 1. The 87% of high-RoM patterns couples were featured by r always higher than 0.99 and m higher than 0.93. Given the division of the box-plots notches, low- and high-RoM groups appear representative of distinctive behaviour of the Xsens system, thus proving that the RoM is a significant parameter affecting the system performance. Further analyses are however required to assess the interaction level between RoM and velocity on Xsens performances. CONCLUSIONS The results prove that the Xsens system is generally accurate for the protocol [2] and able to provide patterns highly correlated and of equal magnitude with the gold-standard. Range of motion, velocity and axis of rotation [4] appear to be factors influencing the Xsens overall performance. REFERENCES 1. Begg R., et al. Computational Intelligence for Movement Sciences, IGP, 2006. 2. Cutti AG, et al. ‘Part 1’ Proc. ISB 2007, submitted. 3. Cutti AG, et al. ‘Part 2’ Proc. ISB 2007, submitted. 4. Cutti AG, et al. Proc. IEEE EMBS, 5912-5915, 2006. 5. Park F, et al. IEEE T Robotic Autom, 10, 717-721, 1994. Journal of Biomechanics 40(S2)
Poster 66
S545
MOTION ANALYSIS OF THE UPPER-LIMB BASED ON INERTIAL SENSORS: PART 3 – ASSESSMENT OF INSTRUMENTAL ACCURACY FOR A NEW PROTOCOL A.G. Cutti1, A. Giovanardi1,2, P. Garofalo1,2, L. Rocchi2, A. Davalli1 INAIL Prosthesis Centre, Vigorso di Budrio, Italy; email: [email protected] 2 DEIS, University of Bologna, Italy
1
INTRODUCTION Inertial Sensors Systems (ISS) are a new generation of low cost, portable, fully wearable motion analysis systems recently become available on the market [1]. ISS sensing units (SU) are lightweight boxes integrating accelerometers, gyroscopes and magnetometers. For each SU a local system of reference (SoR) is defined, and its relative orientation with respect to a global, earth-based SoR is computed by the ISS in real-time. Based on this property, in [2] a protocol has been presented which allows the estimation of elbow, humero-thoracic and scapulo-thoracic 3D kinematics. In [3] it was proved that the protocol, once used in combination with the Xsens MT9B ISS (Xsens Technologies, NL), is based on bone-embedded SoRs representative of the joints functional anatomy. As a further step, the aim of this third abstract was to assess the limits of the Xsens dynamic accuracy during the execution of the protocol [2]. It was proved in fact that the Xsens accuracy is sensitive to direction and velocity of rotation [4] and that it generally depends on the type of motion (Xsens Technical Manual). METHODS For the assessment, we compared the joint kinematics measured by the MT9B system for a healthy subject with that measured by an optoelectronic system (Vicon 460, Oxford Metrics, UK), assumed as the gold-standard. To make the comparison possible, the subject was acquired at the same time with both systems, applying for both systems the protocol described in [2]: in the Vicon system, each MT9B SU was simulated by a cluster of 4 markers, rigidly attached to the SU itself. In order to restrict the source of differences between correspondent joint angles to instrumental errors of the MT9B, a hand-eye calibration [5] was applied, before the experimental session, between each cluster and the correspondent SU. The hand-eye calibration allowed, in fact, to relate the technical SoR of the clusters to the SU local SoR. The subject was asked to perform 5 times all the 7 single-joint, single-degree-of-freedom tasks described in [3], a shoulder internal-external rotation with the arm abducted 90°, a hand-to-nape task in the sagittal plane and a hand-to-top-of-head task in the frontal plane. In each task, the movement was cyclically repeated 5 times. After synchronization, correspondent joint angles patterns measured by MT9B and Vicon were compared by computing the RMS error, the correlation coefficient (r) and the angular coefficient of the regression line (m). Since RMS, r and m do not have a normal distribution, their median values and quartiles were computed for the entire set of data. RESULTS AND DISCUSSION From the experimental session, 450 couples of correspondent angles patterns (as measured by MT9B and Vicon) were obtained. After excluding 24 of them due to recording problems, 426 RMS, r and m values were obtained (Figure 1). For the 99% of patterns couples the RMS ranged between XXI ISB Congress, Poster Sessions, Wednesday 4 July 2007
Figure 1: Box-plots of RMS, r and m, for the whole set of data and for the data in the low-RoM and high-RoM groups. 0.2° and 3.2°; 98% of the couples had r above 0.85 and for 97% of couples m ranged between 0.77 and 1.16. These results prove the considerable accuracy of the system as well as its ability to generate angles patterns highly correlated and of close magnitude with the gold-standard. A visual analysis of r data showed that angles patterns featuring a limited range of motion (RoM) were characterized by a lower correlation. For this reason a threshold was defined to discriminate between low-RoM and high-RoM patterns. For elbow and shoulder the threshold was fixed to 24° while for the scapula it was fixed to 12° to take into account the joints different maximum RoM. Based on the threshold, RMS, r and m data were divided into the low-RoM (247 samples) and the high-RoM groups (179 samples), as shown in Figure 1. The 87% of high-RoM patterns couples were featured by r always higher than 0.99 and m higher than 0.93. Given the division of the box-plots notches, low- and high-RoM groups appear representative of distinctive behaviour of the Xsens system, thus proving that the RoM is a significant parameter affecting the system performance. Further analyses are however required to assess the interaction level between RoM and velocity on Xsens performances. CONCLUSIONS The results prove that the Xsens system is generally accurate for the protocol [2] and able to provide patterns highly correlated and of equal magnitude with the gold-standard. Range of motion, velocity and axis of rotation [4] appear to be factors influencing the Xsens overall performance. REFERENCES 1. Begg R., et al. Computational Intelligence for Movement Sciences, IGP, 2006. 2. Cutti AG, et al. ‘Part 1’ Proc. ISB 2007, submitted. 3. Cutti AG, et al. ‘Part 2’ Proc. ISB 2007, submitted. 4. Cutti AG, et al. Proc. IEEE EMBS, 5912-5915, 2006. 5. Park F, et al. IEEE T Robotic Autom, 10, 717-721, 1994. Journal of Biomechanics 40(S2)