Inertial Sensor based Identification of Human Movements

The scope of this paper is the presentation of experiments which involve measurements and identification of human movements by using the inertial sensors. We describe the purpose, design and obtained results of two experiments, as well as our future plans which include the exploration of the forces acting at spine segments by measurements with inertial sensors. The first experiment implemented the method for measuring the range of motion (RoM) of head in transverse plane (Kuzmanic, 2007). It was done in the Laboratory of Biomechanics and Automatic Control – LaBACS, University of Split. In the second experiment we analyzed the standing – up movement and we used the robot assistive device for the support of human while performing the standing – up task. Measurements for purposes of this experiment were done in the Laboratory of Biomedical Engineering and Robotics, University of Ljubljana. We have proposed the new method which uses the Extended Kalman filtering for combining the data acquired from inertial sensor measurements of standing – up movement with data from the dynamic human body model (Music, 2008). Our plans regarding the next experiment are focused on the identification of the spinal load during sitting and standing, by using the inertial sensors measurement system. 1 MEASUREMENT OF THE HEAD’S RANGE OF MOTION The measurement of range of motion (RoM) and static posture of the head gives important physical parameters for clinical assessment and diagnosis related to cervical spine functions. In literature, this movement is referred as a cervical range of motion. The detection of an abnormal RoM or asymmetrical patterns is an essential for preventing cervical dysfunction (McAviney, 2005; Wu, 2007). One of our aims was to investigate the feasibility of the use of inertial sensors in routine clinical assessments. Therefore, our goal was to design the system based on inertial sensors and to propose the method for measuring the range of motion (RoM) of head in transverse plane. The measurement was performed using single inertial measurement unit MTx XSens sensor (XSens Motion Technologies, Netherlands), Fig. 1. Specialized software for sensor data acquisition, with high visualization abilities has been developed in LaBACAS. MTx XSens sensor can provide useful, noninvasive measurement of head motion in three cardinal planes for the fast evaluation of disturbances related to head/neck problems and cervical dysfunctions. The advantage of the proposed method over standard methods is the ability to measure unilateral RoM of the head. This technique overcomes the limits of ‘gold standard’ measurement devices by estimating the neutral position, which is assumed to be a nontrivial problem in standard RoM measurement. In addition, a proposal for use of sensor for visual feedback RoM assessment is presented. LaBACS MTx Software was developed by in-house, to control the operation of the inertial measurement unit (IMU), acquire the data and display them in the real time. Program was developed under Microsoft Visual Studio 2005, using MFC (Microsoft Foundation Class). Fig. 2. shows a frame of running software. Measurement was done on 6 subjects without any known symptoms of cervical dysfunction. Five repetitions of movements were analyzed and averaged for each subject in order to eliminate the variability during movement recording. In accordance with standard, total RoM is calculated by subtraction of maximal and minimal angle, or by summation of left and right RoM, assuming that the neutral position angle is known. During the measurement neutral position is identified statistically, over time interval of five repetitions of cyclic RoM movement, Fig. 3 b). Figure 1: Measurement setup: Subject with sensor mounted on a cap Figure 2: User interface of LaBACS MTx Software 1.1 Results of the first experiment The results of the measurement on 6 asymptomatic subjects are given in Table 1. Resulting angles of each group are described in terms of mean RoM ± standard deviation [ o ], for the movement on the left (LRoM) and right side (RRoM). The results of the present study demonstrate similar ranges of motion as found in literature (Dvir, 2000), although the existing results are obtained with different instrumentation. Measurement of individual neutral position has a standard deviation ranging from minimally ±1.12 o to maximally ±3.36 o . These results imply that the subjects are able to return the head to a self-defined neutral position. Therefore, the measurement method of head motion based on inertial sensors is valid for current application of RoM in transverse plane and is suitable for measurement of head neutral position, as well. Table 1: RoM results for 6 asymptomatic subjects; left side: LRoM right side: RRoM RoM (total) LRoM/ RRoM 1) 2) 73.02 ± 7.61 74.34 ± 9.44 147.28 ± 15.51 147.36 ± 15.54 1.022 ± 0.096 1) αL – αR where αL is maximal and αR is minimal head angle; 2) LRoM + RRoM