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Gait Analysis Based on Inertial and Magnetic Sensors

Motion analysis often is based on stereophotogrammetry, which estimates the three-dimensional coordinates of points on an object by measurements made in two or more photographic images taken from different positions. While it provides very useful and detailed information about walking, the technique’s full capability of quantitative analysis of movement often is not used.

altThe first reason for this is the cost of instrumentation. Second, the use of instrumentation based on stereophotogrammetry requires measurements to be carried out in a laboratory. This presents another set of problems. The typical dimension of a motion analysis laboratory is 10 meters, meaning that only two gait strides can be measured entirely. It is therefore not clear whether the measurement of such a short path can be representative of actual walking.

Due to the long subject preparation for the measurement (at least 45 minutes) and the fact that the patient should cover the same 10 meters several times in a row, the overall examination becomes tiring: that restricts measurement to no more than 10 walking trials.

The measurement is performed in an “artificial” environment, where the lack of harshness in the floor and the nonslip surface make it far from being a true urban or rural environment. In this laboratory environment, skin markers positioned on the skin and the presence of technical personnel create psychological pressure on the subject, who may find it difficult to use their typical walking pattern.

Improving Motion Analysis

For the above reasons, INAIL Prostheses Centre in Rome, Italy, invested in research and development to move motion analysis to the same place in which prosthetic devices are being designed, or where rehabilitation takes place. In this way, motion analysis can be performed in outdoor environments covering long distances in order to capture as closely as possible the typical walking pattern of the patient.

One of the achievements resulting from this research is the Outwalk Protocol, which was developed to easily measure the thorax-pelvis and lower-limb 3D kinematics on children with cerebral palsy (CP) and amputees while walking in free-living conditions by means of an Inertial and Magnetic Measurement System (IMMS). Outwalk defines the anatomical/functional coordinate systems (CS) for each body segment through three steps: (1) positioning the sensing units of the IMMS on the subjects’ thorax, pelvis, thighs, calves, and feet, following simple rules; (2) computing the orientation of the mean flexion-extension axis of the knees; and (3) measuring the sensing units’ orientation while the subject’s body is oriented in a predefined posture, either upright or supine. If the supine posture is chosen, e.g. when spasticity does not allow the subject to maintain the upright posture, the hips’ and knees’ static flexion angles must be measured through a standard goniometer and input into the equations that define Outwalk anatomical coordinate systems.

One of the fundamental aspects for the complete description of the amputees’ movement is the knowledge about the segment and joint kinematics that describe how the trunk, pelvis, hip, knee, and ankle move during walking, and their mutual coordination. In order to obtain the most representative data, the motion analysis should take place in a real environment, measuring hundreds of gait cycles in one measurement. Furthermore, the system should be able to provide realtime data, generate a final report with “three clicks,” and require a minimum period for the subject preparation.

Currently, the above requirements cannot be satisfied even with advanced and expensive commercial systems. However, this “mirage of motion analysis” appears to be attainable, thanks to the combination of a new system developed by Xsens Technologies (The Netherlands) used in the entertainment, industrial, training and simulation, and movement science fields, and algorithms and software developed at INAIL Prostheses Centre.

altThe system comprises inertial and magnetic sensors contained in a small, lightweight box that can be positioned easily on the body of the patient. Based on such a system, INAIL created software called Outwalk Manager to measure the real-time kinematics of the trunk, pelvis, hip, knee, and ankle, according to all the requirements previously mentioned.

Once the sensing units are positioned on the body, and after a few simple instructions, the patient is asked to maintain a particular upright posture for a few seconds; then, the patient is asked to perform a flexion-extension of the right knee, then of the left knee. If the patient is not able to maintain an upright posture, the system calibration can be performed in a supine position, and the flexion-extension movements of the knee can be performed passively by the operator. Once the calibration steps are completed, the system is ready to measure the subject walking. Using a Bluetooth transmission system, the operator can be up to 150 meters from the patient and observe real-time data about the mobility of the body segments and joints on a laptop computer.

After the measurement of the walking trial, an algorithm developed at the INAIL Prostheses Centre is able to recognize and segment all gait cycles available among the joint kinematics data. Additionally, it is immediately possible to compare different measurements obtained for the same patient, in order to quantitatively evaluate the improvements along the rehabilitation treatment. Finally, part of the kinematic data can be encoded into audio signals, obtaining a biofeedback system to support the rehabilitation process.

This article was contributed by Xsens Technologies. For more information, visit http://info.hotims.com/45600-321.