# Validation of a Wearable IMU System for Gait Analysis: Protocol and Application to a New System
## 2 Materials and Methods
### Specifications of the New Wearable System and Its Data to Be Validated  
The experiment used a commercial wearable 3D accelerometer (Protheo of SXTsrl, Cinisello Balsamo, Milan, Italy), with the following technical specifications: 85 (I) \(\times\) 53 (w) \(\times\) 16 (h) mm of size, 70 g of weight, four digits LCD, on board ARM7 microprocessor, and raw acceleration sampling frequency of 128 Hz. This device is able to log tri-axial accelerations and ECG signals into the internal memory (up to three days of continuous monitoring), which can be downloaded at the end of the acquisition by Bluetooth(r) data transmission. Therefore, the data storage allows the recording of different tests in sequence. Integrity and quality of data have been examined only at the end of the test after downloading raw data and post processing. IMU calibration is performed off line. During this experiment, the system is worn at a sacrum position on the back of the subject, by means of an elastic belt with a pocket to fix the device firmly to the body and avoid artifacts in raw data originating from movements of the device with respect to the COM position. In the pocket of the belt, in addition to the device, during the subject setup, a user-customized paper or plastic element has been placed in order to align the device with the vertical axis along which the force of gravity acts. IMU alignment with the gravity direction is verified with a small bubble. A control and correction procedure with respect to the gravity base line is provided in the data analysis software. The subject removes the wearable device after all four trials of the validation test are completed. Data are downloaded via Bluetooth(r) from the wearable device to a PC for offline processing. Concerning the biomechanical approach, the wearable IMU-based system for kinematic gait analysis implements the pendulum and harmonic oscillator models [9; 10; 18; 29] with an original approach [23; 24]. When the subject walks with the rigid legs of length L and makes a step of a length S, the COM moves forward along a sinusoidal curve both in the vertical and in the medio-lateral planes [31; 32]. In this study, only the sagittal plane is considered (Figure 3). When the subject is stepping in double stance phase, the hip angle is \(\theta_{\text{max}}\), and COM descends from its highest point to the lowest one. During the step cycle, the vertical distance between the maximum and the minimum height of the COM position defines a distance: the maximum amplitude of oscillation hCOM. Each step (either left or right) is carried out following this pattern. The cycle is repeated at every step with its characteristics, (i.e., the pattern is very similar but not exactly the same). The use of a harmonic oscillator model allows us to define a correlation between the cycle of COM positions and the step cycle during walking. One harmonic oscillator is associated with each step cycle; thus, we look at the walking as a set of oscillators describing human kinematics. The oscillation period defines gait frequency and cadence.