## 2 Materials and Methods
### Movit System  
Movit System G1 by Captiks is a wearable wireless system for motion capture and motion analysis. The system is composed of wearable small inertial devices (Movit) (Figure 2) and a receiver (Dongle). Each Movit communicates wirelessly with the Dongle, which is connected to the PC via a USB port. The Movit system exploits a proprietary communication protocol (2.4 GHz, based on IEEE 802.15.4 MAC) that allows synchronization and sending of data from multiple sensors (from 1 up to 16 devices) at a high sample rate (200 Hz in internal storage mode and up to 100 Hz in real time mode). Each Movit (dimension W 48 mm \(\times\) H 39 mm \(\times\) D 18 mm, weight 40 gr) hosts different sensors, in particular, a 3-axis accelerometer and a 3-axis gyroscope (\(\pm\)2 g to \(\pm\)16 g and \(\pm\)250 dps to \(\pm\)2000 dps, respectively, by MPU 9250 by InvenSense, San Jose, CA, USA). A 40 MHz clocked microcontroller (AT32 UC3A4256 by Atmel, San Jose, CA, USA) handles data that can be internally stored and/or transmitted wirelessly. A Li-Po battery supplies the Movit G1 with up to six hours of operational mode. Each Movit, in addition to the raw data of the accelerometer and gyroscope, provides its orientation in the space through a 6 DoF quaternion. Thanks to a patented two-step calibration, the orientation of each sensor can be mapped to the orientation of the body joints, to allow the reconstruction of kinematics and anatomical angles. The first calibration step consists of acquiring 3 positions obtained with 2 rotation movements of 90 deg (using the calibration base) for the definition of a unique reference coordinate system. The second calibration step consists of the acquisition of the T pose (standing position with arms parallel to the ground) to align the Movit System to the body coordinate system. The application of sensors on the patient's body is easy and immediate. The elastic bands are applied directly to the patient's clothes and are equipped with a coating that limits their movement. The sensors are positioned on the bands using plastic support (Dock) equipped with Velcro to make everything applicable on any physical conformation. The Movit System G1 was originally validated through a video based gold standard system [35] (Vicon, by Oxford Metrics). The testing was realized by measuring different subjects performing different motor exercises (walking and joint movements) and the results demonstrated excellent accuracy and repeatability. In particular, the RMSE errors for the joint angles in the sagittal plane are \(<\)3.5 deg, the Pearson correlation coefficients for hip, knee, and ankle in the sagittal plane are \(>\)0.97 and the percentage errors of spatio-temporal parameters in the gait test are \(<\)5%. The system is supplied with: (i) Motion Studio for recording session and system set up; (ii) Motion Analyzer for analyzing session and obtaining a pdf report related to the specific test. The Motion Studio allows for recording the movement from many points of view. The synchronization between raw data (accelerometer, gyroscope), joint angles, 3D animation, and video allows a study at different levels, making the analysis even more complete. It also gives the possibility to set up the transmission frequencies, the accelerometers, and gyroscope full scale range (from \(\pm\)2 g to \(\pm\)16 g and from \(\pm\)250 dps to \(\pm\)2000 dps, respectively), the recording mode (memory or real time mode) and the system calibration. Advantageously (especially in the considered application requiring the computation of temporal gait quantities), it is not mandatory to establish an exact measurement site for the Movit device, so they can be located anywhere but within the same body district. The most significant criterion for sensor position is the symmetricity between the left and right half of the body (i.e., the sensors positioned on the right arm and forearm should be on the same line as sensors on the left arm and forearm, and the same for sensors on the lower half of the body).