## 2 Data Acquisition
### Data Measurement  
To compute the ground-truth joint-angle values of the lower limb, seven individual Awinda sensors were mounted to the lower half of each subject's body. As depicted in Figure 1, one sensor unit per each lower-body bone segment was fixed. The bone segments include the pelvis, the thighs, the shanks, and the upper parts of the feet. To reduce the effect of skin motion artifacts, sensors are mounted in places with less skin movement.  
These include the pelvis bone at the height of the anterior superior iliac spine, the middle of the lateral thighs, the upper parts of the tibiae, and the front upper parts of the feet.  
Here the objective is to estimate the leg kinematics (joint angles, particularly) from any of the sensors fixed to the body as summarized in Figure 2. As the right leg is dominant for most people, the three sensors on the right leg in addition to the waist sensor were investigated and compared in this study. A study by [29] suggested that human locomotor muscle synergies are decoded from slow cortical waves of the brain. They claimed to have formulated a relationship between brain signals and leg kinematics. However, in this study, a noninvasive method with only a single sensor is used to mimic the function of spinal cord signals during locomotion. This is possible because the movement of our leg is manifested in our pelvis motion, presuming the subject always maintains contact with the ground. The pelvis moves forward/backward and sideways during normal walking. Due to maintaining continuous ground contact, the leg motion directly drives the trunk body depending on the speed and direction. This creates a repetitive rhythmic motion. This makes it easier to estimate the repetitive poses of the lower half of the body from various bone segments' inertial data. As an example, Figure 3, shows the inertial data of the pelvis for a single gait leg pose.  
After sensor synchronization, sensor calibration was performed before every experiment by orienting the sensors in one direction on a level surface. Next, sensors were carefully attached to subjects by Velcro tape straps in a similar direction as recommended by the manufacturer. Then, subjects were instructed so that they walk naturally, in any direction, by switching their paces to slow, normal, or fast at their convenience. Hence, diverse data were collected during our experimentation from the 16 subjects. The Awinda station, which is connected directly to an LG Gram 11 th Gen Intel(r) Core(tm) i7 computer, receives the synchronized data from the seven sensors via a wireless transmission. The Awinda station antenna supports wireless communication up to 50 m range in an outdoor area. This made the data-collection process a lot easier. The data collection was made at a sampling rate of 100 Hz for approximately 10 min per subject. Sixteen subjects comprised 13 males and 3 females; an age group of 28 \(\pm\) 7.2 years old; a weight group of 63.3 \(\pm\) 12.2 [Kg]; and a height group of 169.3 \(\pm\) 8.1 [cm]. In this study, the data were collected from walking activity only. The experiment was carried out in a level, open space field which does not have any structures that could pose magnetic interference to the sensor. A Google map of the experimental site is shown in Figure 4.  
Figure 1: A subject wearing seven Awinda sensors during data acquisition.  
Figure 3: Pelvis inertial data of a single gait cycle. Red (\(x\)-axis), green (\(y\)-axis), and blue (\(z\)-axis).  
Figure 2: Summarized diagram of the developed system. Adapted with permission from Ref. [23]. 2022, IEEE.