## 2 Materials and Methods
### Data Collection  
Four young male able-bodied subjects with body mass indexes (BMI) of 18.026, 23.072, 24.509, and 29.761, respectively, participated in the walking experiment. The setup was as shown in Figure 3. Subjects who could walk normally were selected for the experiment and each had never suffered from any spasticity and foot symptoms. Table 1 shows the details of the subjects anthropometric. The experiment was done by having the subjects walked on a treadmill using the PICAFO as a test measurement device, as shown in Figure 4. There were three sensors on the test device, namely a force sensor, rotary encoder, and accelerometer. Data from the sensors were sent to a personal computer through the USB-6211 data acquisition for data logging. Note that there was no applied controller for the PICAFO stiffness; thus, it had zero stiffness for the whole data collection. For each subject, there were three sessions of walking. The treadmill applied different walking speed, for instance, speeds (Sp) of 1 km/h, 3 km/h, 5 km/h represent slow, medium, and fast walking velocity, respectively. In each session, data of 20 steps of walking were collected and repeated two times. Therefore, there were a total of 480 walking steps observations for analysis. As illustrated in Figure 4, the sensor placement and walking steps data collected from the sensors are the followings:  
1. Rotary encoder typed B1-106  
Data collected: ankle position (\(\emptyset\)), ankle angular velocity (\(\omega\)), ankle angular acceleration (\(\alpha\)). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Anthropometric Parameters**} & \multicolumn{3}{c}{**Subject, Sb**} & \multirow{2}{*}{**Mean**} & \multirow{2}{*}{**Standard Deviation**} \\ \cline{2-2} \cline{5-6}  & **1** & **2** & **3** & **4** & \\ \hline Body mass (kg) & 45 & 61.3 & 70 & 97.5 & 68.45 & 21.964 \\ Height (m) & 1.58 & 1.63 & 1.69 & 1.81 & 1.678 & 9.912 \\ BMI & 18.026 & 23.072 & 24.509 & 29.761 & 23.842 & 4.827 \\ Gender & Male & Male & Male & Male & - & - \\ Age (year) & 29 & 25 & 25 & 29 & 27 & 2.310 \\ Foot mass (kg) & 2.353 & 2.589 & 2.715 & 3.114 & 2.693 & 0.276 \\ Foot length (m) & 0.135 & 0.14 & 0.15 & 0.17 & 0.1488 & 0.0134 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Subject’s anthropometric. Figure 3: Experimental setup. Sensor placement: at the ankle joint right after the MR brake in the same axis. The standing position is the neutral position, dorsiflexion is a positive direction, and plantarflexion is a negative direction [17]. 2. 3-axis accelerometer ADXL-335  
Data collected: linear ankle acceleration (a). Sensor placement: near the ankle joint with x-axis on the horizontal sagittal plane and y-axis on the vertical sagittal plane. 3. Force sensor FLX-A201  
Data collected: ground reaction force on the heel (F\({}_{\text{heel}}\)) and toe (F\({}_{\text{toe}}\)). Sensor placement: inside the PICAFO insole with fix position such as 9 cm of toe distance and 6 cm of heel distance from the ankle joint axis in the sagittal plane. This research focused on the investigation of the ankle angular velocity and ankle torque. The ankle angular velocity could be measured directly from the rotary encoder. However, the ankle torque could not be measured directly. Therefore, the estimation of the ankle torque was conducted by using foot dynamic equations [35]. Free body diagram of a foot during walking is shown in Figure 5. Equations (1) and (2) described the net force equation on x-axis and y-axis. Equation (3) describes the net torque around the foot center of mass (CoM), in which Fa is the ankle force; Ma is the ankle torque; Ftoe is the GRF on toe; Fheel is the GRF on heel; m is the foot mass; I is the foot inertia around the center of gyration (CoG); R is the distance of the working force to the CoM, g is the gravity constant; a is a linear acceleration of the ankle; and \(\alpha\) is the angular acceleration of the ankle. The up, right, and counterclockwise (dorsiflexion) vectors are denoted with a positive (+ve) sign.