## 3 Measurements
### FBG Data  
Figure 9 shows the measurements of both systems (data already filtered) in all seven subjects. The sessions were performed with the volunteers walking on a treadmill at a constant speed in order to have a common comparison reference. Although the same treadmill speed allows for a meaningful comparison of kinetic parameters between sessions, deviations in normal gait dynamics, such as cadence, stride length, and knee flexion \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Volunteer** & **Height (cm)** & **Weight (kg)** & **Age (years)** \\ \hline A—Male & 177 & 77 & 35 \\ B—Female & 159 & 60 & 17 \\ C—Female & 158 & 55.5 & 64 \\ D—Female & 163 & 59.5 & 18 \\ E—Male & 185 & 90 & 24 \\ F—Male & 170 & 63.5 & 30 \\ G—Male & 180 & 73 & 58 \\ Mean & 170.29 & 68.36 & 35.14 \\ St. Dev. & 10.70 & 12.26 & 18.84 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Subjects’ physical information and statistical data. Figure 8: The figure shows the raw, unfiltered data measured by the FBG for two full gait cycles at three different speeds. The swing and stance phases of the gait cycle are indicated by the minimum and maximum deflections of the FBG, respectively, and the time between them. This data validates the consistency and reliability of the system [34]. angle [41, 43, 44], still occur. The experimental tests were conducted for 10 s, but only two complete gait cycles were considered to enhance the clarity of the unfiltered data. Figure 8 displays the raw waveforms acquired by the FBG sensor for three different walking speeds: 0.8 km/h (0.22 m/s), 4 km/h (1.11 m/s), and 8 km/h (2.22 m/s). The waveform sharpness increases with the increase in walking speed. However, the running movement is characterized by a quicker step, leading to waveform sharpness and slight kinematic variations, particularly when the front foot lands on the ground and propels the body forward. Nevertheless, the results facilitate the identification of various events that occur during the walking and running movements. To compare the measured period of the data with different steps in a complete cycle, Figure 8 presents the two extreme values acquired by the system during a full step cycle of 2.5 s at 4 km/h. The minimum deflection is observed when the leg is in the stance phase and the knee joint is flexed, corresponding to a value of 1547.76 nm (resonance wavelength). On the other hand, the maximum deflection is observed when the leg is in the swing phase, corresponding to a value of 1548.16 nm. The period between these two extremes of movement is characterized by low-amplitude variations that follow the knee movement when the leg is in contact with the floor. Notably, during the period between the minimum and maximum values, the leg remains in contact with the floor (stance phase), and the FBG sensor is substantially stretched, with minor variations attributable to the elastic knee band. This relatively constant period is highlighted in red in Figure 8. The right leg begins its movement backward, with the ankle touching the floor first, followed by the base of the foot until it starts moving forward with the tip of the toes (the maximum deflection in the figure represents approximately 80% of the gait period) [42]. The FBG sensor system allows for the detection of various movements, such as flexion and extension, which are associated with a complete step, thereby enabling the comparison of results acquired in various scenarios involving different subjects. The periodic nature of the signals and their correlation with gait cycles are easily recognizable and can be  
Figure 9: Demonstration of the FBG wavelength shift signals recorded from seven different subjects. identified in the measured waveforms. The system presented for the knee can be applied to other joints in the human body, with slight modifications required to measure joints with multiple degrees of freedom (DOFs).