## 4 Results and Discussion
The system's flexibility in maintaining a single degree of freedom (DOF) measurement aligns with this study's focus on knee flexion and extension but also opens opportunities to expand the photonic system's DOF with additional FBGs for multi-joint monitoring. This study aimed to create a practical, high-resolution system for monitoring knee gait cycles. The results validate this objective by showcasing the following: (i) Practicality: The lightweight, wireless, and battery-operated sensor modules offer ease of placement and freedom of movement, emphasizing the system's practicality for end-users. (ii) High Resolution: The 10 p.m. sensitivity of the FBG sensor and the 1\({}^{\circ}\) angular accuracy of the inertial sensors deliver the precision needed for effective gait analysis. (iii) Ease of Use: Volunteers reported comfort during use, with no interference with natural movement, which is critical for both clinical assessments and dynamic sports environments. The results also support potential future enhancements, such as integrating the optical fiber sensor within textile materials to reduce movement artifacts at higher speeds. Additionally, refining the inertial sensor interface could further improve the accuracy of angle measurements, enhancing the system's applicability for monitoring multiple joints simultaneously. A comparative analysis of Figure 9 reveals that both the FBG and IMU signals follow a consistent waveform pattern across all subjects, with synchronized peaks and troughs corresponding to the stance and swing phases of the gait cycle. Although minor amplitude differences are observed, the temporal alignment between the two signals indicates a strong correlation in capturing joint motion. Based on the established calibration curve of approximately 10 p.m./\({}^{\circ}\) and the FBG sensor's 10 p.m. resolution, the angular deviation between the FBG and IMU signals is estimated to remain within \(\pm 1^{\circ}\), which is within clinically acceptable limits for gait monitoring. This confirms the hybrid system's capability to deliver coherent and redundant information under standardized walking conditions. While detailed point-by-point statistical regression was not included in this version, future studies will include quantitative analyses such as correlation coefficients, root mean square error (RMSE), and Bland-Altman plots to further validate the agreement between the two sensing modalities. Overall, the experimental data solidify the proposed system's role as a viable and innovative tool for body kinematics monitoring, as it is capable of bridging the gap in the current literature by offering a robust, high-resolution, and versatile solution for both clinical and sports applications. In comparison to traditional optical motion capture systems, which are often considered the gold standard for clinical gait analysis, the proposed hybrid system integratingFiber Bragg Grating (FBG) sensors and wireless inertial modules presents a compelling alternative for both clinical and field applications. Optical systems such as Vicon or Qualisys provide high-precision kinematic data but require expensive infrastructure, controlled environments, marker placements, and skilled technicians, which limits their portability and scalability in real-world settings [18; 45]. By contrast, the FBG-inertial hybrid system described in this paper offers a compact, wearable, and non-invasive solution that maintains high angular resolution (\(\pm 1^{\circ}\)) and strain sensitivity (10 p.m.) while enabling real-time monitoring during natural movement across diverse environments. The wireless modules have shown strong robustness against ambient electromagnetic interference from common sources such as Wi-Fi and Bluetooth, further enhancing their clinical utility. Additionally, unlike traditional systems constrained to lab settings, this wearable system facilitates longitudinal and home-based monitoring, which is essential for rehabilitation, sports medicine, and geriatric care.