## 2 Methods
### Photonic Component  
The system proposed in this paper is based on Fiber Bragg Grating (FBG) technology, which comprises a sensing module and a monitoring module. Figure 1 displays the photonic component of the system, which incorporates a light source and an interrogation system used for monitoring the received light. The FBG is embedded in a PVC foil within the knee band, which is visible in the photograph. The hardware and software components are used for interrogating and analyzing the light signals, which are then displayed on a computer screen. The FBG-based system provides an efficient means of measuring the mechanical parameters of human limb motion, enabling objective and reliable data for applications such as physical therapy and athletic assessments. This light source presents a FWHM (full width at half maximum, e.g., the bandwidth measured at 3 dB) of 60 nm, where a maximum ripple of 0.2 dB was able to produce a broadband light beam in the desired wavelength range. Moreover, this optical source is able to generate optical beams with an output power up to 8 mW. The FBG was inscribed in a hydrogen-loaded standard telecommunication fiber, specifically the Corning SMF-28e+ from Corning Incorporated, headquartered in Corning, New York, NY, USA, using a pulsed excimer laser from Coherent Inc., located in Santa Clara, CA, USA, and the phase mask technique. The resulting grating has a length of 8 mm and a resonance wavelength of 1547.76 nm, corresponding to a core refractive index modulation period in the half-micrometer range. The interrogation monitor allows real-time spectrum monitoring of FBG sensor interrogation systems and uses two transmission gratings to split spatially the wavelength spectrum as well as a parabolic mirror to focus each component to a specific pixel in the imager. The optically sensitive part of the imager is an Andor iDus InGaAs 1.7 detector (Andor Technology Ltd., Belfast, Northern Ireland, UK), operated with a region of interest (ROI) comprising 80 pixels. This configuration provides an intrinsic spectral resolution of 0.4 nm, distributed over the usable spectral range (e.g., 1529-1561 nm). The proposed application benefits from the features of the optical detector, including a dynamic range of 30 dB, a maximum wavelength drift of 2 p.m. per 1\({}^{\circ}\), an input optical power range of 10-50 dBm, and a power consumption of 250 mW. In addition, the manufacturer provides a software that allows for the real-time visualization of waveforms during sensor activation. This system has a spectral resolution of 10 p.m., which was measured in terms of \(-\)20 dBm input power and a low-loss optical circulator. An optical circulator  
Figure 1: Photonic component’s setup, with the knee band already connected. It is composed of a light source, an optical circulator, and an interrogation monitor hardware. was also used for separating the reflected light from the injected spectrum. The optical circulator operates in the 1525-1565 nm range, and it was selected as it allowed all detectable wavelengths to pass. Moreover, the optical device possesses a range of additional attributes, including a typical insertion loss of 0.7 dB (where the power loss remains below 15% of the total), a maximum polarization loss of 0.2 dB (equivalent to a maximum of 5%), a reverse-to-direct input isolation of at least 50 dB (with less than 0.01% of the optical power injected into the reverse input leaking into the direct one), and a maximum optical power capacity of 500 mW. Because this system has a high resolution, it allows for the addition of several more FBGs in the same fiber since all of them can be individually differentiated from one another even if they have a very close resonance wavelength. What limits the system's resolution is the dynamic range of each sensor so that no peak overlaps the adjacent one.