## 4 Discussion  
### Proposed Protocol Evaluation  
Table 1 shows that high CCC values are obtained when comparing the PP with the PiG and with the CMP in the joint variables contained in the sagittal plane. These results highlight the interest of this work since it suggests that, in cycling, similar results to those provided by protocols accepted by the scientific community can be obtained in the sagittal plane, but with a more stable and comfortable placement of markers for the participant and a simpler procedure.  
Outside the sagittal plane, the PP shows a high correlation with the PiG in the hip and ankle with CCC values above 90% for all joint variables. The exception was hip internal rotation where it shows a medium correlation value (53%) and a gait cycle lag of 20%. These results are slightly worse when correlating the PP with PCM for hip variables and worse for ankle variables. However, this difference in results is also apparent when analysing the CCC between the PiG and the CMP. The reduced CCC values observed for hip internal rotation may be attributed to several factors. One such factor, previously discussed in this paper, is the fundamental difference between the protocols: the PP employs a non-optimal pose estimation approach, whereas the CMP utilises a Least-Squares Pose Estimation method. As a result, the PP is highly sensitive to marker placement, and even minor discrepancies in marker positioning can lead to noticeable variations in the calculated kinematics. Conversely, the CMP is more robust to such misplacements due to its reliance on a greater number of markers. However, this increase in marker count may compromise participant comfort during testing. Another consideration, particularly relevant when comparing internal rotation values between the PP and PiG, is the inherently low ROM of this movement, typically around 10\({}^{\circ}\). Such a limited ROM increases the relative impact of minor marker misalignments, potentially affecting both the magnitude and sequence of the observed motion. This underscores the importance of meticulous marker placement when using non-optimal estimation protocols. To mitigate variability introduced by inconsistent marker placement, it is recommended that the same operator apply the markers across all participants. In this work, this methodology was adhered to throughout the experimental protocol. On the other hand, the ankle-foot complex is, along with the shoulder complex, one of the most difficult to analyse accurately in the human body. The results provided by different protocols for the same movement provide a wide dispersion, as shown by [PERSON] et al. [42]. However, this limitation is less pronounced in the present study since the pedalling analysis used shoes with cleats, which considerably reduced the possible movements of the foot-ankle outside the sagittal plane.  
Regarding knee joint variables outside the sagittal plane, it is observed that the PP correlates better with CMP in the frontal plane than with the PiG. This was to be expected since one of the limitations of the PiG is the estimation of knee joint variables outside the frontal plane if devices such as the Knee Alignment Device [43] (KAD) are not used. The KAD is a device which is composed of three reflective markers that form a directional trihedron which is placed on the knee of the participant to be tested. The objective of using the KAD is to assist the PiG in the automatic definition of the direction of the knee flexion axis during a static trial. The correlations of internal rotation of the knee present average values in all three comparisons. Again, this is to be expected due to the reduced ROM in the transverse plane [42].