## 4 Discussion  
To understand complete knee loading during cycling, a comprehensive 3D model of the leg including ankle, knee, and hip joint was developed. To validate such a model, the first step had to be reliability analysis using data from the same trial. Reliability analysis showed high internal consistency as well as good ICC for the vast majority of parameters. Differences between two consecutive samples of the same cycling trial for each subject were within acceptable limits. Overall, the results showed that the model presented in this study can be used in further biomechanical research to understand overuse injuries. To the authors' best knowledge, previous studies performed only one trial and calculated average values and their variability merely for descriptive purposes, whilst reliability analysis remained unaccounted [11,12]. To date, the model made by [PERSON] and [PERSON] [12] is the closest to the presented model, but a different protocol was used to calculate the actual loads. They used seven reflective markers mounted on anatomical points and three pedal markers from which two of them were mounted only during static calibration to evaluate foot and calf kinematics parameters [12]. The model presented in this paper used four kinematics clusters plus three pedal markers to evaluate foot, calf, and thigh kinematics parameters. The presented study is not comparable with [PERSON] and [PERSON] [12] because different calculation protocols were used. The study presented by [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [18] showed that the coordinate reference system of the segment calculated using markers mounted on an anatomical point or kinematic cluster affected the calculated parameters due to soft tissue movement [18]. Another source of error in kinematics data could be during the calibration and markers' digitization. Some knee force and torque parameters had lower ICC and statistically significant differences between the two trials. These were closely related to the parallel differences in the recorded forces and torques at the pedal. We can only hypothesize that this could be due to the change in pedaling technique whilst sustaining the same power output. It has been previously demonstrated that forces and torques increase at the onset of fatigue [19]. Cyclists in this study ride at a steady-state pace for five minutes from which the two one-minute trials were recorded. Because ICC and Cronbach's alpha were negative for peak medial force at the knee and on the pedal, it was not possible to interpret these results. This meant that the scale was not reliable [20]. [PERSON] and [PERSON] (2003) compared the simplified spherical joint and revolute joint models [11]. The main difference was that the spherical joint model assumed that inertial properties around the x- and z-axes and shank medial/lateral acceleration were equal to zero. Furthermore, the revolution joint model assumed all simplified spherical model assumptions with the additional assumption that the knee y-axis had the same orientation as the laboratory coordinate system (GCS). The results showed differences between the two models. The main reasons for the differences between the models were the assumptions described above. They concluded that joint loads were best described using the spherical joint model. Such a model contributed to a better understanding of the overuse injuries [11,12]. The anatomy of lower limb and foot/pedal platform degrees of freedom could affect joint loads [11,21]. Furthermore, the marker setup was simple, while only kinematics clusters needed to be fixed on segment and visible to cameras. It is important to emphasize that the model presented in this paper was a step-by-step model by which the net joint forces and torques determined could not discriminate the individual contributions of the muscle forces and the passive joint reaction force (bone-to-bone contact force and force). In order to achieve this, more in-depth muscle modeling and adequate optimization techniques are needed.