## 4 Discussion
### Position Problem  
The results of the position problem were analysed from two different approaches. First, they were analysed from a physiological point of view by checking the physical meaning of the results and comparing them with results published in the literature obtained undersimilar test conditions. Second, a discussion of the intra-subject and inter-subject variability in the results obtained was carried out to determine the robustness of the PP. The pelvis' movements as a rigid solid (Figure 3) show small rotations in the sagittal and frontal planes and larger rotations in the transverse plane, a behaviour in accordance with that shown by other authors in their works [8; 15]. Similarly, the displacements of the CoM of the pelvis show an oscillatory behaviour around the resting position on the saddle. In particular, the displacements in the AP and CC directions show an oscillation frequency twice that of the displacement in the LM direction. Thus, the pelvis moves to the left during the first phase of the cycle and to the right in the second phase. The pelvis CoM performs a complete cycle of displacement in the AP and CC direction in each half of the cycle because of the extension and flexion mechanism performed in each phase of the pedalling cycle. In general, the movements performed by the pelvis are mainly due to compensatory mechanisms of the body to perform the technical gesture of pedalling. These mechanisms generally tend to be magnified in amateur cyclists and even in professional cyclists because of fatigue [6]. The knee's flexion-extension movement (Figure 4) presents the greatest ROM during the entire pedalling cycle, approximately 75\({}^{\circ}\). This value is similar to that found in the literature [5; 11; 15; 17]. The TE of this movement shows how an extension of the knee occurs from the beginning of the pedalling cycle (0\({}^{\circ}\)) to approximately 160\({}^{\circ}\), in what is known as the power phase [28]. The maximum knee extension occurs just before bottom dead centre (BDC), in concordance with some papers in the literature [5; 9; 10]. Once the power phase is overcome, the pedal recovery phase begins (180-360\({}^{\circ}\)). In this phase, maximum knee flexion occurs when the crank angle is approximately 345\({}^{\circ}\), a value in agreement with the literature [5; 9; 10; 15]. Knee movement in the frontal plane shows an ROM close to 10\({}^{\circ}\), in line with that shown by other works [5; 11; 12; 13]. This movement TE shows that positive values are hardly achieved; so, participants tend to spend the whole pedalling cycle in knee abduction. The abduction presents two maximums: the first with a value of 5\({}^{\circ}\) when the crank is positioned at 90\({}^{\circ}\), and the second approximately 13\({}^{\circ}\) when the crank is close to 270\({}^{\circ}\). This behaviour coincides with that shown by other authors in their work [3; 9; 15; 16]. Between both maximums, abduction presents a valley with a value close to 0\({}^{\circ}\) at the instant when the crank forms 170\({}^{\circ}\) with the vertical, coinciding with the moment of least knee flexion. This fact is due to the internal and external collateral ligaments, which are responsible for stability in the lateral-medial direction of the knee, decreasing the capacity of movement in the frontal plane when the knee is more extended due to the tension of them. In addition to these ligaments, the sartorius, semitendinosus, rectus femoris and tensor fasciae latae muscles act as active ligaments contributing to this lateral-medial stability. The tensor fasciae latae causes an external rotation of the knee when it is extended, as can be seen in Figure 4, where there is a smooth decrease in internal rotation that disappears when knee flexion is initiated again. The instants in the pedalling cycle when the maximum internal rotation of the knee occurs coincide with the moments when the highest values of hip adduction are observed--specifically, when the knee is nearest the midline of the body.