Essentially, and at least to a first approximation, the spike moves in a continuous manner with no preferential states, as clearly shown in the scatter plots in Fig. 3 ▸(b). In this way, the result of a particular instance of image-processing analysis, including a 3D classification, should be regarded as a snapshot of this quasi-continuum of states. In our case, we have shown that a particular meta image-classification approach, implemented through a consensus among different methods in many steps of the analysis, results in classes that are at the extreme of the main axis of variance in PC space. Clearly, PC1, through the analysis of the two extreme classes, reflects a concerted motion of the NTD–RBD–SD1–SD2 thumb, although there are smaller collective movements throughout the spike (see Fig. 2 ▸ and Supplementary Movie S1). In this case, the RBD moves together with the NTD, with a smaller degree of independent flexibility and always in the ‘up’ conformation. The NTD–RBD movement can be characterized to a large degree as a rotation, but the different RBDs present a much more complex pattern of flexibility, indicating an important structural rearrangement [from elastic analysis (Fig. 2 ▸) and PCA (Fig. 3 ▸)]. The presence of quasi-solid body rotation hinges is clearly located between amino acids 318–326 and 588–595, which produce most of the displacement, together with other hinges between amino acids 330–335 and 527–531, which accompany a less pronounced ‘up’ movement of the RBD.