Cells have a protein-based ''skeleton" known as the cytoskeleton, which generates forces that allow them to do various things, in particular to change shape in order to move or divide. The cytoskeleton is formed of three main fibers: microtubules, intermediate filaments and microfilaments (otherwise known as actin filaments). In this study, the researchers concentrated on actin filaments, that are known to play a vital part in the production of the contractile force in cells.An actin filament is a polarized (or oriented) polymer that can arrange itself into different architectural structures: parallel (in which all the filaments lie in the same direction), anti-parallel (in which they lie in opposite directions) and dense networks (in which they are interlaced). Highly localized contractions can occur in the cytoskeleton of each cell, in the same way as happens in the muscles of the body. These contractions are caused by mysosins. These are motile proteins that can move actin filaments and change their shape. The different actin architectures inside a cell are arranged in such a way that it is extremely difficult to study them separately. That is the reason why up until now, it was impossible to study what effect the arrangement of actin filaments had on the force exerted by mysosins.In this study, the researchers created a device that imitates a cytoskeleton and in which the different actin filament architectures are geometrically separated from each other, thus making them easily identifiable. The analyses carried out using this device showed that during contraction, mysosin molecules act in a specific and selective manner on the cytoskeleton. Under the action of the mysosins, the anti-parallel filaments contract rapidly, whereas the networked filaments contract more slowly and the parallel filaments do not contract at all. These selective phenomena suggest that, in the intracellular space, the action of mysosins only induces contraction in certain targeted structures. The results obtained also indicate that, within each cell, the speed of contraction and the change of shape of each actin filament are determined by its architectural arrangement.In addition to the importance of these results in gaining fundamental knowledge about the mechanisms that control certain vital cell properties, this study also constitutes a first phase of the work towards determining the cause of the mechanical intracellular dysfunctioning observed in pathologies as serious as cancer.