A flywheel for example to a sport training or a rehabilitation machine, is linked to a hauling cable through a system of pulleys, including, in a well-known way, at least a disk-shaped part (4) rotating about a central axis (5) and incorporates a series of weights (6) that, depending on their distribution and their own weight provide a given moment of inertia. Starting from this already known configuration, the flywheel (1) is distinguished in that it has a moving coupling means (7) that allows the variation of the position of the weights (6) on the disk (4) of the wheel and to modify the moment of inertia, without it being necessary to withdraw or replace any of the weights (6) or the disk (4).