What happens to mass during the process of flexing knees while running?

When flexing the knees while running, the mass of the body remains constant, as mass itself does not change. However, as the knees flex and the leg moves, the center of mass of the body shifts. Specifically, bending the knees brings the legs closer to the body's center of mass or axis of rotation. This movement means that the distribution of mass relative to the axis is altered, which can actually influence how inertia is experienced by the body during the running motion.

By moving the mass closer to the axis of rotation, such as the body's center of gravity, the moment of inertia decreases. This can facilitate faster movements and agility, enabling quicker changes in direction or acceleration. This principle is important in biomechanics, as a lower moment of inertia generally allows for more efficient and rapid movements, particularly in dynamic activities like running.