Biomechanics of cell motility

In order to move, cells exert force on the environment and, for cellular-sized objects, resistive drag forces are comparable to the forces that a cell can produce. Therefore, inertia is negligible: A cell that stops producing force is almost immediately brought to a halt by the environment. The effective absence of inertia means that the net force on a motile cell is zero. Here there is a subtlety. If a cell exerts force on the environment, then the environment must push back with an equal but opposite force. This is the thrust force that propels the cell. But the environment also resists the motion of the cell and exerts a force on the cell that opposes the motion. It is the environment that allows motion and yet impedes it.