Of the three types of cell fibers, microfilaments are the smallest in diameter. The microfilaments help reinforce a cell and give it shape. They are also involved in cell motility (movement) and changes in the shell shape. In muscle cells, microfilaments are responsible for muscle cell contraction.
Intermediate filaments are made of twisted protein strands that are slightly larger in diameter than microfilaments. They help to hold together a cell if it is pushed and pulled.
Microtubules (video) are sometimes called the “engines” of the cell. They are made of protein subunits that are twisted in a spiral fashion. They are involved in the movement of substances in the cell or the entire cell. They are also involved in the movement of chromosomes during cell division.