Even though membranes have their lipids connected by hydrophobic interactions, which are not very strong, membranes do not break apart when an animal moves. This is due to the support provided to the membrane by proteins (intracellular, extracellular, integral membrane) and by other materials, especially outside the plasma membrane. This last group would include cell wall or extracellular matrix materials.
Work Step by Step
Something weak which withstands stresses must have something else supporting it. Concrete is a poor building material for making vertical walls, but it is used for huge skyscrapers. This works because rebar is added to the vertical concrete to support it. Just look through the chapter to see what non-lipids are part of membranes or contact membranes to find these supports.