When you stretch a rubber band, all of the parts of the rubber band move in unison. When you stretch a more complicated material, this is not necessarily the case.
Below is a video of a 3D collagen gel (which you cannot see) and the fibers of the gel are decorated with small fluorescent particles (bright dots). The video is a minimally artistic compilation of 4 different parts of the gel. Instead of stretching the gel, I have applied a shear deformation by moving a boundary, causing the network to move. In the video, you can imagine the top edge moving to the right, while the bottom edge moves to the left. During this deformation, I am using confocal microscopy to image inside the network.
While this collagen network is deformed, you can see that the particles attached to the fibers move both along the direction of shear and in bursts; sometimes these even move in and out of the plane. This is very different than what is seen in a rubber band. Hopefully by understanding how these particles move, we can gain insight into how structural changes in collagen affect its strength.