Muscle contraction

The sarcomere - the functional unit of muscle contraction

As a muscle contracts, the thin and thick filaments move over one another, thus increasing their relative overlap.  This in turn causes the I bands and H zone to become smaller, although the filaments themselves do not change in size or length.

Adapted from Campbell, N. and Reece, J.  Biology. International 7th Ed. San Francisco: Benjamin Cummings, 2005, p. 1066-1069.