This one is listed last because it is the most accurate and detailed. It is the home page of K.C. Holes at the Max Planck Institute for Medical Research in Heidelberg. It has a rather long essay on the molecular basis of muscle contraction. The best parts are the figures of actin and myosin structure and the muscle movie of actin-myosin interaction.
This is the most detailed and accurate movie, http://www.mpimf-heidelberg.mpg.de/~holmes/
This site, from Cytokinetics, has a movie that begins with the heart contracting and zooms in to show actin and myosin interaction in cardiac muscle. You choose windows or Mac and then be patient while the movie loads.
http://www.cytokinetics.com/cyto/contractility
