The recent publication of the crystal structures of G-actin and of myosin subfragment-1, together with analysis of a time-resolved series of well sampled low-angle 2D X-ray diffraction patterns from bony fish muscle permits the study of the molecular movements in muscle that are associated with generation and regulation of contractile force. Here it is shown that even though low-angle (i.e. low resolution) X-ray diffraction patterns are being used, these patterns are sensitive, for example, to sub-domain movements of as little as 3 A or 4 degrees within the actin monomers of actin filaments. Actin filament diffraction patterns from whole muscle are being used to define actin domain and tropomyosin movements involved in regulation. Myosin and actin filament diffraction patterns are being used together to start to show how the complete "quasi-crystalline" unit cell in the bony fish muscle A-band can be modelled as a series of time-slices through a typical tetanic contraction of the muscle. In this way, the time sequence of images can be used to create "muscle--the movie".