The interference fine structure of the M3 reflection on the low-angle x-ray diffraction patterns of muscle fibres is used for the measurements of axial movements of myosin heads with a precision of 0.1-0.2 nm. We have measured changes in the M3 interference profile during tension rise induced by a 5 to 30 degrees C temperature jump in thin bundles of contracting fibers from rabbit skeletal muscle. Interpreting the data with a point diffractor model gives an estimate for the axial movement of the myosin heads during force rise of less than 0.6 nm. Modifications of the point diffractor model are discussed. We show that our experimental data can be explained by a model where myosin heads bind actin in a number of structurally different states.