Recent x-ray diffraction measurements of the axial periodicities of the actin and myosin filaments in contracting muscles show that they are stretched by small but significant amounts by the developed tension, so that at least one half, possibly more, of the observed compliance of a sarcomere must reside in the filaments themselves. This implies that the movement steps of a crossbridge deduced from quick-release experiments may be shorter than some previous estimates, necessitating at least two steps to account for current in vitro single-molecule measurements. Intensity measurements of the wider angle x-ray reflections also show some unexpected features.