We use X-ray diffraction to investigate how the force and the number of attached myosin motors influence the structure of the actin and myosin filament of contracting muscle. Whole muscles (Sartorius from Rana esculenta) at ca 2.1 mm sarcomere length are vertically mounted at the ID2 beamline of the ESRF Synchrotron (Grenoble, France) in a trough containing Ringer solution at 4 °C, with the upper extremity attached to the lever of a motor/force transducer system (300C-LR, Aurora Scientific Inc.). Muscles are electrically stimulated to produce isometric tetani (T₀), and then ramp lengthening and shortening at different velocities are imposed to attain different forces. 2D diffraction patterns are collected in 50ms time windows at T₀ and during ramps when force reaches a steady value. The spacing of the actin-based reflection at 2.73nm decreases linearly with force, from 1.5 to 0 T₀, with a slope 0.28±0.02 %/T₀. The spacing of the M15 myosin-based reflection at ca 2.9nm decreases linearly with force, with a slope 0.23±0.01%/T₀ between 1.5 and 0.5 T₀, and 2-3 times more steeply for lower forces. These results suggest that spacing changes in the actin and myosin filaments are coupled when the filament force and thus the number of attached motors (Piazzesi et al Cell, 2007, 131:784, 2007) is higher than 0.5 the isometric value. Myofilament periodicities uncouple when the force and number of attached motors are steadily decreased below 0.5 the T₀ values. Supported by CNISM, ESRF.