Structural changes in myosin cross-bridges during shortening of frog skeletal muscle

X-ray diffraction patterns from frog sartorius muscle were recorded during steady shortening with various loads. The intensity of the third meridional reflection from the thick filament decreased on shortening to an extent proportional to the drop in tension. The intensity correlated more closely with the tension than with the shortening velocity. The Bragg spacing of the third meridional reflection decreased in proportion to the decrease in tension. The intensity decrease of the actin layer lines at 1/5.1 and 1/5.9 nm-1 was roughly proportional to the decrease in the load, indicating that the number of cross-bridges decreases similarly. The intensity of the (1,1) equatorial reflection showed a significant decrease only with low loads. Assuming that a steady structural state is attained during steady shortening, the results are consistent with the cross-bridge model in which the number of myosin cross-bridges decreases during shortening.