A time-resolved X-ray diffraction study of muscle during twitch.

1. A position sensitive X-ray counter was connected to a data-collection system which registered the outputs of the counter as a function of time. This enabled us to study time-dependent changes in the intensities of the equatorial reflexions from frog sartorius muscle during an isometric twitch. 2. The intensity ratio of the 1.0 and 1.1 equatorial reflexions (I1,0/I1,1) decreased during a twitch. The time at which the intensity ratio started to decrease coinsided approximately with the onset of tension development. No detectable change in the intensity ratio occurred during the latency relaxation. 3. The intensity ratio reached a minimum level approximately 90 msec before the peak of tension, and stayed at that level until the peak tension. 4. The intensity ratio returned to the 'resting' value 0.7--1.1 sec after the twitch tension had fallen to zero. 5. The observed decrease in the intensity ratio was interpreted as being caused by radial movements of the myosin projections from the vicinity of thick filaments to that of thin filaments. This leads us to conclude that, early in the rising phase of twitch tension, the number of myosin projections in the vicinity of thin filaments reaches the value obtained during a maximum isometric tetanus.