Structural changes in the actin and myosin containing filaments during contraction

Earlier studies on the low-angle X-ray diffraction diagrams given by striated muscles showed that some features of the diagrams remained virtually unchanged during contraction, whereas others changed in a characteristic manner. Thus the sub-unit repeat and the pitch of the actin helices in the thin filaments remained apparently constant, and the subunit repeat in the myosin filaments (the 143 A cross-bridge spacing) remained almost con-stant too (Elliott et al., 1965, 1967; Huxley et al., 1965; Huxley and Brown, 1967), although more extensive measurements revealed a small increase in spacing during contraction, by about 1 % (Huxley and Brown, 1967; Haselgrove, 1967, 1970). On the other hand, there was a very substantial decrease i