J. Mol. Biol. (1983) 169, 469-506 Changes in the X-ray Reflections from Contracting Muscle During Rapid Mechanical Transients and their Structural Implications H. E. HUXLEY I, R. M. SIMMONS 2, A. R. FARUQI I,M. KRESS 1 J. BORDAS 3 AND M. H. J. Koch 3 1Medical Research Council Laboratory of Molecular Biology Hills Road, Cambridge CB2 2QH, England 2Medical Research Council Cell Biophysics Unit 26-29 Drury Lane, London WC2B 5RL, England 3European Molecular Biology Laboratory c/o Deutsches Electronen Synchrotron Notkestrasse 85, D; 2000 Hamburg 52 Federal Republic of Germany (Received 17 February 1983) During normal contractions of vertebrate striated muscle, it is believed that the cross-bridges which produce the sliding force undergo asynchronous cyclical changes in their structure. Thus, an X-ray diffraction diagram from a muscle under these conditions will give structural information averaged over the whole range of cross-bridge states. Such diagrams show characteristic and informative differences from those given by relaxed muscle, but can give little information about changes in the configuration of the cross-bridges at different stages of their working stroke. However, it is possible to effect a partial synchronization of these changes by applying very rapid changes in length, completed in less than one millisecond to an otherwise isometrically contracting muscle. If the amplitude of these length changes is comparable to the length of the cross-bridge stroke (say 100 A per half-sarcomere), then it should bring about a transient but significant redistribution of cross-bridge states, which would show up in the X-ray diagram. We have made use of synchrotron radiation as a high intensity X-ray source in order to record such patterns with the necessary time resolution (1 ms or less) and have found major changes in the intensity of the 143 A meridional reflection accompanying the rapid length changes of the muscle. These changes appear to arise from specific configurational changes in the cross-bridges during the working stroke. A model is suggested in which the 143/1, meridional intensity in a contracting muscle arises mainly from attached cross-bridges and is generated by the part of the myosin head near the $1-$2 junction. During normal contraction, cross-bridges go through their structural cycle asynchronously with each other, since they start at different times, but if the S 2 changes in length rather little, then the configurational changes in the myosin heads are synchronized with the actin filament movement in such a way that the $1-$2 junction remains relatively 41i9 0022-2836/83/260469-38 $03.00/0 9 1983 Academic Press Inc. (London) Ltd.