Axial disposition of myosin heads in isometrically contracting muscles

Meridional x-ray diffraction diagrams, recorded with high angular resolutio n, from muscles contracting at the plateau of isometric tension show that t he myosin diffraction orders are clusters of peaks. These dusters are due t o pronounced interference effects between the myosin diffracting units on e ither side of the M-line. A theoretical analysis based on the polarity of t he myosin (and actin) filaments shows that it is possible to extract phase information from which the axial disposition of the myosin heads can be det ermined. The results show that each head in a crown pair has a distinct str uctural disposition. It appears that only one of the heads in the pair ster eospecifically interacts with the thin filament at any one time.