Time-resolved equatorial X-ray diffraction measurements in single intact muscle fibres

Equatorial X-ray diffraction techniques have been successfully applied to the intact single muscle fibre preparation under length clamp and "fixed end" conditions. 10 and 11 intensity changes and stiffness have been measured in the same preparation. Under isometric conditions, equatorial signals and stiffness led force by 14-20ms during the rise of tetanic tension. During relaxation, stiffness and equatorial signals lagged force. The time course of the intensity changes suggests a low force crossbridge state is present to a greater extent during the rise of tetanic tension and during relaxation than at the tetanus plateau. During isotonic shortening at Vmax, stiffness fell to 30% of its isometric level, while equatorial signals fell to 60%. Since stiffness and equatorial signals are thought to detect attached crossbridges, either the average stiffness per attached bridge measured at 4kHz during shortening is less than at the plateau, or the relation between equatorial intensities and the proportion of attached crossbridges during isotonic shortening differs from that measured under isometric conditions. Active tension also affects the lattice spacing. The myosin lattice was compressed during the development of longitudinal force. This implies a radial component of crossbridge tension. The lattice compression was smaller in a compressed lattice and larger in an expanded lattice.