Rapid load change of isometrically contracting muscle causes a synchronised actomyosin power stroke. This power stroke causes a change in intensity (I_M3) of the meridional X-ray reflection at 14.5 nm, which is thought to indicate both elastic and active changes in the myosin lever arm tilt. During sinusoidal length oscillations (0.1-1 kHz), I_M3 signals are approximately sinusoidal, but distorted at the point of maximum shortening, where I_M3 crosses an intensity maximum (I_M3,max), causing an I_M3 well. As oscillation frequency approaches 1 kHz, this well is attenuated until, above 1 kHz, it is absent and I_M3 signals become undistorted sinusoids (Bagni et al. 2001). The frequency domain in which I_M3 distortion decreases corresponds to that over which the power stroke contribution to I_M3 becomes increasingly attenuated. If both power stroke and elastic components of I_M3 signals arise from lever arm tilting, then I_M3 distortion at high frequencies would be restorable by raising its elastic tilting component through increased length oscillation amplitude.

Intact fibre bundles from a toe muscle (dorsal interossei) of Rana temporaria (killed by decapitation) were mounted horizontally between a moving coil motor and a capacitance force transducer. Synchrotron radiation (from the source Elettra, Trieste, Italy; wavelength 0.15 nm, dimensions 0.3 X 3.0 mm) was admitted to the experimental chamber by a fast shutter mechanism at the plateau of an isometric tetanic contraction, while sinusoidal length oscillations were imposed simultaneously at 2.8 kHz. I_M3 was monitored by a 1D delay line detector at 2.6 m from the preparation, with a sampling time resolution of 17 µs.

Oscillation amplitudes causing a peak to peak force oscillation equal to tetanic tension (P_o) produced an undistorted I_M3 signal. However, when the force oscillation was increased to 1.4-1.6 P_o, distortion became evident (Fig. 1), similar to that seen at lower oscillation frequencies. Simulation of these effects using the molecular structure of myosin support a tilting mechanism model of the power stroke event, and show a mean tilt during oscillations of 70 deg to the fibre axis (based on the crystallographic S1 structure (Rayment et al. 1993), and a displacement of 0.76 ± 0.20 nm (n = 7) from that at I_M3,max.

This work was supported by the E.U. Human Potential Programme.