X-ray diffraction patterns were recorded from single frog muscle fibres during steady shortening at the maximum velocity V₀, imposed at the plateau of an isometric tetanus (T₀), and during isometric force redevelopment following such shortening. During the first 20nm/half-sarcomere (hs) of shortening force decreased to near zero and changes in the X-ray pattern were due to the working stroke in actin-attached myosin heads followed by net detachment from actin. Beyond 20 nm/hs shortening the M3 meridional reflection (associated with the conformation of the myosin heads) and its second order M6 (associated with the thick filament structure) became more like those recorded at rest. At 110 nm/hs shortening the M3 spacing and its fine structure were the same as at rest, while the M3 intensity, the M6 spacing and the intensity of the first myosin layer line from the helical packing of the myosin heads, had recovered about half-way to their resting values, without sign of saturation. Force redevelopment following 110 nm/hs shortening and the associated structural changes were twice as fast as those at the start of electrical stimulation (Reconditi et al., PNAS, 108:7236, 2011), as these changes are no longer limited by the rate of thin filament activation. In both cases the initial force generation involves a small fraction of the myosin heads, whilst the majority are in the resting-like conformation.

Supported by MiUR and FIRB - Futuro in ricerca (Italy), MRC (UK), ESRF.