Do cross-bridges contribute to the tension during stretch of passive muscle?

The tension rise during stretch of passive skeletal muscle is biphasic, wit h an initial steep rise, followed by a subsequent more gradual change. The initial rise has been interpreted as being due to the presence of numbers o f long-term, stable cross-bridges in resting muscle fibres. A point of weak ness with the cross-bridge interpretation is that the initial stiffness rea ches its peak value at muscle lengths beyond the optimum for myofilament ov erlap. To explain this result it has been suggested that despite the reduce d overlap at longer lengths, the closer interfilament spacing and a higher sensitivity of the myofilaments to Ca2+ allows more stable cross-bridges to form. Recently the stretch responses of passive muscle have been re-examin ed and it has been suggested that it is not necessary to invoke cross-bridg e mechanisms at all. Explanations based on a viscous resistance to interfil ament sliding and mechanical properties of the elastic filaments, the gap f ilaments, were thought to adequately account for the observed tension chang es. However, an important property of passive muscle, the dependence of str etch responses on the immediate history of contraction and length changes, thixotropy, cannot be explained simply in terms of viscous and viscoelastic properties. The review discusses the cross-bridge interpretation of muscle thixotropy and the relationship of passive stiffness to filament resting t ension and latency relaxation. It is proposed that cross-bridges can exist in three states; one, responsible for the resting stiffness, requires resti ng levels of calcium. When, during activation, calcium levels rise, cross-b ridges enter a low-force, high-stiffness state, signalled by latency relaxa tion, before they move to the third, force-generating state. It is conclude d that, compared with viscoelastic models, a cross-bridge-based explanation of passive muscle properties is better able to accommodate the currently k nown facts although, as new information becomes available, this view may ne ed to be revised.