Do actin-myosin cross-bridges exhibit a viscoelastic behaviour?

The S2 domain of myosin is the segment that links the myosin head (S1 domain) to the thick filament backbone and elongates to allow S1 to execute the power stroke. It has been suggested that S2 exhibits non-viscous behaviour, implying no lag in reaching steady-state force following power stroke elongation. However, the elongation step in the cross-bridge cycle occurs extremely fast, and the cross-bridge detaches rapidly following it without allowing perceptible changes in force. If a muscle is transferred to a rigor solution following activation, all cross-bridges become stuck in a post power stroke state, which allows for investigation of their viscoelastic behaviour. The objective of this study was to investigate the viscoelastic behaviour of cross-bridges by examining their force following transfer to a rigor solution. Fibres (n = 8) were activated at an average sarcomere length (SL) of 3.0 µm, and then transferred into a rigor solution for 90 s. Since active stretch has the potential to influence the behaviour of cross-bridges, fibres were activated at a SL of 2.4 µm, actively stretched to a SL of 3.0 µm, and at steady-state transferred to a rigor solution for 90 s. Active force and stiffness were compared before and after incubation in rigor, and between purely isomeric and active stretch contractions. Although stiffness, and thus the presumed proportion of attached cross-bridges, increased when fibres were transferred to rigor, force at steady-state decreased by 16.5 +/- 4.0% and 10.0 +/- 1.3%, in the purely isometric and active stretch conditions, respectively. These results suggest that cross-bridges exhibit a viscoelastic behaviour, where their force decreases while they remain attached in a constant conformation. The difference in force decrease between the purely isometric and active stretch conditions suggests that active stretch modulates cross-bridge properties. Further research is warranted to investigate the cross-bridge nanostructure before and after rigor incubation.