Fig. 4.

With stiffer titin and longer SL, cross-bridges produce less myosin filament strain thereby reducing force output and contraction efficiency. The elastic potential energy was calculated (using the equation in the inset) for the myosin filament based on the amount of myosin filament strain imposed by the cross-bridges (XBs) under isometric maximum tension or by titin prior to activation. Each circle on the curve represents the amount of titin-based strain for each SL and titin stiffness. For more compliant titin (b = 4 μm⁻¹), the amount of titin-based strain at SL 3.25 μm is on the same order of magnitude as that delivered by the XBs (left inset). However, for very stiff titin (b = 10 μm⁻¹), the same amount of elastic energy delivered to the myosin filament from the XBs results in <1 nm of strain. As titin delivers more strain to the myosin filament, the XBs become less effective in generating force.