Fig. 2.

Imagine a cross bridge attached at an actin binding site as shown in the top panel of the figure. Its force is given by the force in the linear elastic spring that attaches the cross bridge to the myosin filament, which depends exclusively on how far the cross bridge is from its equilibrium position, given by x. If the cross-bridge attachment occurs during a concentric contraction (C), the muscle is shortening and the actin filament will move to the left relative to the myosin filament, thereby decreasing the cross-bridge x-distance and thus the force exerted by the cross bridge on actin. In contrast, if the cross-bridge attachment occurs during an eccentric action (E), the actin filament will move to the right relative to the myosin filament, thereby increasing the cross bridge's x-distance and thus its force. f = rate function of attachment; g = rate function of detachment; +h = the maximal range of possible cross-bridge attachments to actin; M = the myosin cross bridge.