Unfolding transitions in myosin give rise to the double-hyperbolic force-velocity relation in muscle

This work presents an extension to a recent model of muscle contraction that was based on entropic elasticity (Nielsen 2002 J. Theor Biol. 219 99-119). By using entropic elasticity as the origin of muscle force, various possibilities emerge that can account for the presence of the double-hyperbolic force-velocity relation in muscle that was observed by Edman (1988 J. Physiol. 404 301-21). In the present work, it will be argued that a slight change (elongation) of the contour length of the entropic springs involved in their high-force regions is sufficient to produce such a double-hyperbolic profile. A sudden elongation would correspond to an unfolding event of a small region of the myosin molecule, which causes a sudden reduction of the tension that may be produced by the individual molecule. To obtain the double-hyperbolic profile, it is assumed that a gradual transition occurs in the entropic spring array from being mainly composed of non-unfolded myosin springs that have a short (i.e. normal) contour length to consisting of a mixture of myosin springs with short and long (unfolded) contour lengths.