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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.