An enhanced model of cross-bridge operation with internal elasticity.

A recent study has shown (as reported by Rosenfeld, Eur Biophys J 41:733-753, 2012) that an apparatus consisting of a cycling pump, a lever, and charged beads is able to generate force in accordance with Hill's force-velocity relation. Here, we show that a spring integrated into this microscopic model of a myosin motor allows reproducing, in general terms, the muscle fiber responses to sudden changes in fiber length. The time course of relaxation is governed by the same hindering force that determines the maximal value of muscle contraction velocity. Any single one of the exceptionally simple parts of the proposed model device corresponds to some element of the real myosin head and interacts with any other part in accordance with the laws of Newton, Coulomb, and Hooke. In essence, the model demonstrates that Coulomb repulsion should be understood as the physical source of muscle force. Accordingly, some fictitious master equation with ad hoc postulated rate constants is not needed to explain the essential mechanical characteristics of a muscle. The current model still contains no mechanism that could account for superfast relaxations within periods of about 0.1 ms.