title: A cross-bridge cycle with two tension-generating steps simulates skeletal muscle mechanics.
abstract: We examined whether cross-bridge cycle models with one or two tension-generating steps can account for the force-velocity relation of and tension response to length steps of frog skeletal muscle. Transition-state theory defined the strain dependence of the rate constants. The filament stiffness was non-Hookean. Models were refined against experimental data by simulated annealing and downhill simplex runs. Models with one tension-generating step were rejected, as they had a low efficiency and fitted the experimental data relatively poorly. The best model with two tension-generating steps (stroke distances 5.6 and 4.6 nm) and a cross-bridge stiffness of 1.7 pN/nm gave a good account of the experimental data. The two tensing steps allow an efficiency of up to 38% during shortening. In an isometric contraction, 54.7% of the attached heads were in a pre-tension-generating state, 44.5% of the attached heads had undergone the first tension-generating step, and only 0.8% had undergone both tension-generating steps; they bore 34%, 64%, and 2%, respectively, of the isometric tension. During slow shortening, the second tensing step made a greater contribution. During lengthening, up to 93% of the attached heads were in a pre-tension-generating state yet bore elevated tension by being dragged to high strains before detaching.
authors: School of Physiology and Pharmacology, University of Bristol, Bristol, UK. g.w.offer@bristol.ac.uk
authors: Gerald
authors: G
authors: Offer
authors: K W
authors: KW
authors: Ranatunga
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001001:Anura; D001696:Biomechanical Phenomena; D006868:Hydrolysis; D055595:Mechanical Phenomena; D008954:Models, Biological; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000378:metabolism / Q000502:physiology*; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D000255:Adenosine Triphosphate
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