Myofilament flexibility: a possible role in Hill's model for cardiac and skeletal muscle.

The sarcomeric unit ("sark") is an elastic structure (cf. hard rubber). The sark stretch under load is the sum of the deflections of: (a) the naked thick filament, (b) the joined thick-thin filaments, (c) the naked thin filaments, (d) the parallel array of S1 moieties (bending deflection), (e) the parallel array of S2 rods, and (f) the Z-filaments. Hill's model can be revalidated at the molecular level, if the contractile element is identified as the instantaneous array of contract points between each S1 moiety and the thin filament, such that sark stretch accounts for series elasticity. A matrix array of variously activated and test-rig-damaged sarks can account for large quick-release compliances in cardiac muscles.