Fig. 3. Two-dimensional schematic representation of partial assemblies of contractile units in smooth muscle. (A) In a fully adapted muscle, the thick filaments are assumed to span the entire distance between the associated dense bodies. For simplicity, multiple attachments of thin filaments on one side of a dense body are not depicted. (B) Isometric contraction (1) and isotonic contractions (2,3) against different external loads. (C) Isometric contractions (1,2) at different lengths and isotonic contraction (3) against an isotonic load. Each numbered configuration represents an equilibrium (static) condition where external load equals the force produced by the contractile unit. The solid portion of the thick filaments represents the segment in between the dense bodies that overlaps with both of the thin filaments. It is assumed that only the cross bridges within the solid portion of the thick filaments can interact with the thin filaments properly to generate force. Length of the solid portion of the thick filaments in a contractile unit therefore correlates directly with the ability of the muscle to generate force or carry load during contraction. (See text for more details).