Fig. 3.
Hill's functional model of skeletal muscle. Muscle mechanical function will be represented in terms of the Hill's phenomenological model. By this model, active muscle is composed of three elements: CE, SEE, and PEE. CE represents contractile (sarcomeric) part of muscle, which generates active force and increases overall muscle stiffness. In the relaxed state, this element generates zero tension and zero stiffness. The SEE includes the elasticity of actin and myosin filaments and tendons, and the PEE includes the elasticity of the surrounding connective tissue and the elasticity of the noncontractile part of the muscle cell's cytoskeleton. The lengths of CE, SEE, and PEE in the relaxed state are denoted as Lmo, Lso, and Lpo, respectively. The relaxed muscle length is equal to Lpo = Lmo + Lso. The lengths of the Hill's model components during contraction are tLm = Lmo + tUm, tLs = Lso + tUs and tLp = Lpo + tUp, where tUm, tUs, and tUp are displacements, i.e., change of length from undeformed configuration of CE, SEE, and PEE, respectively.