The molecular basis of elasticity in insect flight muscle has been analyzed using both the mechanism of extensibility of titin filaments (Trombitás et al., J. Cell Biol. 1998;140:853-859), and the sequence of projectin (Daley et al., J. Mol. Biol. 1998;279:201-210). Since a PEVK-like domain is not found in the projectin sequence, it is suggested that the sarcomere elongation causes the slightly "contracted" projectin extensible region to straighten without requiring Ig/Fn domain unfolding. Thus, the extensible region of the projectin may be viewed as a single entropic spring. The serially linked entropic spring model developed for skeletal muscle titin was applied to titin in the heart. The discovery of unique N2B sequence extension in physiological sarcomere length range (Helmes et al., Circ. Res. 1999;84:1339-1352) suggests that cardiac titin can be characterized as a serially linked three-spring system. Two different cardiac titin isoform (N2BA and N2B) co-exist in the heart. These isoforms can be differentiated by immunoelectron microscopy using antibody against sequences C-terminal of the unique N2B sequence, which is present in both isoforms. Immunolabeling experiments show that the two different isoform are co-expressed within the same sarcomere.