Possible Contribution of Titin Filaments to the Compliant Series Elastic Component in Horseshoe Crab Skeletal Muscle Fibers

In horseshoe crab skeletal muscle fibers, the extension of SEC at the maximum isometric force P 0 is about 6% of the slack fiber length L 0 (sarcomere length, 7 μm), i.e., about 210 nm per half-sarcomere, being too large to be explained by the cross-bridge and the thin filament elasticities. Cinematographic studies of isometrically contracting myofibril bundes indicate that the highly compliant SEC mostly originates from the “elastic” thick filament misalignment in each A-band during isometric force generation. Possible contribution of the titin filaments to the “elastic” thick filament misalignment is discussed.