Abstract
Titin is an extraordinary long multidomain protein spanning half of the sarcomeric unit. One end of the titin filament is known to emerge from the Z-line, but the precise localization of the other end is still a matter of debate. Assuming that titin filaments connect the Z-line to the tips of thin filaments in the opposite half of the sarcomere, the behavior of an actively contracting muscle during mechanical stretch, as well as the mechanism of muscle shortening heat generation, can be easily explained if one considers that titin filaments are involved not only in the passive elasticity of the muscle but also in the force and heat generation during contraction.