A mechanical model of the half-sarcomere which includes the contribution of titin

ABSTRACT The evidence, in both resting and active muscle, for the presence of an I-band spring element like titin that anchors the Z line to the end of the thick filament did not yet produce a proper theoretical treatment in a complete model of the half-sarcomere. The textbook model developed by A.F. Huxley and his collaborators in 1981, which provides that the half-sarcomere compliance is due to the contribution of the compliances of the thin and thick filaments and actin-attached myosin motors, predicts that at any sarcomere length the absence of attached motors results in an infinite half-sarcomere compliance, in contrast with the observations. Growing evidence for the presence of a titin-like I-band spring urges the 1981 model to be implemented to include the contribution of this element in the mechanical model of the half-sarcomere. The model described here represents a tool for the interpretation of measurements of half-sarcomere compliance at long sarcomere lengths and for investigations of the possible role of titin as the mechano-sensor in thick filament regulation.