Length-Tension Relations in Cardiac Muscle

The distinguishing mechanical characteristics of cardiac muscle are(1) the presence of a significant and sometimes labile resting tension at functional lengths. Certain invertebrate skeletal muscles also possess this property, but the structural basis seems to be different.(2)Force-velocity characteristics of heart muscle are labile and constitute a mechanism for regulating cardiac performance.(3)Data on quick-stretch and release show that the active state in heart muscle is slow in its onset, probably preceding the development of isometric tension by only a short time.The onset of the active state is labile also, and probably forms the basis of the regulatory function of the force-velocity relation. Measurements of heat show a slow rate of increase of heat during a twitch consistent with the onset of contractility.